Nuclear Physics News - NuPECC

Nuclear Physics News

Volume 17/No. 4

Nuclear Physics News is published on behalf of the Nuclear Physics European Collaboration Committee (NuPECC), an Expert Committee of the European Science Foundation, with colleagues from Europe, America, and Asia.

Editor: Gabriele-Elisabeth Körner Editorial Board T. Bressani, Torino R. F. Casten, Yale P.-H. Heenen, Brussels (Chairman) J. Kvasil, Prague M. Leino, Jyväskylä

S. Nagamiya, Tsukuba A. Shotter, Vancouver H. Ströher, Jülich, Jülich T. J. Symons, Berkeley C. Trautmann, Darmstadt

Editorial Office: Physikdepartment, E12, Technische Universitat München, 85748 Garching, Germany, Tel: +49 89 2891 2293, +49 172 89 15011, Fax: +49 89 2891 2298, E-mail: [email protected] Correspondents Argentina: O. Civitaresse, La Plata; Australia: A. W. Thomas, Adelaide; Austria: H. Leeb, Vienna; Belgium: C. Angulo, Lauvain-la-Neuve; Brasil: M. Hussein, São Paulo; Bulgaria: D. Balabanski, Sofia; Canada: J.-M. Poutissou, TRIUMF; K, Sharma, Manitoba; C. Svensson, Guelph: China: W. Zhan, Lanzhou; Croatia: R. Caplar, Zagreb; Czech Republic: J. Kvasil, Prague; Slovak Republic: P. Povinec, Bratislava; Denmark: K. Riisager, Århus; Finland: M. Leino, Jyväskylä; France: G. De France, GANIL Caen; M. MacCormick, IPN Orsay; Germany: K. Langanke, GSI Darmstadi; U. Wiedner, Bochum; Greece: E. Mavromatis, Athens; Hungary: B. M. Nyakó, Debrecen; India: D. K. Avasthi, New Delhi; Israel: N. Auerbach, Tel Aviv; Italy: M. Ripani, Genova; L. Corradi, Legnaro; Japan: T. Motobayashi, RIKEN; Mexico: J. Hirsch, Mexico DF; Netherlands: G. Onderwater, KVI Groningen; T. Peitzmann, Utrecht; Norway: J. Vaagen, Bergen; Poland: B. Fornal, Cracow; Portugal: M. Fernanda Silva, Sacavém; Romania: V. Zamfir, Bucharest; Russia: Yu. Novikov, St. Petersburg; Serbia: S. Jokic, Belgrade; Spain: B. Rubio, Valencia; Sweden: P.-E. Tegner, Stockholm; Switzerland: K. Kirch, PSI Villigen; United Kingdom: P. Regan, Surrey; USA: D. Geesaman, Argonne; D. W. Higinbotham, Jefferson Lab; M. Thoenessen, Michigan State Univ.; H. G. Ritter, Lawrence Berkeley Laboratory; G. Miller, Seattle.

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Vol. 17, No. 4, 2007, Nuclear Physics News

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Nuclear Physics News

Volume 17/No. 4

Contents Editorial .............................................................................................................................................................. 3 Laboratory Portrait Radioactive Isotope Beam Factory at RIKEN (RIBF) by Tohru Motobayashi and Yasushige Yano .................................................................................................. 5 Feature Articles Alpha Particle Condensation in Nuclear Systems by Y. Funaki, H. Horiuchi, G. Röpke, P. Schuck, A. Tohsaki, and T. Yamada ............................................. 11 Mass Chain Evaluations for the Evaluated Nuclear Structure Data File (ENSDF)—An Urgent Appeal for European Participation by F. G. Kondev, A. L. Nichols, and J. K. Tuli .............................................................................................. 19 Giant Resonance Overtones: Compression Modes of the Nucleus. by Mátyás Hunyadi and Muhsin N. Harakeh ................................................................................................ 24 Neutron Stars and Nuclei: Two Dense Systems. by M. Fallot, M. Grasso, E. Khan, and J. Margueron .................................................................................. 31 Impact and Application Heavy-Ion Beam Pumped UV Laser by Andreas Ulrich ......................................................................................................................................... 37 Meeting Reports International Nuclear Physics Conference INPC2007 by Shoji Nagamiya, Ohru Motobayashit, and Makoto Oka .......................................................................... 40 Nuclear Physicists Meet in the Land of the Incas by Ricardo Alarcon........................................................................................................................................ 43 International Symposium on Physics of Unstable Nuclei (ISPUN07) by Dao Tien Khoa and Nguyen Van Giai ..................................................................................................... 45 Facilities and Method New Promises for the Determination of the Neutrino Mass? (A Brainstorming Meeting at GSI, Darmstadt) by H.-Jürgen Kluge and Yuri Novikov ......................................................................................................... 48 News from EPS/NPB ........................................................................................................................................ 51 Calendar ............................................................................................................................................................ 52

Cover illustration: tk.

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Nuclear Physics News, Vol. 17, No. 4, 2007

editorial 1

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The Canadian community of nuclear and particle physicists has just published its vision for the next decade, outlining the present strengths of and the future objectives for the national research program (“Perspective on Subatomic Physics in Canada for 2006–2016”). In Canada, nuclear and particle physics are sponsored by a common program of NSERC called subatomic physics, and have a common national laboratory, TRIUMF. The Long Range Planning Committee (LRPC) report takes an in-depth look at the current set of activities and proposes bold recommendations for seizing new scientific opportunities. It then describes the level of resources needed to realize these objectives. To capture the most benefit, the study concluded, the level of support would need to double over the course of (N) years. This situation is quite similar to that in other countries, indicating a healthy global program in nuclear physics. A good number of other nations are making significant commitments to new facilities as well. In this article, I will be focusing on the nuclear physics elements. The Canadian report looks particularly carefully at the development of graduate students in subatomic physics. More Canadian students are choosing careers in subatomic physics! The study shows a strong growth (45%) in the number of graduate students in the period of the survey (2001–2005). The students are about equally split between the two subfields. This increase can be explained by a large faculty renewal (35%) due

to retirement (young faculty tend to be more research and graduate student hungry), some special Canadian programs to attract high profile faculty into the country (such as Canada Research Chairs), and capital investments in new facilities such as ISAC at TRIUMF, SNO, and SNOlab. The LRPC also examines the global standing of the Canadian facilities. It finds that these facilities are highly competitive and world-leading. Based on these observations, the report makes five recommendations to optimize the scientific return for Canada. The one relevant for this audience: Full exploitation of the high intensity radioactive beams for nuclear physics and nuclear astrophysics at ISAC and ISAC-II at TRIUMF. Again, this projects a very positive outlook into the future. We here in Canada are extremely fortunate with the facilities and experimental devices we have. Things become a little more difficult when one looks into what resources will be available to operate the facilities, to build the upgrades for new and more radioactive beams at TRIUMF, which one needs to optimally utilize the world-class facilities, and to pay for researchers and the aforementioned students, who are flocking in ever-increasing numbers, into our nuclear physics labs. In the case of Canada, the LRPC investigated different scenarios. The findings of the committee are very clear; if we want to exploit the already-made recent major capital investments in the highest priority projects, substantial new funds must be allocated. How can

this be achieved, and what is the role that we as individuals in Canada but also the worldwide nuclear physics community can play? The fact is, these are not unique problems to Canada. On a recent visit from a delegation of the German Ministry of Research and Education, the State Secretary who is in charge of evaluating the available budget for nuclear physics and facilities in Germany, asked me if it is more sensible to have ten Ferrari cars parked in the garage (note, Ferrari represents a state-of-the-art Formula 1 race car, not the luxury item!), with little money for gas, or whether one should rather buy nine and save the extra money to drive the others more. The answer is probably: Buy ten and look for extra money elsewhere. And this is where the worldwide community comes in. The European countries have since long worked together to come up with coherent plans for all its countries, but now one would hope, we can go a step further and try to bring even more communities together—the Americas, Asia, and Europe. We need more overlooking planning or communication to firstly come up with sufficient arguments why we want to extend certain fields or facilities, and why this in not done elsewhere, but secondly strengthen these by providing arguments coming from people outside the country who say they are interested and willing to contribute. Planning and communication on a global scale is more widely spread in the high energy physics community, and everything points

The views expressed here do not represent the views and policies of NuPECC except where explicitly identified.


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editorial toward the fact that we are entering this phase in nuclear physics as well. We have to make sure not everybody is buying the same ten Ferraris and then looking elsewhere for help. There has to be a balance between competition and complementarities. Currently, TRIUMF, Canada’s National Laboratory for Particle and Nuclear Physics, is preparing its next five-year plan. TRIUMF operates in five-year cycles and the present one ends in 2010. For the new plan, major upgrades and new facilities are being considered and we will again face the same dilemma: balancing new capital investments—which one needs to stay competitive—against support for the operation of existing facilities—which one needs to realize the full scientific potential of these world-leading devices. TRIUMF is approaching these questions in a truly global fashion by opening dialogues with other countries. For example, TRIUMF is hosting a joint workshop with the Oak Ridge National Laboratory at the U.S. APS DNP meeting, to evaluate common goals and strategies for an electrondriven photo-fission facility at ISAC.

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In another example, TRIUMF is hosting a workshop in the spring of 2008 to look at joint ventures with Germany’s GSI/FAIR and possible Canadian participation through TRIUMF’s next five-year plan. These are examples of ad-hoc strategies from one country (Canada), and certainly other nations do similar things. However, a more orchestrated approach is bound to be even more successful. The recent INPC in Tokyo was an ideal occasion to bring together the worldwide community to initiate such planning. We should follow that example with INPC2010 in Vancouver, but my hope is that we will not wait that long: more global communication and planning is possible right now! The IUPAP Working Group on International Cooperation in Nuclear Physics (ICNP) is an international body with the goal of coordinating the global program. Also, as for global planning, the OECD, as another global body, recommends regional planning, although it is still not happening. For example, the U.S. and Canada could get together on ISOL-based and fragmentation-based isotopes production,


and develop a coherent plan. I think we should all support these efforts. Moreover, we as individuals have to work toward this end within our own countries to encourage our decision makers to enter the dialogue with other countries and to start regional planning (and eventually global planning), at least for the next generation of facilities. Nuclear science is presently in an excellent position on a global scale, but real work, for keeping it that way, lies ahead of us. JENS DILLING TRIUMF Vancouver

laboratory portrait Radioactive Isotope Beam Factory at RIKEN (RIBF) and Superconducting Ring Cyclotron (SRC) with K = 2600 MeV. The first primary beam, 345MeV/nucleon 27Al10+, was extracted on December 28, 2006. The first experimental result, production of a new neutron-rich isotope 125 Pd, was obtained in May 2007 with 238 U beams. The RIBF is located in the RIKEN Wako Campus in Wako-shi, Saitama, Japan (see Figure 1). A schematic view of the facility is shown in Figure 2.

Introduction The RIKEN RI Beam Factory (RIBF) has started its operation after ten years of construction [1]. It is designed to provide beams of various kinds of unstable nuclei with the world’s highest intensities. The institute RIKEN was founded in 1917, and covers a variety of research fields such as physics, chemistry, medical science, biology, and engineering. RIKEN has a long history of constructing accelerators. The first cyclotron in Japan was built in 1937 by Yoshio Nishina in the former RIKEN campus, and the RIKEN’s 4th cyclotron completed in 1966 was the first heavy-ion cyclotron in Japan, which accelerated light nuclei to the energy around 7 MeV/nucleon. Since 1987, RIKEN has provided intermediate-energy light- and heavyion beams by a four-sector ring cyclotron (RIKEN Ring Cyclotron, RRC) with K = 540 MeV coupled with two injectors, an azimuthally varying field (AVF) cyclotron with K = 78 MeV and a 16 MV variable-frequency linear accelerator (RILAC). The maximum energy for light heavy-ions such as 16 O is 135 MeV/nucleon. Vector- and tensor-polarized deuteron beams are also available. To provide the beam matched to requirements from the new RIBF accelerators, several improvements have been made. For example, a variable-frequency RFQ linac (FCRFQ) [2] and an 8-MV fixed-frequency booster linac (CSM) [3] were installed before and after the RILAC. Thus, intense ions at several MeV/ nucleon energies became available in

the RILAC experimental hall. In 2004, two events indicating the production of the isotope 278113 by the 209Bi(70Zn, n) fusion reaction have been observed [4]. The experiment was performed with an intense beam of 5 MeV/ nucleon Zn from the RILAC. The beams have been used also for various applications to nuclear chemistry, bio and medical science, and materials science. Among them, production of light-mass RI-beams by a projectilefragment separator (RIPS [5]) is one of the characteristic features of the facility. The new facility RIBF uses the accelerators RILAC and RRC, and successively boosts the beam energy up to 345 MeV/nucleon by three newly-built cyclotrons, the fixed-frequency Ring Cyclotron (fRC) with K = 570 MeV, Intermediate stage Ring Cyclotron (IRC) with K = 980 MeV,

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Figure 1. View of the RIKEN Wako Campus. The picture was taken in May 2005, when the RIBF was under construction.

The word “RI beam” is an abbreviation of Radioactive Isotope beam.

