Experimental studies of low-lying E1 and E2 transitions in 12Be are .... ing of 13 âE- and 16 E- plastic scintillators, located 5.0m ... Be moving with v/c â 0.3.
RIKEN Review No. 39 (September, 2001): Focused on Physics at Drip Lines
Experimental studies of E1 and E2 transitions in 12Be Hironori Iwasaki,∗1,∗2 Tohru Motobayashi,∗3 Hiromichi Akiyoshi,∗4 Yoshiaki Ando,∗3 Naoki Fukuda,∗1 ¨ ∗4,†1 Kevin I. Hahn,∗4,†2 Yoshihide Higurashi,∗3 Masaaki Hirai,∗1 Hideki Fujiwara,∗3 Zsolt Ful ¨ op, Isamu Hisanaga,∗3 Naohito Iwasa,∗4,†3 Takeshi Kijima,∗3 Alberto Mengoni,∗4,∗5 Toshiyuki Minemura,∗3 Takashi Nakamura,∗1,†4 Masahiro Notani,∗2 Shuichi Ozawa,∗3 Hiroyuki Sagawa,∗6 Hiroyoshi Sakurai,∗1 Susumu Shimoura,∗2 Satoshi Takeuchi,∗3 Takashi Teranishi,∗2 Yoshiyuki Yanagisawa,∗4 and Masayasu Ishihara∗4 ∗1 ∗2
Department of Physics, University of Tokyo
Center for Nuclear Study, University of Tokyo
∗3
Department of Physics, Rikkyo University ∗4 ∗5
∗6
Radiation Laboratory, RIKEN
Applied Physics Division, ENEA, Italy
Center for Mathematical Sciences, University of Aizu
Experimental studies of low-lying E1 and E2 transitions in 12 Be are presented. By utilizing the in-beam γ-ray spectroscopic technique, we have studied the inelastic scattering of 12 Be on 208 Pb, 12 C, and (CH2 )n targets. The spectroscopic results on the low-lying structure of 12 Be are briefly discussed in relation to the modification of the nuclear structure near the drip line.
Introduction One of the most predominant results from experimental studies on nuclear properties is evidence of nuclear shell closure. The resultant magic numbers have provided useful references to achieve a proper balance in the use of two contrasting aspects of nuclear properties: single-particle motions and collective deformations. In spherical nuclei around the shell closure, nuclear properties are well described by referring to the aspect of single-particle motions, whereas collective defomations manifest themselves in nuclei far from the shell closure. However, there is no well-founded approach to predict the structure of nuclei in which the numbers of protons and neutrons are drastically unbalanced, since the experimental evidence for nuclear shell closure is limited to the region at and near the stability line where the numbers of both nucleons are well balanced. It is thus essential to explore the nuclear properties in nuclei lying far from the stability line in order to establish a unified understanding of the nuclear structure. In the present work, we have investigated the low-lying structure of the neutron-rich 12 Be nucleus. The 12 Be nucleus has drawn much attention due to the possible shell quenching in light neutron-rich nuclei around N = 8.1) Thus far, indications of the N = 8 shell quenching in 12 Be has been obtained by various studies on its ground-state properties.1–3) Thus, we performed two spectroscopic studies on low-lying excited states in 12 Be, since more direct information on shell quenching may be obtained from the low-lying excitation †1 †2 †3 †4
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On leave from ATOMKI, Debrecen, Hungary Present address: Department of Science Education, Ewha Woman’s University, Korea Present address: Department of Physics, Tohoku University Present address: Department of Physics, Tokyo Institute of Technology
properties. Initially, we searched for the low-lying 1− state by intermediate-energy Coulomb excitation.4) Information on the shell gap can be extracted from the excitation energy of the intruder 1− state. Second, we studied the quadrupole deformation of 12 Be in terms of proton inelastic scattering 5) exciting the 2+ 1 state. Measurement The experiment was performed at the RIKEN Accelerator Research Facility. We studied the inelastic scattering by using the in-beam γ-ray spectroscopic technique. We measured the de-excitation γ-rays in coincidence with scattered particles in order to identify the inelastic channel. Angleintegrated cross sections for inelastic scattering were obtained from the observed γ-ray yields. A radioactive 12 Be beam was produced and separated by the RIKEN projectile-fragment separator (RIPS),6) using a 100 AMeV 18 O beam incident on a 1.11 g/cm2 9 Be target. The particle identification of the incident beam was carried out event-by-event by time-of-flight (TOF) information obtained with two 1-mm-thick plastic scintillators (PLs) placed 5.3 m apart along the beam line. The light-output signals of the PLs were incorporated in the particle identification. A typical intensity of the 12 Be beam was 2×104 counts per second, and a purity was approximately 96%. The incident angle of the beam was monitored by two parallel-plate avalanche counters. The angular spread of the 12 Be beam was 0.54 degrees in r.m.s. for the horizontal direction and 0.61 degrees in r.m.s. for the vertical direction. The secondary 12 Be beam bombarded a secondary target at the final focal plane of RIPS. In the present experiment, we used three kinds of targets for the study of inelastic scattering. The Coulomb excitation of 12 Be was studied using a 208 Pb target. Nuclear contribu-