RI Beams at RIKEN At RIKEN, RI beams have been produced by the projectile fragmentation scheme since 1990. Due to the large angular momentum acceptance and high bending power of the separator RIPS [5], the RI beam intensities are highest in the world for many light neutron-rich nuclei. By using this


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laboratory portrait

Figure 2. Bird’s-eye view of the RIBF.

high-intensity capability, the very neutron-rich fluorine isotope 31F was found to be particle-stable [6]. Besides these intermediate-energy RI beams, beams of light unstable nuclei at lower energies, typically 5 MeV/nucleon, are also available in the CRIB facility constructed by CNS [7]. The CRIB uses in-flight direct reactions, such as (p,n) and (3He,n), to produce RI beams, which are mainly used for studying low-energy reactions of astrophysical interest. Studies Using the Fast RI Beam The RI beams from the RIPS have been used for various experiments. Following the pioneering works at LBL in the 1980s [8], studies of interaction cross-section have been performed in RI-beam facilities including RIKEN and GSI, and neutron halo and neutron skin structures have been established in some light neutron-rich nuclei [9]. Certain neutrons have extended spatial distribution outside of the core where neutrons and protons are equally distributed. This is the first indication that some neutrons can be decoupled from protons in spite of the strong p–n interaction. Disappearance of magic numbers is another interesting phenomena. The

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first Coulomb excitation experiment with fast RI beams has been performed for the neutron-rich 32Mg nucleus with the N = 20 magic number [10]. The extracted large B(E2) value supports the idea of disappearance of the N = 20 shell closure in 32Mg. A new method of γ-ray spectroscopy, measurement of γ-rays from fastmoving excited nuclei in coincidence

with reaction products with particle identification, was applied to this experiment, and many experiments using this technique have been performed so far for studying nuclear structures of nuclei around the shell closure at N = 8 and N = 20. Recently, the decoupling of protons and neutrons has been revealed also in excitation of the 16C nucleus to its 2+ state. The 2+ lifetime measured in a new recoil shadow method [11], inelastic scattering with 1H [12], and the one with 208Pb [13] all point to an anomaly: Neutrons almost solely contribute to the 2+ state excitation whereas protons have little contribution. This picture might be related to the low electric quadrupole moments measured for 15B and 17B using the β-NMR technique, which require little neutron effective charge in a shellmodel calculation [14]. Studies of particle-unbound states are another highlight. The Coulomb

Figure 3. Superconducting Ring Cyclotron (SRC).


laboratory portrait

Figure 4. Superconducting RI beam separator (BigRIPS).

dissociation to simulate astrophysical (p,γ) reactions [15] and the neutron halo structure [16] has been extensively studied. These are examples of the studies at RIKEN using the fast RI beams. Encouraged by their success, the RIBF project has been launched to extend the research opportunities provided by the use of RI beams. SRC, Big RIPS, and Zero Degree Spectrometer The part of RIBF that has been completed is the cyclotron complex and the superconducting RI beam separator Big RIPS [17] (see Figure 2). The Zero Degree spectrometer will be completed in fall 2007. Figure 3 shows the picture of the Superconducting Ring Cyclotron (SRC), which is the world first superconducting ring cyclotron with the largest bending power. It consists of six sector

magnets and four RF cavities. It has superconducting main- and trim-coils. The valley regions between the sector magnets are covered with 1-m thick soft iron slabs in order to reduce the stray magnetic-field that otherwise deflects the beam to the opposite direction. The total weight of the SRC amounts

to 8300 tons. The mean injection and extraction radii are 3.56 m and 5.36 m, respectively. The SRC can boost the energy of output beams from the IRC up to 440 MeV/nucleon for light ions and 345 MeV/nucleon for ions up to uranium. The primary beams are converted to intense RI beams by the BigRIPS (see Figure 4 for the picture) with the help of in-flight fission and/or projectile fragmentation of heavy-ions including uranium. A plan view of the BigRIPS is shown in Figure 5. The BigRIPS consists of fourteen superconducting quadrupole triplets and six room-temperature dipoles. It employs a two-stage separation scheme. The first stage serves to produce and separate RI beams with a wedge-shaped degrader inserted at the momentumdispersive focus F1. The second-stage identifies RI beam species event-byevent and tags the secondary beam that still contains various different ions. The horizontal and vertical angular acceptances are designed respectively to 80 mrad 100 mrad vertically, while the momentum acceptance is 6%. These angular and momentum acceptances enable one to collect about half of the fission

Figure 5. Plan view of the Big RIPS and Zero Degree spectrometer together with the two cyclotrons IRC and SRC of the RIBF accelerate complex.


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laboratory portrait

Figure 6. Nuclear chart covered by the RIBF project. The thick solid curves indicate the limit of RI productions of 1 particle per day. An expected r-process path is also shown.

fragments produced by a 350 MeV/ nucleon uranium beam. The Zero Degree spectrometer (ZDS), also shown in Figure 5, analyzes secondary reaction products emitted in the beam direction. Its typical use is for γ-ray measurements in coincidence with fast-moving excited nuclei discussed earlier. The ZDS is used to identify the reaction product. For other applications, such as β-decay measurements with stopped RIs, the ZDS is also useful. The RI beam yield estimated by the code EPAX2 [18] with a primary beam of 1 particle μA intensity and 350 MeV/nucleon energy is illustrated in Figure 6. For example, the intensities of the doubly magic nuclei 78Ni, 132Sn, and 100Sn are expected to be 10, 100, and 1 particles/s, respectively, encouraging detailed studies of nuclei far from the stability valley.

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Status of RIBF As mentioned, after the first extraction of the beam, the accelera-

tors have been tuned to improve their performance, and 238U ions were successfully accelerated in March 2007. Commissioning of RI beam production started in March and the first attempt of new-isotope production with uranium beams was performed. A 345 MeV/nucleon 238U86+ beam was delivered to a beryllium production target of 7-mm thick. The parameters of the BigRIPS were matched to neutron-rich fission products with the atomic number around 50. Figure 7 shows a correlation plot for the atomic number Z and the mass-tocharge ratio A/Q. These quantities are obtained from the energy-loss, total energy, velocity (or time-of-flight) and magnetic rigidity measured by beam-line counters with the help of track-reconstruction. The yield for Z = 46 (palladium) is plotted in Figure 8. Observed double peak structures are due to the mixture of ions not fully stripped. A peak corresponding to the new isotope 125Pd,

Z Z = 46

A/Q

Figure 7. Correlation plot for the atomic number Z and the mass-to-charge ratio A/Q of fission products produced by the 238U+9Be interaction at 345 MeV/nucleon.


laboratory portrait indicated by the arrow, is clearly seen with 26 counts. The data were taken with the beam current of 4 × 107 particle/s on an average, which is about 10−5 of the goal intensity, 1 particle μA, and the data acquisition time is about one-day. This indicates high potential of the Big RIPS separator and future possibility of the RIBF in accessing a large amount of unknown unstable nuclei. Because the RI beam intensities achieved so far is not enough for secondary reaction studies, various efforts are ongoing including the improvement of the beam transmission in every stage of the accelerator complex. The use of the 48Ca and 86Kr beams as well as 238U is currently assumed in the first series of experiments. Information on the RIBF status including the call for proposal is found on the Web page http://www.nishina.riken.jp/UsersGuide/. Major Experimental Installations To fully exploit the research opportunity provided by the variety of RI beams from the RIBF, several programs to construct experimental equipment are being considered or in progress. The SHARAQ spectrometer [19] is a high-resolution spectrometer for missing-mass measurement with RI beams. It is of a QQ-D-Q-D configuration. By employing dispersion-matching optics with a specially designed beam line from the BigRIPS, the momentum resolution of 15,000 will be achieved. The SHARAQ spectrometer is under construction and will be installed in 2007. The SLOWRI [20] aims at conducting various experiments using slow or trapped RIs. RI beams from the Big RIPS will be efficiently stopped and extracted by a gas-catcher system with the RF ion-guide technique.

The SAMURAI is a spectrometer [21] with a large solid angle and a large momentum acceptance dedicated to particle-correlation studies. It is of a QQQ-D configuration, where the dipole D is a superconducting Htype magnet with 6.7 Tm rigidity. Its large gap (80 cm) is useful for measurements of projectile-rapidity neutrons. The e-RI ring is for electron-RI scattering experiments using a SelfConfining Radioactive Ion Target (SCRIT [22]). RI ions are transversely confined due to the attractive force caused by the electron beam itself. A mirror potential is applied externally to achieve longitudinal confinement. Test experiments to examine the confinement mechanism are ongoing at an existing electron ring. The rare RI mass ring [23] is designed to measure the mass of rare (with the production rate of 1 particle per day, for example) exotic nuclides

in 10−6 accuracy. Each ion is injected individually to the ring by a trigger signal provided from a counter in the Big RIPS. The ring is precisely tuned to achieve the isochronous condition, and a time-of-flight of the ion in the ring is measured. To allow for running an RI-beam based experiment and a super heavy element search simultaneously, construction of a new injector [24] is being considered. Another possibility to extend the research opportunity is to build a beam line that brings back the IRC beam to the RIPS separator. Various nuclear moment studies and condensed matter research are planned. Summary The RIKEN RI Beam Factory (RIBF), one of the new-generation RI beam facilities, has started its operation. Potential of RI-beam production by the RIBF cyclotron complex coupled with

Figure 8. Yield distribution obtain by selecting the events in Figure 7 by a “Z = 46” gate. The arrow indicates the peak position for the new isotope 125Pd.


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laboratory portrait the superconducting RI beam separator Big RIPS was demonstrated by production of the new isotope 125Pd. When the RIBF reaches to its full performance, it provides a fascinating opportunity to artificially produce and experimentally study almost all nuclides that have been created and are being created now in the universe. Continuous attempts to improve the primary beam intensity together with construction of various experimental equipment will open a new domain of nuclear physics, and hopefully create a new view on atomic nucleus as well as on the element genesis in the universe, incorporated with the world efforts for realizing RI-beam facilities being constructed or planned.

References 1. Y. Yano, Nucl. Instr. Meth. B 261 (2007) 1009.

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2. O. Kamigaito et al., Rev. Sci. Instr. 76 (2005) 0133061-1. 3. O. Kamigaito et al., Rev. Sci. Instr. 70 (1999) 4523. 4. K. Morita et al., J. Phys. Soc. Jpn. 73 (2004) 2593. 5. T. Kubo et al., Nucl. Instr. Meth. B 70 (1992) 309. 6. H. Sakurai et al., Phys. Lett. B 448 (1999) 180. 7. Y. Yanagisawa, et al., Nucl. Instr. Meth. A 539 (2005) 74. 8. I. Tanihata et al., Phys. Rev. Lett. 55 (1985) 2676. 9. T. Suzuki et al., Phys. Rev. Lett. 75 (1995) 3241. 10. T. Motobayashi et al., Phys. Lett. B 346 (1995) 9. 11. N. Imai et al., Phys. Rev. Lett. 92 (2004) 062501. 12. H.J. Ong et al., Phys. Rev. C 73 (2006) 024610. 13. Z. Elekes et al., Phys. Lett. B 686 (2004) 34. 14. H. Ueno et al., Nucl. Phys. A 738, 211 (2004).


15. for example, T. Motobayashi, Nucl. Phys. A 693 (2001) 258. 16. for example, T. Nakamura et al., Phys. Rev. Lett. 96 (2006) 252502. 17. T. Kubo et al., IEEE Trans. Appl. Superconductivity 17 (2007) 1069. 18. K. Suemmerer and B. Blank, Phys. Rev. C 61 (2000) 034607. 19. T. Uesaka et al., CNS Annual Report 2004 (2005) 42. 20. M. Wada et al., Nucl. Instr. and Meth. A 532 (2004) 40. 21. Y. Sasamoto et al., CNS Annual Report 2004 (2005) 85. 22. M. Wakasugi et al., Nucl. Instr. and Meth. A 532 (2004) 216. 23. Y. Yamaguchi et al., CNS Annual Report 2004 (2005) 83. 24. O. Kamigaito et al., RIKEN Accel. Prog. Rep. 39 (2005) 261.

TOHRU MOTOBAYASHI AND YASUSHIGE YANO RIKEN Nishina Center

feature article Alpha Particle Condensation in Nuclear Systems Y. FUNAKI1, H. HORIUCHI2, G. RÖPKE3, P. SCHUCK4,5, A. TOHSAKI2, AND T. YAMADA6 1 The Institute of Physical and Chemical Research (RIKEN), Wako, Saitama 351-0198, Japan 2 Research Center for Nuclear Physics (RCNP), Osaka University, Ibaraki, Osaka 567-0047, Japan 3 Institut für Physik, Universität Rostock, D-18051 Rostock, Germany 4 Institut de Physique Nucléaire, CNRS, UMR 8608, Orsay, F-91406, France 5 Université Paris-Sud, Orsay, F-91505, France 6 Laboratory of Physics, Kanto Gakuin University, Yokohama 236-8501, Japan Introduction Quantum condensation of particles is one of the most amazing phenomena exhibited by many-body systems. Familiar yet striking examples known for many decades include superconductivity of metals at low temperatures and superfluidity of liquid 4He. More recently the realization of Bose-Einstein condensation of ultracold atoms in traps has created an exciting new field of quantum manybody physics. Also, atomic nuclei and neutron stars can experience quantum condensation of fermion pairs and display superfluid properties. However, in nuclear physics the most tightly bound light cluster is not a pair but a quartet, namely the alpha particle. Can we then expect α-particle condensation in nuclei? Before pursuing to this question, let us make some general remarks. It is a fact that quartetting is more pronounced in nuclei than in most other Fermi systems. And the dominance of quartetting can be traced to the fact that nucleons can exist in four different internal states: proton and neutron, each with spin up or down, all attracting each other. Therefore, a shell model picture in which the α-particle is the first doubly magic nucleus, with a filled 0S–level, is valid (see Fig. 2 below). Moreover, the α-particle is especially stiff, with its first excited state lying quite high, at ~ 20 MeV. The search is now on for quartets in systems other than nuclei. There has long been talk about bi-excitons [1], but with the rapid development of cold atom physics [2], one can hope that soon four different species of fermions will be trapped, giving rise to quartetting of atoms. Several theoretical papers and proposals along this line have already appeared [3]. Quartetting is likely to emerge as an important topic in quantum many-body physics. Let us return now to the nuclear case. The only nucleus having a pronounced quartet-cluster structure in its ground state is 8Be. In Fig. 1(a) we show the result of an exact calculation of the density distribution in the laboratory frame

Figure 1. Contours of constant density (taken from [4]) for 8 Be(0+). The left side (a) is in the laboratory frame while the right side (b) is in the intrinsic deformed frame.

based on a realistic N-N interaction, with the distribution in the intrinsic, deformed frame shown in Fig. 1(b). We observe that the α-structure is very pronounced and leads to the very low average density r ~ r0/3 apparent in Fig. 1(a), r0 = 0.17 fm−3 is the nuclear saturation density. The nucleus 8 Be is very large with an rms radius of ~3.7 fm, to be compared with the value R = r0 A1/3 ~ 2.44 fm given by nuclear systematics. The nucleus 8Be is definitely unusual. One may ask what happens when a third a-particle is brought alongside 8 Be. We know the answer: the 3-a system collapses to the ground state of 12C which, within its small radius of 2.4 fm, is much denser than 8Be and cannot accommodate three a-particles just touching one another. Like virtually all other nuclei, 12C in its ground state is essentially a Fermi-gas,


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feature article b

0s 0S B

Figure 2. Pictorial representation of the THSR wave function for n = 3 ( 12C). The three a-particles are trapped in the 0S-state of a wide harmonic oscillator (B) and the four nucleons of each a are confined in the 0s-state of a narrow one (b). All nucleons are antisymmetrised.

describable in mean-field approximation with effective forces. This does not exclude the presence of α-like correlations, but they would produce only a small correction. On the other hand, one may ask whether the dilute three a configuration 8Be-a or a − a − a can form an isomeric or excited state of 12C. Continuing in this way, one could think of adding a fourth a-particle, a fifth, and so on. These prospects will be the main subject of the following considerations. a-Condensate States in Self-Conjugate 4n-Nuclei We will now show that α-particle condensation most likely occurs in nα nuclei at energies near the n α-particle break-up threshold. Ample experimental data will allow us to identify at least one nucleus, namely 12C, in which such an α-particle condensed state exists. We then discuss the likelihood and the indications that such states are very naturally also present in other nuclei, particularly 16O. The phenomenon may in fact be quite general in nuclear systems [5]. We begin with strong arguments that the 0+2 state at 7.654 MeV in 12C is, indeed, a state of α-particle condensate nature. First, it should be pointed out that the 0+2 state in 12 C, like the ground state of 8Be, is actually particleunstable, being situated about 400 keV above the three α-break up threshold. This state only is stabilized by the Coulomb barrier. It has a width of 8.7 eV and a corresponding lifetime of 7.6 × 10 −17 s. As is well known, this state is of great astrophysical and biological importance owing to its crucial role in the synthesis of the 12 C present in the universe. Partly on anthropic grounds, the astrophysicist Fred Hoyle predicted the existence of this state in 1953. He argued that the high abundance of 12C in the universe, essential for life, can

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only be explained if the triple alpha reaction α + α + α → 8Be + α → 12C* is strongly accelerated by the presence of a resonance state in 12C at the right energy [6]. Hoyle deduced the position of this state very precisely from thermodynamic equilibrium considerations, and it was discovered experimentally three years later by Willy Fowler and his collaborators [7]. Now well known as the Hoyle state, it is notoriously difficult to describe and reproduce within conventional nuclear structure theory (see B. Barrett et al. [8]). For example, the most modern no-core shell-model calculations predict the 0+2 state in 12C to occur at around 17 MeV, more than twice the actual excitation energy [8]. This fact alone tells us that the Hoyle state must have a very unusual structure. Should this state indeed consist of three loosely bound a-particles, one can easily understand that a shell-model approach would have great difficulties in explaining its properties. The first theoretical idea on the structure of the Hoyle state that was widely discussed in the community came from Morinaga. He postulated the “linear chain state” in which three α-particles are lined up one after the other [9]. At a later stage, other authors pointed out that the state may instead be a loosely bound configuration of three α-particles or a two-alpha 8Be nucleus with one more alpha orbiting around it. Hackenbroich et al. [10] and Horiuchi et al. [11] were forerunners in advocating this picture. In fact, the latter group was able to make a

0.8 0.7

S1 D1 G1 S2 D2 G2 S3 D3 G3

0.6 0.5 0.4 0.3 0.2 0.1 0

Ground state

Hoyle state

Figure 3. Occupation probabilities of the k-th a-orbits with S, D and G waves, which are denoted by Lk for the L-waves, for the ground and Hoyle states of 12C obtained by diagonalizing the density matrix r(R, R¢).


feature article

Figure 4. Momentum distribution r(k) (a) and k2 r(k) (b) of the a-particle for the 0+1 (black line) and 0+2 (grey line) states. quite precise prediction of the position of the 0 +2 state, using the so-called Orthogonality Condition Model (OCM) [12]. Then, in the mid-seventies, two very important works on the alpha-cluster structure of 12C appeared. The Japanese physicists, M. Kamimura [13] and K. Uegaki [14], together with their collaborators, independently and almost simultaneously reproduced the Hoyle state within microscopic theory, i.e., employing a twelve-nucleon wave function together with a Hamiltonian containing an effective nucleon-nucleon interaction. At the time, these remarkable works did not attract the widespread attention they deserved. Only recently the significance of the achievement of the Kamimura and Uegaki groups is being appreciated more widely. The two groups started from practically the same ansatz for the 12C wave function, having the following three a-cluster structure: 〈 r1 L r12 |12 C 〉 = A [( R, s)φ1φ 2φ 3 . Here, A is the antisymmetriser acting on all nucleons and f i is an intrinsic a-particle wave function of prescribed Gaussian form, i.e.

φ ( r1 − r2 , r1 − r3 ,L) = exp ⎡ −(1 8b 2 ) ⎢⎣

∑

4 m>n

( rm − rn )

2⎤

⎥⎦

where the size parameter b is adjusted to fit the rms value of the free a-particle radius. The factor χ(R, s) is the threebody wave function for the center of mass motion of the three a’s, with R and s the corresponding Jacobi coordinates. This function was determined using both a Generator

Coordinate Method (GCM) [14] and Resonating Group Method (RGM) [13], assuming Volkov I and Volkov II nucleon-nucleon forces. Thirty years back, the precise solution of this complicated three-body problem was truly pioneering work. The position of the Hoyle state as well as other experimental properties including inelastic form factor and the transition probability were successfully reproduced. Other states of 12C below and around the energy of the Hoyle state were also well described. Importantly, it was recognized that the three α’s in the Hoyle state form an α-gas like state, a feature which had already been pointed out by H. Horiuchi [11] prior to the works of [13] and [14]. All of these authors concluded from their studies that the linear chain state of three α-particles had to be discarded as a description of the Hoyle state. The treatments of [13] and [14] were later extended to reaction theory in [15]. It should also be mentioned that a very recent effort [16] has again reproduced all the features of the Hoyle state, starting from a realistic bare N-N force. Although several authors of the early papers stressed the resemblance of the Hoyle state to an alpha-particle gas, two key aspects were not recognized at the time. The first and fundamental aspect is that since, all three α’s move with their c.o.m. in the same S-wave orbit, one is dealing with an α-condensate state (albeit not in the macroscopic sense) and that α-particle condensation may be a quite general phenomenon in nuclear physics. The second and practical aspect is that the complicated three body wave function χ(R, s) can be replaced by a structurally and conceptually very simple microscopic three α wave function of the condensate type which has practically 100 percent overlap with the wave functions constructed previously [17]. To establish and explain the latter feature, it is instructive to exploit an analogy with the Cooper-pair BCS wave function of ordinary pairing. The component of this wave function with given particle number can be written,

〈 r1 L rN | BCS 〉 = A [φ ( r1 , r2 )φ ( r3 , r4 )φ ( rN −1 , rN )] where f(r1, r2) is the Cooper-pair wave function, including spin and isospin, to be determined variationally by the well known BCS equations. As before, A is the antisymmetriser. The condensate character of the BCS ansatz is born out by the fact that we have a product of N/2 times the same pair wave function f. Formally it now is a simple matter to


13

feature article wave function of Gaussian form as used in [13], [14] and written out above. Of course, in (1) the c.o.m. coordinate Xcm of all α’s, i.e. of the whole nucleus, should also be eliminated. This is easily achieved by replacing R by R−Xcm in each of the α wave functions in (1), utilizing a helpful property of Gaussian functions [18]. The α-particle condensate wave function (1) with (2), as proposed and applied to 12C and 16O in [19] (see also Ref. [10]) and henceforth called THSR-wave function, now depends only on two parameters, B and b. The expectation value of the microscopic Hamiltonian

Figure 5. Present result of the inelastic form factor compared with experiment [32]. RGM result corresponds to Ref. [13].

generalize the pair-condensed state |BCS〉 to α-particle condensation. We write

〈 r1 L rN | Φ nα 〉 = A[φ ( r1 , r2 , r3 , r4 )φ α ( r5 ,L , r8 )L φ α ( rN − 3 , rN )]

(1)

where φα is the wave function common to all α-particles. Of course, finding the variational solution for φα (r1, r2, r3, r4) from (1) is considerably more complicated than finding the Cooper pair wave function. However, in the case of the α-particle and more generally for quartets and for relatively small clusters, the complexity of the problem can be reduced dramatically. As already recognized in the early works [13] and [14], an excellent variational ansatz is provided by an intrinsic wave function of the α-particle of Gaussian form, with only the size parameter b to be determined. In addition, and here lies the essential and crucial aspect of our wave function, even the center-of-mass motion of the various α-particles can very well be described by a Gaussian wave function. This introduces an additional size parameter B, with B >> b, to account for the motion over the whole nucleus. We therefore write

φ α ( r1 , r2 , r3 , r4 ) = e −2 R

2

B2

φ ( r1 − r2 , r1 − r3 ,L)

(2)

where R = (r1 + r2 + r3 + r4)/4 is the c.o.m. coordinate of one α-particle and φ(r1 − r2, . . .) is the same intrinsic α-particle

14

H ( B, b) = 〈 Φ nα ( B, b) | H | Φ nα ( B, b) 〈 Φ nα | Φ nα 〉 (3) can be evaluated, and the corresponding two-dimensional energy surface [20] can be quantized using the two parameters B and b as Hill-Wheeler coordinates [21]. The Hamiltonian was taken to be that of Ref. [22], containing an effective nucleon-nucleon force of the Gogny type, with parameters adjusted to fit α − α scattering phase shifts. Our theory is therefore free of adjustable parameters. Before presenting the results, we should note that the THSR wave function reproduces two important limits exactly: for B = b, it is a pure Slater determinant [23], and for B >> b, the density of α-particles is so low that the antisymmetriser in front of (1) can be neglected. In the latter case, Eq. (1) becomes an ideal Bose condensate of α-particles, i.e. a pure product state (see Fig. 2). Results for Finite Nuclei As already pointed out, the THSR wave function constructed from the Hill-Wheeler equation based on Eqs. (1), (2), has practically 100 percent overlap with the wave functions of [13], [14], once the same Volkov force is used [17]. It is therefore not astonishing that we arrive at very similar results. For 12C we obtain two eigenvalues, corresponding to the ground state and the Hoyle state. Theoretical values for positions of energy, rms values, and transition probabilities compared with the data, are given in Table 1. From the comparison of the rms radii we see that the volume of the Hoyle state is a factor 3 to 4 larger than that of the ground state of 12C. This is the dilute-gas aspect we highlighted at the outset, the density of the Hoyle state at the center of the nucleus being reduced to r0/2 (!) [16]. Constructing an


feature article Table 1. Comparison of the binding energies, rms radii (Rrms), and monopole matrix elements M(0+2 → 0+1) for 12C obtained by solving the Hill-Wheeler (H. W.) equation based on (2) [17] and by RGM [13]. Volkov II as the effective two-nucleon force is adopted in the two cases, for which the 3α threshold energy E3α is calculated to be −82.04 MeV (experimental value: −84.9 MeV). Absolute values E are shown for the ground state in parentheses.

E – E3α (Mev)

Rr.m.s. (fm)

M(0+2 → 0+1) (fm2)

condensate w.f. (H. W.) [17]

RGM [13]

Exp.

0+1

−7.48 (−89.52)

−7.36 (−89.4)

−7.3 (−92.2)

0+2

0.25

0.34

0.38

0+1

2.40

2.40

2.44

0+2

3.83

3.47

6.45

6.7

a-particle density matrix r(R, R′), integrating out of the total density matrix all intrinsic a-particle coordinates, and diagonalizing the result, we find that the corresponding 0S a-particle orbit is occupied to more than 70 percent by the three a-particles [24], [25]. This is a huge percentage, affirming the almost ideal α-particle condensate nature of the Hoyle state. By contrast, even at zero temperature only 10 percent of the particles in superfluid 4He belong to the condensate (which is nevertheless a macroscopic supply of condensed particles). To add further perspective to the picture, in the ground state of 12C the α-particle occupations are equally shared between S, D and G orbits, thus invalidating a condensate picture for the ground state. The occupation numbers for the ground and Hoyle states are shown in histogram format in Fig. 3. The difference between the Hoyle state and the ground state is seen to be spectacular. In the Hoyle state the 0S-occupancy is at least an order of magnitude (!) higher than for any other orbit. This is one of the main typical features of Bose-Einstein condensation, even in strongly correlated Bose systems where there may exist a strong depletion of the condensate, like in superfluid 4 He. On the other hand, the ground-state occupancies can be explained quite well with the standard shell model [25]. It should also be noted that that ground state of 12C is reasonably well reproduced by our theory (see Table 1). A further strong indication of the condensate-like behavior of the a-particles in the Hoyle state is their momentum distribution, which is much narrower, almost δ-function-like, than in the ground state (see Fig. 4). An interesting fact in this respect is that in infinite nuclear matter, a-particle Bose-Einstein condensation (BEC) only exists at low density and that there is no analogue to the BCS phase with a very large coherence length, which allows for

5.4

pairing also at high densities. a-particle condensation only exists in the BEC-phase, i.e. at low density! [26]. Let us now discuss what is, to our mind, the most compelling evidence that our description and interpretation of the Hoyle state is the correct one. Just as the authors of [13], we reproduce very accurately the inelastic form factor 0+1 → 0+2 of 12C. This is shown in Fig. 5. The agreement with experiment as such is already quite impressive. Additionally, however, the following study was made. We artificially varied the extension of the Hoyle state and studied the influence on the form factor [27]. It was found that the overall shape of the form factor, and in particular the minimum, shows little variation. Rather, we found a strong dependence on the absolute magnitude of the form factor. A 20 percent increase of the rms radius of the 0+2 state decreases the height of the first maximum by a factor of two! This strong sensitivity of the magnitude of the form factor makes us confident that the agreement with the actual measurement is in effect a proof that the calculated wide extension of the Hoyle state corresponds to reality. Additionally, it should be kept in mind that the reproduction of the strong monopole transition is by no means trivial, as explained in a recent work by Yamada etal. [28]. The Hoyle state can be considered the ground state of the α-particle condensate configuration. On nuclear scales it has a very long lifetime. Exciting one α-particle from the condensate into the 0D orbit reproduces the experimentally measured position at 9.9 MeV of the 2+2 state in 12C [29, 30]. Without going into details, the 1.0 MeV width of this state is well reproduced [29] by the THSR ansatz. It is tempting to imagine that the 0+3 state at 10.3 MeV, which experimentally is almost degenerate with the 2+2 state, is obtained by lifting one a-particle from the 0S into the 1S