tions to the excitation were evaluated using data taken with a 12 C target. A (CH2 )n target was also used to study the inelastic proton scattering. The contribution from 12 C in the (CH2 )n target was subtracted using the data obtained with the 12 C target to determine the result for the proton scattering. The thicknesses of the 208 Pb, 12 C, and (CH2 )n targets were 350.8, 89.8, and 90.2 mg/cm2 , respectively. The beam energies in the middle of the targets were calculated to be 53.3, 54.0, and 53.8 AMeV, respectively, for the 208 Pb, 12 C, and (CH2 )n targets. A measurement with the target removed was also made to evaluate background contributions. The scattered 12 Be was detected by a hodoscope consisting of 13 ∆E- and 16 E- plastic scintillators, located 5.0 m downstream of the target. The particle identification was performed by the standard TOF-∆E and TOF-E methods. This was carried out to reject events from the breakup of the projectile in the secondary target. De-excitation γ-rays were detected with 55 pieces of NaI(Tl) crystal scintillators surrounding the target from 67 to 148 degrees with respect to the beam axis. Each scintillator crystal has a rectangular shape with a size of 6 × 6 × 12 cm3 coupled with a 5.1-cmφ photomultiplier tube. The high granularity of the setup allowed us to measure the angle of the γ-ray emission with approximately 20◦ accuracy. The angular information was used to correct for large Doppler shifts of γ-rays from the excited 12 Be moving with v/c ≈ 0.3. The energy and efficiency calibrations of each NaI(Tl) detector were performed using standard 22 Na, 60 Co, and 137 Cs sources. Search for the low-lying 1− state by intermediate-energy Coulomb excitation
by the coupled-channel calculation code ECIS79.7) In the calculation, the deformation length was taken to be the same for Coulomb and nuclear potentials. The excitation energy and the incident energy of 12 Be are fixed at 3 MeV and 50 AMeV, respectively. As shown in Fig. 1 (left), the total cross section (solid line) for the E1 transition depends largely on the target atomic number, since both Coulomb (red histogram) and nuclear (blue histogram) excitations are strongly suppressed with the carbon target. On the other hand, the total cross sections for the E2 transition (Fig. 1 (right)) are expected to be in the same order between lead and carbon targets, since the total cross sections are mainly determined by nuclear contributions. Thus, if a state is observed with both targets, it is likely the 2+ state excited by the E2 transition, while the 1− state is not populated with the carbon target effectively. On this basis, we performed inelastic scattering with lead and carbon targets in order to populate and identify the 1− state in 12 Be. Figure 2 shows energy spectra of γ-rays measured during the inelastic scattering of 12 Be with 208 Pb (top) and 12 C (bottom) targets, where Doppler shifts are corrected. In addition to the γ-ray peaks from the 2+ 1 state, another peak is clearly observed at 2.68 MeV in the 208 Pb spectrum. On the other hand, no significant transition is observed with the 12 C target. This target dependence strongly suggests that the excitation occurred via an E1 transition, leading to an assignment of J π = 1− for the 2.68 MeV state. From the excitation cross section (46.5 (11.5) mb) obtained + with the 208 Pb target, the B(E1;0+ g.s. →1 ) value was deduced to be 0.051(13) e2 fm2 . This value corresponds to 0.05 W.u. for the γ-decay strength, which is the first example of a strong low-lying E1 transition in even-even nuclei. Figure 3 plots
In the present work, we studied the inelastic scattering with lead (Z = 82) and carbon (Z = 6) targets at intermediate energies of about 50 AMeV. Comparison of the cross sections between lead and carbon targets provides a useful way in which to distinguish the E1 (l = 1) and E2 (l = 2) excitations, since the dependence on the target atomic number Z is very different for lighter projectiles as 12 Be between these multipolarities. Figure 1 shows the target Z dependence of the 12 Be excitation in the case of E1 (left) and E2 (right) transitions calculated
Fig. 1. Target Z dependence of inelastic cross sections for the 12 Be scattering. Left and right panels show excitation cross sections in the case of E1 (l = 1) and E2 (l = 2) transitions, respectively. Total excitation cross sections are shown by solid lines, while Coulomb (nuclear) contributions to the excitations are shown by the red (blue) histogram.
Fig. 2. Doppler-corrected γ-ray energy spectra observed during the inelastic scattering of 12 Be with 208 Pb (top) and 12 C (bottom) targets.