15

feature article orbit. Initial theoretical studies [31] indicate that this view might indeed be realized. However, its width (~ 3 MeV) is very broad, making a theoretical treatment rather delicate. Further investigations are necessary to validate this picture. At any rate, it would be very satisfactory if the triplet of states, with quantum numbers 0+2, 2+2, 0+3, could all be explained from the a-particle point of view, since those three states of even parity are precisely the ones that cannot be explained within a (no core) shell model approach [8]. In conclusion, so far as 12C is concerned, we have assembled convincing evidence that the Hoyle state is described remarkably well by the THSR ansatz of Eqs. (1), (2), and that it can legitimately be called an α-particle condensate state. In saying this, we are aware that using the word “condensate” for only three particles may amount to a certain abuse of the word. On the other hand, one must remember that also in the case of nuclear pairing, only a few Cooper pairs are sufficient to produce clear signatures of superfluidity in nuclei! (An example is seen in the chain of Sn-isotopes [33].) Still, with just three α-particles condensed into the 0S-state, we would like to have more. We are thus led to ask about α-particle condensation in heavier nuclei? Once one accepts the idea that the Hoyle state is essentially a state of three free α-particles held together only by the Coulomb barrier, it is hard to believe that analogous states should not also exist in heavier n α nuclei like 16O, 20Ne, 24Mg, . . . . Indeed, our calculations, using the THSR ansatz, systematically always show a 0+-state close to the nα-particles disintegration threshold [20]. For example, in 16O we obtain three 0 +-states in [19], namely the ground state at E 0 = −124.8 MeV (compared with the experimental value −127.62 MeV), a second state at excitation energy E0+2 = 8.8 MeV and a third at E0+3 = 14.1 MeV. The four α-particles threshold in 16O is at 14.4 MeV. Unfortunately, the experimental situation in 16O is far less complete than in 12C. However, there exists a 0+ state at 14.04 MeV [34] with a strong E0 transition probability of M = 3.3 fm2, whose magnitude is close to the E0 value of the calculated 0+3 state, that is M = 2.5 fm2. As already mentioned, such an agreement is non-trivial [28]. This state is, therefore, a very strong candidate to be the analog to the Hoyle state in 16O. Recently Wakasa [35] identified a new 0+-state at 13.6 MeV in 16O, which might also have a strong α-condensate component [36]. Further work is necessary to clarify the situation in 16O, but it is very likely that one of the two 0+ states at 13.6 MeV or at

16

14.04 MeV will be the α condensate state. Similar states in 20Ne, 24Mg, . . . , are yet to be discovered. Additional theoretical investigations along the lines of Ref. [16] would be opportune. Outlook Further topics to be investigated in the future in the context of α-particle condensation are numerous. An interesting question is how many α’s can maximally be in a self bound α-gas state. In this respect, a schematic investigation using an effective α − α interaction in an α-gas mean field calculation of the Gross-Pitaevsky type was performed [37]. Because of the increasing Coulomb repulsion, the Coulomb barrier fades away and our estimate yields a maximum of about eight a-particles that can be held together in a condensate. However, a few extra neutrons can have a strong additional binding effect (see 9Be and 10Be [38, 39]) and may stabilize larger condensates. Another exciting possibility is to observe expanding a-particle condensate states. Imagine that one excites 40Ca, via a heavy-ion collision, to about 60 MeV, i.e. to the total α disintegration threshold. The α condensate, being formed with a certain probability, will start expanding, since there no longer exists any Coulomb barrier to confine it. With multiparticle detectors such as INDRA or CHIMERA, all decaying α-particles could be detected in coincidence, and the coherent state could be identified by its very low energy in the c.o.m. system. This would then be analogous to an expanding atomic condensate after switching off the confining trap potential [40]. Experiments in this direction are being analysed at IPN-Orsay [41]. Another interesting idea concerning α-particle condensates was put forward by von Oertzen and collaborators [5, 42]. a-particles outside a strongly bound core (e.g. 40Ca) can form a condensate at the multi-α-particle threshold [5]. For the condensate with a fixed particle number, the emission of two a’s and three α’s must be enhanced. In fact the observation of the emission of 12C in the 0+2 state from the compound nucleus 52 Fe has been observed [43] and a very strong deviation from statistical model predictions is observed. Similar ideas have been advanced by Ogloblin [44], who hypothesizes a three a-particle cluster state on top of 100Sn, and earlier by Brenner etal. [45] who reports evidence of a gaseous a-particles in 28Si and 32S on top of an inert 16O core. Also, very interesting recent experimental work on loosely bound a-structures in light nuclei has been performed by T. Kawabata etal. [46]. In finite nuclei we never will have a macroscopic condensate of α-particles. The situation in this respect is, as


feature article mentioned, the same as for ordinary nuclear superfluidity. However, there may exist the exciting possibility that in collapsing stars or protoneutron stars a macroscopic a-particle condensate be formed. Shen and Toki et al [47] and Lattimer and Swesty [48] give the possible a-phases in compact stars, see also [49]. In an earlier work, we predicted rather high critical temperatures for a-particle condensation [26] and, therefore, this must be considered as a possibility. It should, however, be kept in mind that although the critical temperature is well understood in the low density limit, the theory needs to be extended to higher densities, where the α-particles dissolve. Remarks and Conclusion In conclusion, we see that the idea of a-particle condensation in nuclei and other nuclear systems, where several a-like clusters move coherently in the same c.o.m. orbit, has already triggered many new works and ideas, in spite of the fact that so far strong identification of such a state only exists in 12C, and potentially also in 16 O. The possibility that there exists a completely new (low density) nuclear phase where a-particles play the role of quasi-elementary bosonic constituents is surely fascinating, and one may predict that many more a-particle condensed states will be detected in the near future. It also can be expected that one soon will find condensed quartets in other systems [3]. For example it may be feasible to trap fermionic atoms in four different magnetic substates, giving rise to a situation quite analogous to nuclear physics. References 1. S. A. Moskalenko, D.W. Snoke, Bose-Einstein Condensation of Excitons and Bi-Excitons, Cambridge University Press, 2000. 2. I. Bloch, J. Dalibard, W. Zwerger, arXiv: 0704.3011. 3. A. S. Stepanenko and J. M. F. Gunn, arXiv: cond-mat/ 9901317; B. Doucot, J. Vidal, Phys. Rev. Lett. 88, 227005 (2002); H. Kamei and K. Miyake, J. Phys. Soc. Jpn. 74, 1911 (2005); S. Capponi, G. Roux, P. Lecheminant, P. Azaria, E. Boulat, S.R. White, arXiv: 0706.0609. 4. R. B. Wiringa, S. C. Pieper, J. Carlson, and V. R. Pandharipande, Phys. Rev. C 62, 014001 (2000). 5. W. von Oertzen et al, Eur. Phys. J. A 29, 133 (2006). 6. F. Hoyle, D. N. F. Dunbar, W. A. Wenzel, W. Whaling, Phys. Rev. 92, 1095 (1953). 7. C. W. Cook, W. A. Fowler, C. C. Lauritsen, T. B. Lauritesen, Phys. Rev. 107, 508 (1957).

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44. A. A. Ogloblin et al, Proceedings of the International Nuclear Physics Conference, Peterhof, Russia, June 28-July 2, 2005. 45. M. W. Brenner et al, Proceedings of the International Conference “Clustering Phenomena in Nuclear Physics”, St. Petersburg, published in ‘Physics of Atomic Nuclei (Yadernaya Fizika), 2000. 46. T. Kawabata et al., Phys. Lett. B 646, 6 (2007). 47. H. Shen, H. Toki, K. Oyamatsu, and K. Sumiyoshi, Prog. Theor. Phys. 100, 1013 (1998). 48. J. M. Lattimer, F. D. Swesty, Nucl. Phys. A 535, 331 (2001). 49. C. J. Horowitz, A. Schwenk, Nucl. Phys. A 776, 55 (2006).


feature article Mass Chain Evaluations for the Evaluated Nuclear Structure Data File (ENSDF)—An Urgent Appeal for European Participation F. G. KONDEV,1 A. L. NICHOLS,2 AND J. K. TULI3 1 Nuclear Engineering Division, Argonne National Laboratory, Argonne, IL 60439, USA 2 Nuclear Data Section, Department of Nuclear Sciences and Applications, International Atomic Energy Agency, Wagramer Strasse 5, A-1400 Vienna, Austria 3 National Nuclear Data Center, Brookhaven National Laboratory, Upton, NY 11973-5000, USA Introduction Reliable nuclear structure and decay data represent the fundamental building blocks of nuclear physics and astrophysics research, and are also of vital importance in a significant number of applied nuclear fields such as power generation and associated fuel cycle operations (e.g., fuel manufacture, transport, reprocessing, and waste management), materials analysis, medical diagnosis, and radiotherapy. There is a continuous demand for good quality data formulated and recommended through the speedy assessment and incorporation of new and improved measurements. Systematic studies of the fundamental properties of atomic nuclei represent the quest to understand the origins, evolution, and structure of the universe. Although various theoretical models predict the possible formation of over 6,000 radionuclides, many of them have proven to be difficult to produce in a laboratory environment. Although nearly 3,000 of these nuclides have been generated and characterized in the laboratory, more than 3,000 remain unknown and ill-defined. Most new discoveries can be identified with the neutron-rich side of the valley of stability, where many properties of the resulting nuclei can be expected to change significantly. These studies of neutronrich nuclei are challenging the accepted frontiers of modern nuclear structure physics and astrophysics. Existing and future experimental facilities, such as FAIR (Europe), FRIB (USA), RIEKEN (Japan), and ISAC-TRIUMF (Canada), will provide a surge of new data to impact the field significantly. Such paths to new discoveries benefit greatly from systematic studies of the accumulated knowledge of nuclear physics research, and from the ability to access these data promptly. Under these circumstances, the rapid availability of comprehensive, up-to-date, and well-ordered

databases is an essential requirement for the nuclear physics research community and applications specialists who need reliable data at the press of a key. Such credible databases also act as a bridge between science, technology, and society by making the results of basic nuclear physics research available to a broad audience of users, and hence having a profound effect on the socioeconomical applications of modern nuclear science. Experimentally determined nuclear structure and decay data for all known nuclei are evaluated and incorporated into the Evaluated Nuclear Structure Data File (ENSDF) database [1]. This database contains comprehensive nuclear structure data from various nuclear reactions and decay processes, and recommends best values for a range of nuclear properties that are derived by critical analysis of all experimental information. The net result is a treasury of data of immediate use to the world-wide nuclear physics community (Table 1). Imagine, for example, a scientist who is interested in studying the properties of the doubly magic nucleus 24Mg and their relevance to various nucleosynthesis phenomena. With more than 1,900 journal articles published on the subject (as retrieved from the bibliographical Nuclear Science References (NSR) database [2]), a preliminary review would involve a large amount of time and effort to identify and extract the required information. However, using ENSDF and with the simple operation of a computer mouse, evaluated data can be accessed and displayed in a matter of seconds. Last year, more than 1 million electronic retrievals were made from ENSDF and other derivative databases, with the main users being scientists in the USA and European Union (Figure 1). Although the recommended data are sufficiently complete and precise for many applications, the contents can be used as the starting point


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feature article Table 1. Nuclear parameters in ENSDF: adopted levels, nuclear reaction and decay data sets. Nuclide: Q(β−) and Q(α)

β− decay energy and α decay energy for the ground state

S(n) and S(p)

neutron and proton separation energies

XREF

cross-reference assignments for the various experimental data sets

Nuclear level: E(level)

excitation energy relative to the ground state

Jπ

spin and parity

T /2or Γ

half-life or total width in centre of mass

decay branching

of ground states and isomers (T1/2 ≥ 0.1 s)

Q,μ

static electric and magnetic moments

XREF

flags indicate in which reaction/decay data sets the level is seen

configuration assignments

—

band assignments

also possibly band parameters

isomer and isotope shifts

only literature reference is given

charge distribution

for ground states—only literature reference is given

deformation parameters

—

B(E2), B(M1)

electric and magnetic excitation probabilities

1

γ and E0 transitions: level scheme

placement of levels

Eγ

measured γ-ray or E0 transition energy

Iγ

relative photon intensity

normalization factor

converts relative to absolute photon intensity per 100 decays of the parent

Mult, δ

electric or magnetic multipole character; mixing ratio

CC

internal-conversion coefficients (when significant)

B(E2)W, B(M1)W

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reduced transition probabilities in Weisskopf units

Figure 1. Geographical usage of the nuclear data services of NNDC, BNL, USA. Data source www.nndc.bnl.gov/usndp.

toward more extensive studies. Thus, ENSDF is a primary source of data for many specialized databases, sophisticated computer codes, search engines, and publications, some of which are indicated in Figure 2. ENSDF evaluations can be particularly useful in the identification of contradictory results that exist in the literature, and hence can stimulate new and improved measurements to assist in resolving such discrepancies. Last but not least, ENSDF data are frequently used in the planning of various stages of many nuclear physics experiments, including dedicated research and development activities, interpretation of results from new measurements, and in the preparation and review of journal articles. ENSDF is a crucial database in a wide range of important nuclear activities. International Network of Nuclear Structure and Decay Data Evaluators The compilation and evaluation of nuclear structure and decay data stretches back to the 1930s, with the publication of the first known tabulations of recommended nuclear parameters by Curie et al. [3] and Fea [4]. These tabulations were followed in 1940 by what was essentially to become the popular Table of Isotopes as conceived originally by Livingood and Seaborg [5]. Clearly, this type of extremely useful work has developed and expanded considerably since those times. These and other parallel evaluation efforts continued, and the familiar presentational style of


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Figure 2. ENSDF: major data sources and derivatives.

Nuclear Data Sheets appeared in early 1966, while Ewbank and co-workers at the Oak Ridge National Laboratory in Tennessee, USA, developed the database format in the 1970s. Since 1980, the ENSDF master database has been maintained by staff at the National Nuclear Data Center (NNDC) of Brookhaven National Laboratory, sponsored by the Office of Science, U.S. Department of Energy. Evaluated data from ENSDF have been used as the primary input for Nuclear Data Sheets, Table of Isotopes, Nuclear Wallet Cards, NuDat, and other products familiar to the nuclear physics community (see Figure 2). Nuclear structure and decay data are compiled and evaluated by means of a collaborative program organized through the International Network of Nuclear Structure and Decay Data Evaluators (NSDD) established in 1974 under the auspices of the International Atomic Energy Agency (IAEA). This network began at a time when the workload was heavily reliant on American input. A more equitable involvement of other national laboratories and universities was envisaged, and partially achieved. At different times prominent nuclear physicists, such as F. Ajzenberg-Selove, R. G. Helmer, C. W. Reich, S. Raman (USA), P. M. Endt, C. van der Leun, P. J. Twin and A. H. Wapstra (Europe) and many others, have been involved in the resulting compilation and evaluation activities. Several countries have contributed over a long period of time, including Belgium,

Canada, China, France, Japan, Kuwait, Russia, and the United States of America. Recently, new evaluation groups have emerged in other countries, such as Australia and India. The total NSDD evaluation effort is equivalent to about 9 full-time equivalent scientists per annum (FTE), albeit approximately 12 FTE are required to maintain the desired currency and quality of ENSDF. Other specific nuclear properties are comprehensively compiled and evaluated by individual members of the network without limiting their efforts to a particular mass chain. Most recently, these horizontal evaluations include atomic masses [6], nuclear magnetic and electric moments [7], electric monopole strengths [8], and capture gamma rays [9]. An additional database entitled XUNDL [10] provides a rapid means of electronic access to the most recent publications and pre-prints of experimental nuclear structure data before they are included in the ENSDF database. An additional useful source of information is the bibliographical database entitled Nuclear Science References (NSR) [2]. There are many distinct advantages associated with maintaining a healthy ENSDF database by means of a multinational network: (a) ensure the maintenance of a well-defined archive of nuclear structure and decay data for future generations,


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feature article (b) valuable interplay between nuclear reaction and decay data studies to define the best nuclear parameters, (c) constructive impact of recommended nuclear structure data on developing nuclear theories, (d) assistance in the resolution of contradictory results, (e) identification of requirements for and stimulation of new measurements, (f) benefits to users in many applied areas—nuclear medicine, analytical science, environmental monitoring, nuclear engineering, and so on. Members of the NSDD network can take great personal pride in assisting such an impressive array of basic and applied nuclear physicists with their everyday nuclear data needs. Future Perspectives There is no doubt that future advances in nuclear physics and other related areas will require large and increasingly sophisticated databases that are based on modern computer technologies. Popular use and adoption of data within any electronic database is largely determined by the following: (a) reliability and credibility—data must be correctly evaluated and incorporated into the database, (b) comprehensive—the database should include all measured quantities and their uncertainties, (c) up-to-date—results from all measurements should be promptly incorporated into the database, (d) accessibility—easy and rapid availability in userrequired formats. The main goal of the NSDD network is to improve the existing nuclear physics databases in all these aspects. Furthermore, the development of specialized data modules will continue, such as calculations of internalconversion coefficients, log ft values, and so on that are tailored to the various needs of the nuclear physics research community. Special attention will also be paid to the development of new evaluation methodologies and to the prompt introduction of modern computer technologies in order to improve the quality and efficiency of access to the existing databases. Improved impact of the resulting nuclear data evaluations in advanced research areas will be sought by carrying out specialized horizontal evaluations and topical reviews in collaboration with scientists from the nuclear physics research community. All of this work will require dedicated effort by nuclear

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data physicists and the full support of the international nuclear physics community. Challenges The organizers of the International Network of Nuclear Structure and Decay Data Evaluators have become aware in recent years of an increasing problem in maintaining and updating ENSDF evaluations with the necessary regularity. Evidence of a shortfall in effort has been detected over the previous ten years as evaluators in Europe have retired without any obvious replacements. Although some progress has been made in recruitment through the commitment of nuclear physics institutes in India and elsewhere for this essential work, these welcome additions are not fully commensurate with the losses experienced in Europe, a region of the world that might have been expected to ensure some re-generation of expertise in this vital area of research and development. The survival and maintenance of the quality of ENSDF depends on the recruitment of new data evaluators to replace the ageing nuclear physicists undertaking this important work. Unless new blood can be introduced soon, there is a serious danger that the current loose confederation of dedicated participants will fade away and as a consequence the core nuclear physics databases will become hopelessly outdated. An urgent need has arisen for younger scientists to join the NSDD evaluation network and to contribute to the nuclear data activities. There can be no doubt that the assistance of the worldwide nuclear physics research community is urgently required to ensure the survival of ENSDF at the necessary level of credibility, reliability, and quality. Anyone with the necessary expertise, supportive infrastructure, and personal interest in undertaking mass chain evaluations for ENSDF should contact Jagdish Tuli at NNDC, Brookhaven National Laboratory, USA. Acknowledgements The authors express their gratitude to all colleagues within the International Network of Nuclear Structure and Decay Data Evaluators for their enthusiastic efforts to maintain the quality of the existing nuclear structure databases. F. G. Kondev and J. K. Tuli are supported by the U.S. Department of Energy, Office of Nuclear Physics, Office of Science, under contracts DE-AC02-06CH11357 and DE-AC02-98CH10886, respectively.


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References

1

1. ENSDF database (Evaluated Nuclear Structure Data File), www.nndc.bnl.gov/ensdf 2. NSR database (Nuclear Science References), www.nndc.bnl.gov/nsr 3. M. Curie, A. Debierne, A. S. Eve, H. Geiger, O. Hahn, S. C. Lind, St. Meyer, E. Rutherford, and E. Schweidler, The radioactive constants as of 1930, Rev. Mod. Phys. 3 (1931) 427. 4. G. Fea, Tabelle riassuntive e bibliografia delle transmutazioni artificiali, Nuovo Cimento 12 (1935) 368. 5. J. J. Livingood and G. T. Seaborg, A table of induced radioactivities, Rev. Mod. Phys. 12 (1940) 30. 6. A. H. Wapstra, G. Audi, and C. Thibault, The AME2003 atomic mass evaluation, Nucl. Phys. A729 (2003) 129.

7. N. J. Stone, Table of nuclear magnetic dipole and electric quadrupole moments, At. Data Nucl. Data Tables 90 (2005) 75. 8. T. Kibédi and R. H. Spear, Electric monopole transitions between 0+ states for nuclei throughout the periodic table, At. Data Nucl. Data Tables 89 (2005) 77. 9. H. D. Choi, R. B. Firestone, R. M. Lindstrom, G. L. Molnár, S. F. Mughabghab, R. Paviotti-Corcuera, Z. Révay, A. Trkov, V. Zerkin, and C. Zhou, Database of prompt gamma rays from slow neutron capture for elemental analysis, International Atomic Energy Agency, Vienna, Austria (2007), ISBN 92-0101306-X. 10. XUNDL database (eXperimental Unevaluated Nuclear Data List), www.nndc.bnl.gov/ensdf/xunindex.jsp


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feature article Neutron Stars and Nuclei: Two Dense Systems M. FALLOT,1 M. GRASSO,2,3 E. KHAN,2 AND J. MARGUERON2 1 Subatech, 4 rue Alfred Kastler BP 20722, F-44307 Nantes Cedex 3, France 2 Institut de Physique Nucléaire, Université Paris-Sud, IN2P3-CNRS, F-91406 Orsay, France 3 Dipartimento di Fisica e Astronomia and INFN, Via Santa Sofia 64, I-95123 Catania, Italy Introduction: Neutron Stars, Nuclei, and Nuclear Matter Neutron stars are the remnants of core collapse supernovae [1,2]. They are the most compact stellar objects after black holes. Some of their properties, such as masses, rotation frequencies, and emission of radiations are measurable, whereas other signals like gravitational wave emission are planned to be in the next years. The properties that are not directly linked to observations, such as the internal composition or temperature, require the development of theoretical models. Fortunately, some of the missing information can be obtained from the study of the other dense nuclear systems, atomic nuclei, which are accessible to experimental facilities. Traditionally, the link between neutron stars and bulk nuclei is made via the nuclear matter: an ideal infinite system equally composed of interacting neutrons and protons where Coulomb interaction has been switched off. For instance, the central density of heavy nuclei is very close to the equilibrium density of nuclear matter, called the saturation density ρ0. Moreover, the nuclear matter concept can be extended to isospin asymmetries. Asymmetric nuclear matter is rather similar to the nuclear matter found in neutron stars. Coulomb potential energy at those densities is usually small compared to kinetic energy and the main interaction between particles is driven by the nuclear force. Recently, more direct relations between neutron-rich nuclei and neutron star matter have been proposed. Indeed, some of the exotic neutron-rich nuclei produced in nuclear facilities are also located in the outer crust of neutron stars, while the inner crust is composed by drip-line nuclei immersed in a neutron gas. Before entering into this discussion, we should present in more detail the physics of neutron stars. Discovery and Observation of a Large Variety of Neutron Star Systems Landau as well as Baade and Zwicky suggested the existence of neutron stars in the early 1930s. Their existence remained conjectural until 1968 when Jocelyn Bell

and her thesis advisor Anthony Hewish discovered radio pulsars, characterized by radio emission with a periodicity that lies between a few seconds and few tens of milliseconds. Radio pulsars are interpreted as spinning neutron stars with an intense magnetic field misaligned with the rotation axis. Radio waves are thought to be emitted by the electrons accelerated along the polar magnetic fields. Hence, the radio waves are not isotropically emitted but focussed and the rotating neutron star is emitting a pulsed signal like the lighthouses that guide the boats along the coasts (Figure 1). The vast majority of radio pulsars are isolated neutron stars because in binary systems the accretion disk tends to screen the signal. In addition to radio emission, neutron stars are also found in interacting binary systems that emit intense X-rays. In such binaries, a neutron star closely orbits a normal optically visible star and draws gas away from it. The infalling accreted gas is heated to millions of degrees and emits X-rays. Rapidly rotating and relatively young radio pulsars are also found in the visible spectrum (Crab pulsar, Vela pulsar). Some neutron stars are also strong high-energy (greater than tens of MeV) gammaray sources. Besides, the strongest known magnetic fields of the present Universe have been found in neutron stars where surface magnetic fields are of the order of 107 T. In few young neutron stars, much more intense magnetic fields have been observed and may exceed 1011 T. The usual dynamo effect is here unable to produce such intense

Figure 1. Rotating pulsar with its magnetic fields and the focussed radio beams.


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feature article magnetic fields. A possible phase transition to strongly spin polarised matter could be responsible, but this is still a speculation in dense matter. From time to time, probably due to the twist of its magnetic field, magnetars emit a giant flare like the one that reached the Earth on December 27, 2004 and has interrupted all radio broadcasts for a few seconds. Measurements of masses and radii of neutron stars still represent an observational challenge. The most accurate measurements of masses are obtained in binaries of neutron stars applying the Kepler laws. Observed values are typically around 1.4 solar masses. The time derivative of the rotation velocity, associated to the luminosity, provides an estimate of the moment of inertia that, combined with the value of the mass, gives a measure of the radius. This leads to a typical radius of a tenth of kilometers. These measurements cannot reach the accuracy required to disentangle between the models used to describe neutron stars. Other methods are then proposed, like the one based on black body radiation but it has been found that neutron stars may have a non-uniformly distributed surface temperature. This complicates the interpretation of the black body emission. Up to now, about 1,500 neutron stars have been identified so far and, as shown, they participate to a large variety of observed systems that are characterized by their electromagnetic emission going from visible spectrum to gamma rays. Could those emission processes provide information about the internal composition of neutron stars? The Equation of State of Dense Stellar Matter On the theoretical side, the mass and the radius are determined by solving the hydrostatic equilibrium equation. In the framework of the general relativity the equilibrium of a spherical object is described by the Tolman-Oppenheimer-Volkov equations, and for completeness, the equation of state (EoS) is required. The density increases from 106 g/cm3 at the surface (starting point of the crust), to several times the saturation density (ρ0 is 3·1014 g/ cm3) in the core. The number of neutrons in neutron stars exceeds by far that of protons. The net isospin asymmetry δ = (N − Z)/(N + Z) can reach 0.95 in the interior of the stars. The equation of state relies on the composition of dense matter in the star for which very scarce information are available: Where are localized the phase transitions between matter composed of neutrons, protons, and electrons, and more massive hadrons such has hyperons? There is a global consensus that nuclear matter will convert to quark matter, but at which density? Does mesons (pions, kaons)

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condensation occur? Several equations of state have been derived in order to investigate the observational consequences of the composition of dense matter. The maximum masses and the radii predicted by those models can be quite different (Figure 2). On the experimental side, investigations on the atomic nuclei like the measurement of giant monopole resonances, masses, and central densities allow one to probe the equation of state around the saturation density. Heavy ion collisions, hot giant resonances, and exotic nuclei properties, attempt to explore more extreme regions of the phase diagram. However, the improvement of EoS’s at lower and higher densities than ρ0 and for strong isospin asymmetries is still required (Figure 3). In the latter case, the density dependence of the symmetry energy is convenient to explore the relation between isospin symmetric and asymmetric equation of state: it can be shown that the density dependence of the symmetry energy is equivalent to the isospin dependence of the incompressibility modulus. The symmetry energy therefore plays a central role in determining the structure and the evolution (cooling) of the stars. The future facilities producing exotic nuclei will allow one to test this isospin dependency for values of the asymmetry parameter, δ, larger than 0.2. This asymmetry is smaller than the asymmetry in neutron star, but may provide at least additional constraints for the theoretical models.

Figure 2. Mass-radius diagram for typical EoS, depicted with observational constraints (see J. M. Lattimer and M. Prakash, Ap. J. 550 (2001) 426 for notations and further explanations).


feature article containing the most probable nuclear cluster, the neutron and the electron gases. For densities higher than 1013g/cm3, the nuclear clusters are close enough to begin a dissolution process and deformed structures appear. They are commonly called the pasta phases because the matter is arranged in noodle shapes like lasagne or spaghetti, or Swiss cheese. At this stage, the proton fraction has decreased down to 0.1. This process results in the formation of homogeneous nuclear matter. In the inner core, where the density is greater than the saturation density, exotic particles such as strange hyperons and/or Bose condensates (pions or kaons) may become abundant. It is possible that a transition to a mixed phase of hadronic and deconfined quark matter develops. Figure 3. Equation of state for various values of the isospin d (from D. T. Khoa etal. Nucl. Phys. A602 (1996) 98).