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Fig. 3. E1 transition strength observed between the ground state and the bound excited state in light nuclei. The γ-decay strength is plotted as a function of the mass number A. The open circles represent the data obtained for even-even nuclei, while the cross symbols show the data for the other nuclei.
the experimental data of the E1 strength observed between the ground state and the bound excited state.8) The γ-decay strength in W.u. is plotted as a function of the mass number A. The transition between the 1/2+ ground state and the first excited 1/2− state in 11 Be is a famous example, having the strongest E1 strength ever observed between bound states. As shown in the figure, the present result on 12 Be has anomalously large strength, representing the strongest transition ever observed for even-even nuclei. The experimental value agrees well with the recent theoretical value of B(E1) = 0.063 e2 fm2 studied by large-scale shell model calculations.9) Using the present result on the 1− state, we can discuss the modification of the shell structure near the drip line, since the 1− state may correspond to the excitation of the 1p1/2 state to the 2s1/2 state in a naive shell model picture. First, we plot the energy difference between 2s1/2 and 1p1/2 states, (1/2+ ) – (1/2− ), in Fig. 4 (top) as a function of the proton number Z for N = 7 isotones. As pointed out by Talmi and Unna, the energy gap decreases as the proton number decreases.10) The near degeneracy between the 2s1/2 and 1p1/2 states is achieved at 11 Be. Next, we make a similar plot for N = 8 isotones. The excitation energies of the lowest 1− states, (1− ) – (0+ ), are plotted in Fig. 4 (bottom) as a function of Z. The energy gap similarly decreases in 12 Be, showing that the N = 8 shell gap is significantly narrowing in 12 Be. Recently, spin assignments for the positive-parity states of 13 B at approximately 3 MeV have been proposed based on the knockout-reaction measurement.11) These states in 13 B may correspond to the 1− excitation from the negative-parity ground state (J π = 3/2− ), suggesting that 13 B also follows the trend of the lowering of the 1− states in the N = 8 isotones. Thus, we can conclude that the N = 8 shell quenching widely occurs in neutron-rich nuclei around 12 Be.
Quadrupole deformation of scattering
12
Be studied via proton inelastic
The near degeneracy of the 2s1/2 and 1p1/2 states may result in the enhancement of the quadrupole deformation in the
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Fig. 4. Energy levels in the Be and C isotopes with the neutron numbers N = 7 and N = 8. In the top panel, the energy differences between the 1/2+ state and the 1/2− state are plotted as a function of the atomic number Z.10) The bottom panel shows the plots of the excitation energies of the lowest 1− states. The relevant levels in the B isotopes are also plotted.
N = 8 nucleus 12 Be. This is our motivation for the study of the 2+ 1 excitation by proton inelastic scattering. In the case of 12 Be, the 2+ 1 state is located at the excitation energy of 2.10 MeV, which is lower than that of 10 Be (3.37 MeV). This reduced energy may be a signature of shell quenching, as in the case of the N = 20 nucleus 32 Mg.12, 13) In the experiment, the angle-integrated inelastic cross section for the 2+ 1 state was obtained by detecting de-excitation γrays in coincidence with scattered particles. This is the first application of the in-beam γ-ray spectroscopic technique to the study of inelastic proton scattering. We thus performed the same measurement on 10 Be and compared our result with the previous data 14) in order to confirm the validity of our new γ-ray method. Figure 5 shows γ-ray energy spectra measured during the proton inelastic scattering of 12 Be (top) and 10 Be (bottom), for which Doppler shifts are corrected. In the figure, one can see single dominant peaks at 2.10 MeV and 3.37 MeV, respectively, for 12 Be and 10 Be. The cross section of the inelastic 12 proton scattering to the 2+ Be (10 Be) obtained 1 state of from the photo-peak yield is 27.0 ± 4.0 mb (17.6 ± 3.2 mb). From a coupled-channel analysis, the deformation lengths 12 for the 2+ Be and 10 Be were determined to be 1 states in
Summary In summary, we have carried out in-beam γ-ray spectroscopy of 12 Be using intermediate-energy reactions of Coulomb excitation and proton inelastic scattering. Anomalous properties of the low-lying structure of 12 Be were revealed; an intruder 1− state, accompanied by a very large E1 transition, was observed at the low excitation energy of 2.68 MeV, and large + quadrupole deformation was found for the 0+ g.s. →21 transition. The present results provide considerable evidence of the N = 8 shell quenching near the drip line. This paper was prepared on the basis of recent experiments performed at RIKEN.4, 5) The present study was partly supported by the Ministry of Education, Culture, Sports, Science and Technology by a Grant-in-Aid for Scientific Research under program number (B) 08454069.
References
Fig. 5. Doppler-corrected γ-ray energy spectra observed during the proton inelastic scattering of 12 Be (top) and 10 Be (bottom).
1) 2) 3) 4) 5) 6) 7) 8)
2.00 ± 0.23 fm and 1.80 ± 0.25 fm, respectively. Note that the value of 1.80 ± 0.25 fm for 10 Be agrees well with the previous results of 1.84–1.99 fm deduced from several works on inelastic proton scattering at Ep = 12.0–16.0 MeV.14) The deformation length is known for the adjacent isotope 9 Be, where the results of 1.61–2.13 fm were obtained from α scattering.15–17) Comparison of these deformation lengths indicates that a tendency towards strong quadrupole deformation is preserved for these nuclei and that the singly closed shell structure does not persist in 12 Be.
9) 10) 11) 12) 13) 14) 15) 16) 17)
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