Anatomy of the Star Neutron stars are quasi-spherical objects composed of six major regions: the inner and outer cores (~99% of the mass) where nuclear matter is homogeneous and that are usually sufficient to understand the main properties of neutron stars; the inner and outer crust (1–2 km width) composed of inhomogeneous nuclear matter (nuclei or nuclear clusters), which screens the core from observations (even from neutrinos), the envelope (few meters), which influences the transport and the release of thermal energy from the surface, and finally the atmosphere (few centimeters), which plays an important role in shaping the emergent photon spectrum (Figure 4). At the surface defined by the interface between the outer crust and the envelope, 56Fe atoms are arranged as in a solid. Going toward the interior, the atoms are ionized and in the outer part of the crust one can find nuclei with numbers of nucleons up to A = 200 arranged in a Coulomb lattice in the presence of an electron gas. Due to electron capture processes, these nuclei become richer in neutrons with increasing density (109 to 1011 g/cm3). Neutrons start to leak out of nuclei at densities above the neutron drip density—the equivalent of the neutron drip line in a stellar environment (finite pressure, beta-equilibrium): 4 1011g/cm3 in the inner crust. Nuclei are located at the sites of a crystal immersed in a super-fluid of neutrons and relativistic leptons. The lattice can be modelized by its elementary constituents, the Wigner-Seitz (WS) cells, each of them

Cooling, Glitches, and Vortices: The Life of a Neutron Star With time neutron stars evolve and new phenomena occur. In the following we report on some of those phenomena that are directly related to the properties of nuclear matter and in particular to the pairing properties of nuclear matter. Being at the end point of stellar evolution, neutron stars do not produce energy but lose the gravitational energy gained during the core collapse by neutrino emission.

Figure 4. The basic structure of a neutron star (from G. Röpke, Univ. Rostock).


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feature article Indeed, because of their very weak coupling with matter, neutrinos and anti-neutrinos mainly produced by beta decay and inverse beta decay could carry out the energy of the core and cool down the temperature. This is called the URCA process [3], by reference to the name of a casino existing in the mid 1950s in Rio di Janeiro, known by the promoter of this process, G. Gamow. According to him, the efficiency of this casino in spoiling the money of the gamblers was comparable to the URCA process in cooling down the star. Later on, it was discovered that the URCA process is strongly suppressed by energy and momentum conservation unless a minimum amount of proton, around 11% of the baryonic density, is present [4]. This minimal amount is strongly correlated with the symmetry energy as. Relativistic models, having large values of as, satisfy this criterion around the saturation density while most non-relativistic models, with a lower as value, do not. The difference between non-relativistic and relativistic models predictions should then be investigated. Anyhow, the URCA process is too efficient to explain the slowing down of the surface temperature with time that is observed for a dozen of stars: neutron stars are visible by thermal emission during a few millions of years. Several improvements have been proposed: superfluidity leading to the presence of a neutron gap may quench cooling from the URCA process. A modified URCA process is also considered where adding an additional nucleon as a spectator of the process allows momentum and energy conservation. Other processes are also considered like neutrino bremsstrahlung, pair breaking emission, and so on. It should be noted that the specific heat in the crust is also important in cooling modelization. It depends on the excitation spectrum, which is different in the super-fluid phase than in the normal phase [5]. Neutron stars are also fast rotating stellar objects and we know, from Earth laboratory experiments on Helium 4 for instance, that a rotating super-fluid produces vortices. In the case of finite nuclei, surface effects forbid the formation of vortices. In other words the rotational energy needed is too high and nuclei vaporize at lower energies. In the case of neutron stars, gravitational pressure maintains the nucleons together and vortices can be formed in the core as well as in the inner crust. Those vortices link together two layers of the star (core and crust) and impose a rigid rotation. As the neutron star releases energy, the vortex must be destroyed from time to time and this is the possible origin of observed “giant glitches.” A “giant glitch” is a brutal variation of the rotation period of the star. One possible scenario that explains the existence of the glitches is that

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the neutron fluid forms vortices that can pin on the nuclear clusters in the crust. The unpinning would generate the angular momentum transfer from the core to the crust, which is at the origin of the glitches. The pinning force depends on the neutron pairing gap in the crust. Hence, cooling and giant glitches require accurate modelization of the pairing gap in the crust of neutron stars made of non-homogeneous matter. Most of the actual models are based on the Wigner-Seitz approximation since the seminal work of Negele and Vautherin [6]. This allows straightforward application of the Hartree-Fock BCS or Hartree-Fock-Bogoliubov models built for the description of atomic nuclei. It has recently been shown that those models are valid if the density of states around the Fermi surface is averaged over a few 100 keV by temperature effects or energy exchanged during reaction processes [7]. For temperature below a few 100 keV, it is necessary to improve the modelization of the continuum states. For that, based on the ideas developed in condensed matter, first band theory type approaches have been built [8] and represent certainly the new generation of models. Nevertheless, experimental probes of the pairing gap in nuclei are necessary but still very difficult. With respect to the importance of such knowledge, nuclear physics investigations should be pushed in this direction.

Nuclei: A Possible Laboratory for Neutron Stars Recently, several empirical relationships have been found that are directly correlated to some properties of nuclei to neutron star physics. For instance, the neutron skin thickness nuclei has been linked to the pressure of pure neutron matter at sub-nuclear densities [9] and consequently to the neutron star radius [10]. Indeed, the pressure is related to the derivative of the symmetry energy [11] and the neutron skin thickness of nuclei is an observable that yields some information about low-density neutron-rich matter and, in particular, about the density dependence of the symmetry energy. In neutron stars, this question is essential: the density dependence of the symmetry energy determines the proton fraction and the threshold density at which direct URCA process occurs, as discussed in the previous section. Moreover, it governs the threshold densities of other particles such as hyperons, pions, kaons, quark, and so on, which trigger phase transitions and cooling processes. This example illustrates that articulations can be drawn in which nuclear physics experiments could bring useful


feature article constraints. The perspectives offered by the next generation of radioactive beam facilities are in this sense very attractive. Identifying the experimental methods and choosing the relevant observables for the future exotic beam facilities is a strong challenge and requires an important interplay between experimental and theoretical fields. This is the motivation of a series of workshops called Exotic Nuclei and Neutron Stars [12]. These workshops associate nuclear experimentalists, theoreticians, and astrophysicists in five working groups. The aim is to draw physics cases for the experiments relevant to neutron star properties. After the second meeting, held in May 2007 at the Institut de Physique Nucléaire d’Orsay, some tasks have already been defined. We briefly mention some of them. •

•

Working group 1: collective excitations in exotic nuclei. Among the different ways to measure neutron skins, collective vibrational modes furnish very efficient constraints on the models used to compute the neutron skins. Some of them are even more directly related to the neutron skin thickness such as the spin dipole mode. In GSI a pioneering work has been recently performed relating the neutron skin thickness to soft dipole modes [13]. The study of collective modes in exotic nuclei is also relevant because the excitation spectra of neutron-rich nuclei in the crust of the stars can influence the cooling of the star. Furthermore, the incompressibility modulus of nuclear matter can be deduced from Isoscalar Giant Monopole Resonance and Giant Dipole Resonance properties; their study in exotic nuclei will constitute essential piece of information to constraint the symmetry energy and its isospin dependence. As a complementary constraint on the models, charge radius measurements using laser spectroscopy in very neutron rich nuclei will be performed accurately. Working group 2: pairing in exotic nuclei. The main question lies on the pairing interaction itself [14]. How is pairing generated? What is the contribution of phonon coupling to the pairing? How do medium effects such as isoscalar or isovector densities influence the pairing field? The answer may come from a global and unique description of pairing effects going from halo nuclei to heavy nuclei via low density neutron matter in the crust of neutron stars. The experimental study of two-neutron transfer would constitute an interesting tool, but theoretical developments are required to analyze such data. For instance, to check the dependence of the results

•

•

•

(energies and cross-sections for the rotation and vibration pairing modes) on the properties of the chosen pairing interaction. Working group 3: EoS dependence on density and temperature. Probing the phase diagram away from standard nuclei constitute the Graal of experimental investigations such as fusion-evaporation and multi-fragmentation experiments. Those are also privileged tools to access to level densities at finite temperature. Experiments are already conducted in this aim, but future facilities will allow to probe the nuclear EoS in more asymmetric matter, together with 4π arrays such as FAZIA [15]. Nuclear models predict different isospin dependence and further theoretical investigations are required in the coming year(s). Working group 4: nucleosynthesis in neutron star mergers. Both type II supernovae and neutron star mergers are candidates to be the location of nucleosynthesis through rapid neutron capture process [16]. The paths are known to be dependent on nuclear inputs such as the symmetry energy. Again, the density dependence of symmetry energy is fundamental to furnish precise and reliable predictions. A lot of measurements and theoretical calculations are required for r-process study. Optical potential determinations should be performed at very low energy. From the theoretical point of view many topics are of relevant importance, such as the determination of neutron capture and beta-decay rates and the study of fission processes. Working group 5: hyper-nuclei. The presence of hyperons in dense matter softens the EoS. Hyperons contribute more to the energy density than to the pressure because they have larger masses and smaller Fermi momenta. Their presence also enhances the neutrino cooling of the core because they can participate in rapid URCA reactions such as λ⇒p + e + ve. Furthermore, they increase proton to neutron ratio and trigger the URCA process involving nucleons. Unfortunately, very few experimental data exist in this field to discriminate the different theoretical predictions. Many data are expected in the future thanks to experiments such as HyPhi in GSI or J-PARC in Japan. Among those data is expected the production of a large variety of σ hyper-nuclei as well as hyper-nuclei having several hyperons. Together with mean-field models, those data should help in understanding the difference between σ-N and λ-N interaction as well as the λ-λ interaction.


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feature article The next stage of the working groups is to propose concrete experiments on present or future facilities, which will be discussed during the next workshop. In order to prepare these proposals, predictions are needed to investigate the relevance of future measurements. For instance, the sensitivity of two neutron transfer cross-sections on the pairing functional has to be established. Both experimental and theoretical contributions are necessary. Any interested physicist is encouraged to contact the authors of this article, in order to join the next workshop. We would like to stress that the activity of these workshops and working groups relies on the important contributions of our speakers and group coordinators: Didier Beaumel (IPN, Orsay, France), Brandon Carter (LUTH, Meudon, France), François De Oliveira (Ganil, France), Hans Emling (GSI, Germany), Lydie Giot (Subatech, France), Stephane Goriely (IAA ULB, Brussels, Belgium), Francesca Gulminelli (LPC, France), François Le Blanc (IPN, Orsay, France), Nicolas Le Neindre (IPN, Orsay, France), Marek Lewitowicz (Ganil, France), Patricia Roussel-Chomaz (Ganil, France), Alan Shotter (TRIUMF, Vancoucver, Canada), Take Saito (GSI, Germany), and Heinrich Johannes Wörtche (KVI, The Netherlands).

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References 1. N. K. Glendenning, Compact Stars, Springer-Verlag, New York (1997). 2. H. Bethe, Rev. Modern Phys. 62 (1990) 801. 3. G. Gamow and M. Schoenberg, Phys. Rev. 59 (1941) 539. 4. J. M. Lattimer, C. J. Pethick, M. Prakash, and P. Haensel, Phys. Rev. Lett. 66 (1991) 2701. 5. P. M. Pizzochero et al., Astro. J. 569 (2002) 381; C. Monrozeau, J. Margueron, and N. Sandulescu, Phys. Rev. C (2007), in press. 6. J. W. Negele and D. Vautherin, Nucl. Phys. A207 (1973) 298. 7. N. Chamel, S. Naimi, E. Khan, and J. Margueron, Phys. Rev. C (2007). 8. B. Carter, N. Chamel, and P. Haensel, Nucl. Phys. A748 (2005) 675. 9. S. Typel and B. A. Brown, Phys. Rev. C64 (2007) 027301. 10. C. J. Horowitz and J. Piekarewicz, Phys. Rev. C64 (2001) 062802. 11. J. M. Lattimer and M. Prakash, Phys. Rep. 333 (2000) 121. 12. http://snns.in2p3.fr/ 13. A. Klimkiewicz et al., submitted to Phys. Rev. Lett. 14. H.-J. Schulze et al., Phys. Rev. C63 (2001) 044310. 15. http://fazia.in2p3.fr/documents/ LoI_SPIRAL2_ThermoDyn_v7.pdf 16. S. Goriely et al., Nucl. Phys. A758 (2004) 587.


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meeting reports International Nuclear Physics Conference INPC2007 The 23rd International Nuclear Physics Conference, INPC2007, was held from June 3 (Sunday) through June 8 (Friday) at Tokyo International Forum in Japan. This conference is the largest conference in nuclear physics and it is held every three years. In Japan, the previous one was held in 1977, exactly 30 years ago. In spite of an anticipated rainy season in Japan at this time of year, we fortunately had very comfortable weather with clear skies throughout the conference period. About 800 participants attended this conference from 38 countries. Yukawa Session The conference started with a public symposium on June 3 for the centennial celebration of the birth of Professor Hideki Yukawa. Seven invited speakers—H. Sato, T. Yamazaki, T. D. Lee, A. Zichichi, A. Arima, Y. Nambu, and J. P. Schiffer (shown in Figure 1)—presented a variety of public lectures, covering historical stories behind the birth of the Yukawa theory on nuclear interactions to frontier sciences after the Yukawa theory. Half the conference participants plus 400 public audience members spent the entire afternoon listening to these interesting lectures. Opening Ceremony On June 4 the Opening Ceremony of the INPC2007 was held under the presence of the Emperor and Empress of Japan. After the welcome greeting by the conference chair (Shoji Nagamiya), the President of the Japanese Physical Society (Masako Bando), the President of the Science Council of Japan (Ichiro Kanazawa), and the President

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of IUPAP (A. Astbury) gave short speeches. Then, the Emperor delivered an impressive speech (Figure 2). He mentioned first how the Japanese people were encouraged and pleased by the news of the Noble Prize for Hideki Yukawa at the time when Japanese people had been devastated by the War. Then, he referred to Dr. Nishina who built the first cyclotron in Japan and sympathized with him on the occasion when his cyclotron was thrown into the Tokyo Bay after the War. Finally, he touched both positive and negative sides of science by referring to nuclear power, and spoke of his strong desire that the research in nuclear physics must contribute to world peace and the happiness of the humankind. The entire message is written in Ref. [1].

After the Opening Ceremony a Tea Party was held by inviting selected participants. They chatted with the Emperor and Empress at a very warm atmosphere (see Figure 3). Because it is not common to have the Majesty at a scientific conference, the INPC2007 was selected as the most important conference in 2007 among conferences that were sponsored by the Science Council of Japan. Scientific Program The scientific session of the conference was started by an opening talk delivered by W. Wiese. Then, almost all subjects in nuclear physics were discussed in subsequent scientific sessions. About 30 plenary speakers spoke about the most recent progress in the fields covering neutrinos, hot

Figure 1. Speakers at the Yukawa session held on the first day. (Photo for T. Yamazaki is missing from this poster photo.)


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Figure 2. Speech by the Emperor of Japan at the Opening Ceremony of INPC2007.

Physics) decided to create the “IUPAP Young Scientists Prize.” It was also agreed that methods of how to grant this prize should be decided individually by each commission. The C12 commission, which is a commission for nuclear physics, decided to grant three prizes to three young physicists at every INPC. At INPC2007 three physicists, as introduced in Figure 5, received the first IUPAP prizes, and they gave invited talks in a plenary session. At a large conference like INPC it is not common to select young scientists as plenary speakers. Therefore, our trial to set aside a session for the three awardees was well received by the participants of the conference.

and dense QCD matter, hadron structure, nuclear structure, nuclear reactions, astrophysics, applications, and so on. In parallel sessions, over 1,000 contributions were submitted, out of which about 300 were selected for oral presentations. Among submitted contributions 300 papers were for nuclear structure and 200 for nuclear reactions, showing that two traditional fields are still growing in a healthy manner. Many new results were reported in the fields of hot and dense nuclear matter, hadrons in nuclei, astrophysics, and neutrinos. Furthermore, on the fourth day of the conference an exciting new result was reported from a new accelerator in Japan, the RI Beam Factory at RIKEN. All the talks can be downloaded from http://inpc2007.riken.jp/ IUPAP Prize, and So On A few years ago the IUPAP (International Union for Pure and Applied

Figure 3. Tea party with the Emperor and Empress.


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Figure 4. Scenery for a plenary session.

In addition, the Elsevier publishing company decided to give two prizes, one for the most impressive oral presentation and the other for the most impressive poster presentation. The award ceremony for these two prizes was held at the same time as the IUPAP Session.

RI Beam Factory and J-PARC In Japan, two new accelerators will be open to international users. One is the RI Beam Factory at RIKEN, which started its operation from the end of 2006, and the other is the J-PARC high-intensity proton accelera-

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tor at KEK and JAEA, which will start to operate from the end of 2008. The organizers feel strongly that it is extremely important to open these two new facilities to international communities. Having these accelerators was, therefore, a strong motivation for Japanese nuclear physicists to host this important conference in Tokyo at this time. Pre- and post-conference symposia were thus devoted to exploring scientific opportunities at these two facilities. Tours of the two facilities were organized before the conference. It was extremely well received from those who participated in the pre-symposia.


Unification of the Field Along with the expansion of nuclear physics, the field has also been fragmented. Accordingly, topical conferences have become popular in recent years, more than a general nuclear physics conference. Therefore, when we decided to organize this INPC conference in Tokyo, the first goal was to set to reunification of fragmented sub-fields. This effort is very important, because nuclear physics is an exciting field and it has a common goal to study many-body isolated systems in vacuum. At INPC2007, this goal was achieved rather successfully, primarily

meeting reports be kept in future conferences at INPC. At the end of the conference, it was announced that the next INPC conference would be held in Vancouver in 2010.

Reference 1. S. Nagamiya, AAPPS Bulletin 17 (2007) 2.

SHOJI NAGAMIYA J-PARC Center Figure 5. Three IUPAP Prize winners: R. J. Fries (left), Y. A. Litvinov (middle), and K. Sekiguchi (right).

because of excellent speakers at the conference. All the organizers feel

strongly that this trend to reunite subfields in nuclear physics must

OHRU MOTOBAYASHIT RIKEN Nishina Center MAKOTO OKA Tokyo Institute of Technology

Nuclear Physicists Meet in the Land of the Incas From June 11 to 16, 2007, nuclear physicists from around the world met in the historic city of Cusco, Peru to celebrate the VII Latin American Symposium on Nuclear Physics and Applications. And a celebration it was! Cusco, the ancient capital of the Incas, provided an awe-inspiring setting for the symposium. The city welcomed the scientists with open arms beginning with an inauguration ceremony that took place at the Paraninfo Universitario, a beautiful colonial cloister at the Plaza de Armas. This was followed by an exclusive visit to the Incan Museum of Cusco and culminated in the evening with a reception at the Palacio Municipal where the participants were escorted around the Plaza de Armas by an outstanding music band of students. What followed was a truly exciting week that gave nuclear physicists the opportu-

nity to enjoy a momentous scientific program in the sacred land of the Inca civilization, in which Cusco shined through its ancient ruins, monuments, and the music and smiles of the Incan people. This conference continued the series initiated in Caracas, Venezuela (1995, 1997), with subsequent meetings in San Andrés, Colombia (1999), Ciudad de México, Mexico (2001), Santos, Brazil (2003), and Iguazu, Argentina (2005). The Symposium provided a forum for the promotion of Nuclear Physics and its applications among Latin American laboratories and the international community. The symposium series has evolved from a meeting on Nuclear Structure and Heavy-Ion Reactions in 1995 to the present format, where all major research frontiers of nuclear science are represented together with a strong

emphasis in the applications of the field and its broad impacts on society at large. Our aim is to hold an event that clearly shows the vitality and


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1

significance of our field both to physics itself as well as to the well-being of society through its numerous and powerful applications. More than 120 scientists and students representing a total of 20 countries that included most of Latin America, the United States, Canada, the European Union, Croatia, Australia, and Russia attended the Cusco Symposium. The sessions were held at the Department of Physics of the Universidad Nacional San Antonio Abad del Cusco (UNSAAC). The meeting consisted of plenary and parallel sessions and covered six main topics: Applications of Nuclear Physics, QCD in Nuclear Physics, Fundamental Symmetries and Neutrinos, Nuclear Structure, Nuclear Astrophysics, and Advances in Nuclear Physics. Thanks to grant support from the NSF, a total of 16 graduate students from U.S. institutions participated in

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the Symposium. They were joined by a similar number of students traveling from Latin American countries and by local Peruvian students. They presented 20-minute talks on their research and each submitted a 4page paper for publication in the upcoming AIP proceedings. A spe-


cial student session was held where they discussed issues like future interactions and opportunities for graduate school in the United States. The Symposium was also a time for acknowledgments as a special session was devoted to honor the scientific career of Professor Ettore

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meeting reports Gadioli of the University of Milano. Professor Gadioli has a long-standing relationship with Latin American nuclear physicists and he was instrumental in launching the first Symposium in 1995. The last day of the Symposium, June 16, was reserved for an orga-

nized trip to Machu Picchu, probably the most familiar symbol of the Inca Empire and recently recognized as one of the wonders of the modern world. This exciting trip was the perfect gift after a week of talks, events, excursions, cold weather, altitude sickness, and many more adventures. The Sym-

posium series will continue in 2009 with Chile already selected as the most likely host. RICARDO ALARCON Arizona State University

International Symposium on Physics of Unstable Nuclei (ISPUN07) The second International Symposium on Physics of Unstable Nuclei (ISPUN07) was held from July 3 to 7, 2007, in a sea side resort close to Hoi An, an ancient town on the central cost of Vietnam that is protected by UNESCO as a World Heritage. The first meeting of this series took place in 2002 at Halong Bay, also a famous destination in Vietnam, with the aim to encourage nuclear physics research in that part of the continent and to give an opportunity for the new generation of Vietnamese physicists to meet their colleagues from abroad. An early attempt to hold a conference in Hanoi in 1994 has also been quite successful and got a good response from the international nuclear physics community. The success of ISPUN02 led to the idea of having a regular cycle of such meetings in Vietnam, focusing on current issues of nuclear structure and reactions studies with unstable nuclei. ISPUN07 was organized by Institute for Nuclear Science & Technique (INST) and Institute of Physics and Electronics (IPE) in Hanoi, and it has strongly benefited from the encouragement and support from various organizations both at home and

abroad: the Natural Science Council of Vietnam and Vietnam Atomic Energy Commission, GANIL, CEA Saclay, GSI Darmstadt, RIKEN, and the EU Asia-Link network on nuclear physics and astrophysics. In particular, the involvement of the latter network has allowed the attendance of a number of Asian students (6 from China and 3 from Vietnam) who participated actively in the Asia-Link network workshop held on July 2 in Hoi An, and in ISPUN07 after that. It should be noted that several of them did not hesitate to travel all the way by train from Beijing or Lanzhou (a 3day trip each way) to Hoi An for this purpose. The attendance was quite satisfactory with a total of 90 participants, about 80% coming from abroad and the rest from Vietnamese universities and institutes in Hanoi, Ho Chi Minh City, and Dalat. The ISPUN07 scientific program has been rather heavy with a total of 66 oral presentations. In addition, a poster display was set up for the whole duration of the meeting. Nevertheless, the atmosphere was quite relaxed and there were quite good working conditions in beautiful natural surroundings, with the weather conditions varying from heavy tropi-

cal downpours to hot sunny days. In the middle of the week a half-day excursion around the ancient Hoi An by rickshaw (the local taxi) was organized for the whole group of participants in order to give everyone a chance to discover the ancient city of Hoi An which was open to the World in the 17th century by the Portuguese missionaries and settled down by the Japanese and Chinese merchants later on in the 18th and 19th centuries. The excursion was concluded by a relaxing boat ride along the Thu Bon River, down to where the river merges into the sea. The scientific program of the symposium can be divided into three main parts: construction of the new facilities and instruments, present status, and new problems in the experimental studies of unstable nuclei, status, and progress of theoretical approaches. Concerning the new facilities and instruments, there were several general presentations. The RIBF at RIKEN was presented, with the first beam extracted in December 2006 and the first experiment with the RI beam separator Big RIPS performed in May 2007, where results on the new isotope 125Pd have been obtained. The SPIRAL2 project at GANIL is


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meeting reports expected to produce radioactive beams of unprecedented intensities (two orders of magnitude above the existing facilities) in the mass range from A = 60 to A = 140, at energies from a few keV up to 25 MeV/ nucleon. The FAIR facility at GSI has an ambitious program of studying exotic nuclei by means of direct reactions at low momentum transfer. The EXL set-up is a high efficiency and high resolution universal detection system, and its first feasibility study with prototype detectors at the ESR storage ring of GSI was presented. A brief presentation of the Radioactive Ion Beams Facility in Brazil (RIBRAS) was also given. News was given on the successful commissioning of the large acceptance spectrometer MAGNEX and the forthcoming commissioning of the high resolution spectrometer SHARAQ. We had a presentation of the ICHOR project, which will be of great help for the study of spin-isospin excitations in unstable nuclei. There was an overview of the REX-ISOLDE physics program and an outlook for the upgrade of energy, beam intensity, and quality of the ISOLDE complex (HIE-ISOLDE). An interesting presentation of the experimental set-up for the cosmic ray shower detection in Hanoi was given by a member of the VATLY laboratory at INST, which is an associated member of the international Pierre Auger Project in studies of ultra-high energy cosmic rays. On the experimental side, the main emphasis is of course on the studies of nuclei far from stability. Because of this, the topic of giant resonances is less actively explored whereas the interest is now on low-lying states, pygmy states, and resonances at lower energies. Results from invariant mass spectroscopy of halo nuclei—11Li and

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Be—were shown. Recent measurements of 18–21N isotopes and excited states of 18–21O by beta-delayed neutron and gamma emission were presented. Other issues of current interest were discussed in several talks: the changes in subshell closures around N = 28 and Z = 50, N = 82; core plus particle structure in Carbon isotopes; transitions in equilibrium shapes of nuclei. A presentation was given on the subject of superheavy nuclei produced in 48Ca induced reactions, and the evolution of fission times in the region Z = 114–124 was also discussed in another talk. Another important issue is the symmetry energy term of the nuclear Equation of State, and there were two talks discussing it based on the latest heavy ion fragmentation measurements. In a short but very interesting section of nuclear astrophysics, stellar nucleosynthesis has been discussed in several talks: a study of the 7Be(p,γ)B stellar reaction, a measurement of the alpha spectroscopic factor of the 6.356 MeV state in 17O, a preliminary measurement of the 21Na(a,p)24Mg reaction for investigating the abundance of 22 Na in novae. In the nuclear reaction studies with unstable beams, there were interesting presentations on the results obtained at GSI from the fragmentation of 208Pb beam, analyzing power measurements at RIKEN of elastic proton scattering off 6He, reaction mechanisms in Be + 64Zn systems around the Coulomb barrier. It was also stressed that no enhancement is observed in the 6He + 64Zn sub-barrier fusion. On the theory side, several talks were devoted to clustering phenomena in stable and unstable nuclei using AMD and AMD + GCM approaches, and to the 4-alpha condensate structure of the 4th 0+ state in 16O. The


issues of magnetic properties of light neutron-rich nuclei, and the structure of medium mass exotic nuclei were discussed in the framework of the snhell model. A comprehensive overview of the developing V-low k approach was given. Several presentations dealt with the self-consistent mean field approach in non-relativistic and relativistic framework, applications to the description of superheavy nuclei and deformed exotic nuclei, the screening of pairing interaction induced by low-lying surface vibrations. The pairing correlations near the neutron drip line, or at finite temperature, were also discussed. Nuclear excitations treated by RPA or QRPA were the subject of a series of presentations, with emphasis either on continuum effects, deformation effects, or damping effects due to particle-vibration coupling. Nuclear reactions were well represented with talks on the MultiChannel Algebraic Scattering theory, studies of three-body resonances, spectroscopy of exotic nuclei by knock out reactions, and the presentation of a classical stochastic model of break up. Nucleon-nucleus and nucleusnucleus optical potentials calculated from Dirac-Brueckner-Hartree-Fock approach using the folding model technique are shown to describe rather well nucleon– and nucleus–nucleus elastic scattering data. Transport models were shown to be quite successfully in studying the isospin dynamics in nuclear collisions. Finally, the effects of three-body forces on the Equation of State of nuclear matter, and on the structure of hybrid (hadrons and quarks) neutron stars were reported. The ISPUN07 symposium will give a long-lasting memory to all its participants. Especially, for the young Asian students it has been surely a

meeting reports precious experience of having the opportunity to meet and discuss physics with many world experts of the field, a benefit that certainly overrides the hardships of traveling. For others, it was not only just a scientific conference but also a unique occasion to discover the beauty of a far away country as well as the hospitality of its friendly people. DAO TIEN KHOA INST Hanoi NGUYEN VAN GIAI IPN Orsay

DAO TIEN KHOA


NGUYEN VAN GIAI

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facilities and methods New Promises for the Determination of the Neutrino Mass? (A Brainstorming Meeting at GSI, Darmstadt) Neutrino physics has followed a long way, full of striking discoveries, since Wolfgang Pauli postulated the existence of the ghostly particle in his famous letter to “Liebe radioaktive Damen und Herren.” The last fascinating discovery concerns the observation that neutrinos oscillate, that is, change flavor, which illustrates that the neutrino must have mass. However, oscillation experiments themselves are not able to obtain the neutrino rest mass. They can determine only the mass differences squared. The absolute mass must be determined by different measurements. Over the last 60 years heroic attempts have been undertaken to find a neutrino mass different from zero. A dramatic decrease in the upper mass limit from 1000 eV down to 2 eV was obtained over this long period of time for the electron antineutrino in the tritium decay experiments, which are most sensitive. The advances in the neutrino mass measurements were not so impressive and the knowledge of the neutrino mass presently is stuck at the level of 225 eV obtained in measurements of the internal bremsstrahlung spectra from the decay of 163Ho by electron capture. This nuclide has the smallest decay Q-value known so far and could be considered as the mostly favorable for the neutrino mass determination. What is the reason for such a large neutrino–antineutrino difference in the mass limit? Do we need to improve the neutrino mass value? And if, how to achieve a dramatic increase in precision of mass determination in the electron-capture sector? All these cornerstone questions have been dis-

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cussed in an exchange of ideas taking place at GSI on March 8–9, 2007. The idea of neutrino mass determination by electron capture was put forward a long time ago. One of the first attempts was made at ISOLDE/CERN at the beginning of the 1980s. The upper limit of 1300 eV for the neutrino mass was determined by observing the M-shell x-rays and Auger electrons in the decay 163Ho ⇒ 163Dy and by additional measuring a decay Q value in a single-nucleon transfer reaction. Attempts performed during the subsequent decade by different experimental groups improved this limit only by a small factor, two orders of magnitude less stringent than in the case of the antineutrino mass value. In that time the technical possibilities did not keep pace with requirements for more precise measurements. However, over the last decade the progress achieved in both the techniques of mass measurements in Penning traps as well as of microcalorimetry of atomic de-excitations, allows one to resurrect attempts for neutrino mass determination at a new level of accuracy. The difference between the masses of parent and daughter atoms in the electron capture process is shared between the captured-electron binding energy and the total neutrino energy, which includes also the neutrino rest mass. In order to determine the latter, the atomic masses of the parent and daughter atoms and the electron binding energies should be measured as accurately as possible. There exist different scenarios how to proceed experimentally as dis-


cussed by Yu. Novikov (PNPI and GSI) in his contribution. Mass differences can be measured by state-of-theart Penning traps with an accuracy of the order of 10−11 as it was pointed out by K. Blaum (Mainz and GSI) in his review talk about ion traps and their impact on physics research. Measurements of atomic de-excitation spectra, occurring after filling the vacancy in the atomic shell, can be carried out by microcalorimeters, which detect the total energy release from electrons and photons but neutrinos. As noted by F. Gatti (Genoa) and L. Gastaldo (Heidelberg) in their presentations about the unique potential of microcalorimetry, the energy position of peaks in these spectra, that is, the electron binding energies, can be determined with an accuracy better than 1 eV for a sum energy of about 1 keV. This corresponds to M-electron binding energies in the region of holmium and dysprosium. Two different approaches exist, in both of which the determinations of calorimetric spectra and of Q values are essential; One involves the determination of branching ratios for electron capture from different atomic orbits, the other method the shape analysis of the calorimetric sum peak. In the first approach, the branching ratios must be determined with an accuracy of about 10−4 and the Q values should reach an accuracy of approximately 0.1 eV. Both requirements can not be met presently. As discussed by W. Quint (GSI), even the very auspicious Penning trap facility for highly charged ions, HITRAP, presently being installed at

facilities and methods the Experiment Storage Ring ESR at GSI, will not allow Q value measurements with such precision in the foreseeable future. More promising looks to be the second approach whose idea was put forward many years ago by A. De Rujula: The tails of the peaks of the atomic de-excitation spectra depend on the neutrino mass and the peak nearest to the Q value is most sensitive to its value. In principle, analysis of the spectral peaks in the capture channel has a big advantage in comparison to observation of the spectrum of the β− decay near the endpoint: For the example of the 163Ho decay, one expects for the 2 eV part at the edge of the 2.047 keV peak normalized to the full peak area a ratio ranging from 10−10 to 10−11 if the Q value varies between 2.3 and 2.8 keV. This corresponds to a gain in sensitivity by about two orders of magnitude as compared to the tritium decay with the decay Q value of 18.59 keV. In the second approach, the neutrino mass or mass limit deduced from electron capture in 163Ho also strongly depends on both the accuracy of decay Q value as well as on the absolute value of Q. The required data acquisition time is estimated to range between 60 and 600 days for deriving a 2 eV neutrino mass limit from measurements with a multi-pixel microcalorimeter. Another crucial question discussed during the brainstorming at GSI was whether or not other candidates exist appropriate for capture measurements if Nature will not be so kind to provide a favorable Q value for 163Ho. The search for new candidates should be supplemented by precise spectroscopic measurements of atomic and nuclear excitation energies by bolometers whose features were presented

by P. Egelhof (GSI). A search of existing data reveals that there are about a dozen of relevant pairs of nuclides for whom the decay Q values could be very small if also nuclei with decays from ground states of the mother nuclides to excited states of the daughters are included. However, an assessment of how small the Q-values are and if those nuclides are at all suitable for neutrino mass determination is presently not feasible because of the large uncertainties, in many cases exceeding even 10 keV. In order to search for additional candidates for a determination of the neutrino mass one does not need to start with ultra-precise mass measurements from the very beginning. For these purposes the existing Penning trap systems, which are on-line with the various radioactive beam facilities, can initially be used. In presentations given by F. Herfurth (GSI) and A. Herlert (CERN) the possibilities for atomic mass measurements of such candidates at SHIPTRAP/GSI, ISOLTRAP/CERN and other facilities have been discussed. Measurements at these installations can be considered as mutually complementary and as a prerequisite for later ultra-accurate mass measurements at the HITRAP facility at GSI. Plans for a new Penning trap mass spectrometer were presented by S. George (Mainz and GSI). It consists of four cryogenic traps in the very same superconducting coil and is presently designed at Mainz University by K. Blaum et al. The mass uncertainty is expected to reach a level of 1 eV for heavy nuclides if ions in high charge states are used. As discussed by O. Kester (GSI), those can be produced by stripping of relativistic projectiles by a massive target or by impact of high-energy electron.

The domain of antineutrino mass determination in continuous β− decay was not omitted at the brainstorming. The flagship project KATRIN was presented by Ch. Weinheimer (Münster) who expects a mass limit or mass value at the level of 0.2 eV to be reached in the measurement campaign planned for 2010 to 2015. A. Fässler (Tübingen) announced similarly low (but model-dependent) limits from the analysis of the Heidelberg-Moscow measurements of the half-life of neutrinoless double β-decay of 76Ge. The WITCH experiment installed at ISOLDE/ CERN was discussed by N. Severijns (Leuven). It allows the search for a heavy electron neutrino with a mass value reaching up to a few MeV. A review of the impressive potential (but also of conceptual problems) of neutrino physics was given by M. Lindner (Heidelberg). Accurate neutrino mass measurements will lead to new physics beyond the present Standard Model of elementary particles and the eventual deviations of the neutrino and antineutrino parameters would result in rather radical theoretical changes such as CPT violation. Among the wide variety of studies of neutrino properties, mass measurements are very ambitious ventures that are of paramount importance for large-scale physics and cosmology. Therefore dissimilar approaches using novel scenarios and completely different techniques (and, thus, suffering from completely different systematic uncertainties) should be strongly encouraged. As a result of extensive discussions at the meeting, the following milestones in the neutrino mass program can be outlined:


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facilities and methods •

•

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Masses for the pair 163Ho – 163Dy should be measured with utmost accuracy at the new-generation four-trap Penning mass spectrometer, which is under design for the HITRAP facility at GSI. The longlived radioactive nuclide 163Ho can be produced either at ISOLDE/ CERN and then moderately ionized in the MAXEBIS (electron beam ion source) of HITRAP or on-line produced in highest charge states at GSI. Similarly, the mass of moderately or highly charged stable 163Dy can be measured. The expected uncertainties for mass values are about 1 eV, which is adequate to provide a neutrino mass determination on a level of a few eV. An exact Qε value for the pair 163Ho-163Dy is a very crucial starting point for subsequent analysis of microcalorimetric atomic de-excitation spectra. Independent of the mass measurements the atomic de-excitation spectra after electron capture by 163 Ho should be measured off-line by multi-pixel microcalorimetric detectors with an uncertainty in the peak position of about 1 eV. This is presently achievable, for example, at the Universities of Genoa and

•

•

Heidelberg. The 10−6 s time resolution of the detector is sufficient to reach the required statistics within a few weeks to a few months of measuring time as determined by the exact Qε value for 163Ho provided by Penning trap measurements. In parallel to these investigations, new candidates for the neutrino mass determination by electron capture should be searched for. As the mass uncertainties for the about ten candidates with small Q values are still too large precursory measurements at the different existing traps (ISOLTRAP, SHIPTRAP, JYFLTRAP, etc.) should be performed with subsequent ultra-precise mass measurements of the most promising candidates at the HITRAP facility. In addition, trap-assisted nuclear spectroscopy of selected nuclides can be performed for very accurate determination of decay channels. The analysis of the experimental data should be accompanied by very accurate theoretical calculations of the atomic structure of the atomic systems under investigation. Here, the electron wave functions of the innermost atomic


shells of neutral atoms and those of highly charged ions are of supreme importance and can be scrutinized by atomic theorists from St. Petersburg. From this list of required work it is obvious that a campaign for neutrino mass measurements with an accuracy of a few eV (two orders of magnitude better than known now) requires the collaboration of multidisciplinary experts and demands well-coordinated common efforts. A very attractive aspect of this innovative activity is that the experiments can be performed at existing facilities or new instruments coming soon into operation. It can be expected that the tools and approaches of atomic physics (atomic masses, atomic de-excitation spectra, atomic electron binding energies, etc.) will enable in the near feature a new access to a fundamental property of a fundamental particle. H.-JÜRGEN KLUGE GSI, Darmstadt YURI NOVIKOV PNPI, St. Petersburg

news from EPS/NPB Call for Nominations for the Lise Meitner Prize for Nuclear Science of the European Physical Society, 2008 The Nuclear Physics Board of the EPS invites nominations for the “Lise Meitner Prize” for the year 2008. The award will be given to one or several individuals for outstanding work in the fields of experimental, theoretical, or applied nuclear science. The Board welcomes proposals that represent the breadth and strength of European nuclear sciences. Nominations need to be accompanied by a filled-in nomination form, a brief curriculum vitae of the nominee(s) and a list of major publications. Letters of support from authorities in the field that outline the importance of the work of the nominee(s) are also helpful.

Nominations will be treated as strictly confidential and although they will be acknowledged there will be no further communication from the selection committee. Nominations should be sent to: Selection Committee Lise Meitner Prize c/o Chairman Prof. Hartwig Freiesleben Institut für Kern- und Teilchenphysik Technische Universität Dresden 01069 Dresden, Germany Phone: +49 (0)351 46335461; Fax: +49 (0)351 46337292 E-mail: [email protected]

For the nomination form and more detailed information go to the website of the EPS, Nuclear Physics Division: http://ific.uv.es/epsnpb/ or the website of the EPS: www.eps.org (EPS Prizes, Lise Meitner Prize) The deadline for the submission of nominations has been set for January 11, 2008. HARTWIG FREIESLEBEN Chairman, EPS-Nuclear Physics Division


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calendar September 13–19 Hebden Bridge, West Yorkshire, UK. 12th Geant4 Collaboration Workshop http://indico.cern.ch/conference Display.py? confld=10311 December 4–6 Sapporo, Japan. 10th International Symposium on Origin of Matter and Evolution of Galaxies (OMEGO7) http://nucl.sci.hokudai.ac.jp/~omego7

2008 Jan 7–10 Ringberg, Germany. International Workshop on Astronomy with Radioactivities http://www.mpe.mpg.de/gamma/ science/lines/workshops/ AwRVI/AwRVI.html Jan 20–26 Bormio, Italy. LXVI International Meeting on Nuclear Physics http://wspig2.physik.unigiessen.de:8000/bormio/ February 4–10 Jaipur, India. Quark Matter 2008 http://www.veccal/ernet.in/qm 2008.html

June 8–14 Crete, Greece. 9th International Conference on Applications of Nuclear Techniques. http://www.crete08.org June 9–15 Kyiv, Ukraine. 2-nd International Conference on Current Problems in Nuclear Physics and Atomic Energy (NPAE-Kyiv2008) http://www.kinr.kiev.ua/NPAEKyiv2008/ July 18–22 Barcelona, Spain. Euroscience Open Forum ESOF2008 http://www.esof2008.org July 20–25 Debrecen, Hungary. 11th International Conference on Nuclear Microprobe Technology and Applications http://icnmta.atomki.hu/ July 27–August 1 Mackinac Island, Michigan, USA. 10th Symposium on Nuclei in the Cosmos (NIC X) http://meetings.nscl.msu.edu/nic2008/

August 25–29 Cologne, Germany. 13th International Symposium on Capture Gamma-Ray Spectroscopy and Related Topics. http://www.ikp.uni-koeln.de/cgs13/ September 1–7 Zakopane, Poland. Zakopane Conference on Nuclear Physics http://zakopane2008.ifj.edu.pl/ September 7–13 Ryn, Portland. ENAM http://enam08.fuw.edu.pl September 15–20 Lanzhou, Gansu, China. 7th International Conference on Nuclear Physics at Storage Rings STORI’08 http://ribll.impcas.ac.cn/conf/stori08/ November 9–4 Eilat, Israel. 18th Particle and Nuclei International Conference PANIC 08 http://www.weizmann.ac.il/ conferences/panic08

2009 August 10–15 Fort Worth, Texas, USA. 20th International Conference on the Application of Accelerators in Research and Industry CAARI 2008 http://www.caari.com

June 2–5 Mackinac Island, Michigan, USA. 3rd International Conference on “Collective Motion in Nuclei under Extreme Conditions” (COMEX 3) http://www.meetings.ncsl.msu.edu/ COMEX3/

More information available under: http://www.nupecc.org/calendar.html

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