Available online at www.sciencedirect.com
ScienceDirect Physics Procedia 66 (2015) 520 – 523
C 23rd Conference on Application of Accelerators in Research and Industry, CAARI 2014
Study of Neutron Induced Reactions on 7Be using Large Angle Coincidence Spectroscopy Jiri Vacika* , Vladimir Hnatowiczb, Ulli Kösterc a
Nuclear Physics Institute (NPI), Academy of Sciences of the Czech Republic, 250 68 Rez, Czech Republic b
Institute of Laue-Langevin, BP 156, 6, Rue Jules Horowitz, 38042 Grenoble, Cedex 9, France
Abstract Neutron induced reactions on the radioactive nuclei 7Be represent a unique tool for study of the 8Be nuclear structure in the highenergy region. The interactions of thermal neutrons with 7Be is dominated by the strong 7Be(nth,p)7Li reactions. While 8Be in the ground state disintegrates rapidly into two D particles, the alpha decay from the 2- neutron capture state is prohibited because of the parity violation. The study of the 7Be(nth,D)4He reaction (with two 9496 keV D particles) is then a unique tool to study parity violation in strong interactions. The test was carried out on the collimated thermal neutron beam at NPI Rez using the large angle coincidence spectroscopy. The 7Be nuclei were prepared at the ISOLDE facility by implantation of the 60 keV 7Be ions into 5 μm Al foil. The results showed that the ratio of the cross sections is σ(p1)/σ(p0) = (1.58 ± 0.06) x 10-2, and in the coincidence measurement simultaneous emission of two 8775 keV D particles is observed. This reaction channel corresponds to a two-stage process comprising transition from the neutron capture state (18991 keV) to an intermediate state by emission of J photon that is followed by the decay into two alphas. The intermediate state in the highly excited 8Be nuclei is the level at 17640 keV (1+), which is in contrast to the previous reported data. In the experiment no D particles with the energy 9495 keV were observed. © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license © 2014 The Authors. Published by Elsevier B.V. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selectionand andpeer-review peer-review under responsibility the Organizing Committee of 2014 CAARI 2014. Selection under responsibility of theof Organizing Committee of CAARI Keywords: Radioactive nuclei, Berylium, Thermal neutrons; Coincidence spectroscopy
* Corresponding author. Tel.: +420-266-173-129; fax: +0-000-000-0000 . E-mail address:
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1875-3892 © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and peer-review under responsibility of the Organizing Committee of CAARI 2014 doi:10.1016/j.phpro.2015.05.068
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1. Introduction Neutron induced reactions on radioactive 7Be (EC, EJ = 477.6 keV, T1/2 = 53.22 d) play an important role both in nuclear physics and in astrophysics. In nuclear physics they represent a good tool to investigate 8Be nuclear structure in the high-energy region, Tilley et al. (2004). In astrophysics, the reactions are considered for their possible role in creation of 7Li during primordial nucleosynthesis, Coc et al. (2004). The interaction of 7Be with thermal neutrons is dominated by the reaction 7Be(nth,p)7Li, the cross section of which is enhanced by the presence of 2 - (l = 0) highly excited resonance at 18.991 MeV. The reported cross sections for two proton reaction branches are 38400 ± 800 and 420 ± 120 b, respectively, Tilley et al. (2004). While 8Be in the ground state decays rapidly into two alpha particles, such decay from the above mentioned 2- capture state is prohibited because of parity violation. For that reason, an observation of the reaction 7Be(nth,4He)4He with emission of two alpha particles would be a direct prove of parity violation in strong interactions. The neutron induced reactions on 7Be accompanied with alpha particle emission were investigated by Coc et al. (2004) using a coincidence technique, but only an upper limit of the cross section of 0.1 mb was estimated for the parity violating channel. Instead the reaction 7Be(nth,gamma,4He)4He was observed with a cross section of about 155 mb. According to Coc et al. (2004) the reaction represents a two stage process, comprising electric dipole gamma transition from the capture state at 18.991 MeV (1-) to the state at 16.62 MeV (2+) that is followed by the decay into two alpha particles. Gamma transition to other lower lying states could also be possible but it is claimed to be less probable, Coc et al. (2004). Because possible parity violation in strong interaction is of fundamental significance for nuclear and particle physics, the NPI team performed new measurement of the neutron induced reactions on 7Be under improved experimental conditions. In this report information on the preliminary results are given which could be of interest for field specialists. 2. Experimental The study of neutron induced reactions on 7Be is complicated by its radioactivity and relatively short life time (T1/2 = 53.22 d). 7Be is produced in a charge-particle induced nuclear reaction and its available total amount is strongly limited. In the present case the 7Be target was prepared at ISOLDE (CERN) by implantation of 60 keV 7Be ions either into a 5 μm thick Al foil, or into 100 Pm Si wafer. A high purity Al foil was chosen to minimize target contamination with boron (10B) and lithium (6Li). The initial number of the 7Be atoms in the target was determined from the emission rate of 478 keV gamma rays measured on a calibrated HpGe detector (see below). The target was placed in to a vacuum chamber situated at the end of the 6 m long neutron guide that was installed on horizontal neutron beam of the LWR-15 research reactor (operated by Research Centrum Rez). Typical thermal neutron flux at the sample position was 107 cm-2s-1 (with a cadmium ratio ~ 105). Reaction products were registered with a windowless Hamamatsu PIN diode (10 x 10 mm2 sensitive area) placed at the distance of 10 mm from the target (detector solid angle was ω = 7 x 10-2). Acquisition of the energy spectra was carried out using a standard spectrometric system. To reduce unwanted background, however, a coincidence measurement was utilized. For a thin Al target two identical PIN diodes placed symmetrically on opposite target sides at the distance of 5 mm (ω = 2 x 10-1) were used. Acquisition of 2048 x 2048 two dimensional spectra was accomplished by a fast/slow coincidence system based on the FAST ComTec MPA-3 Multiparameter Multichannel Analyzer. Typical time resolution of the MPA-3 system was 1 Ps. Energy calibration of the single and two dimensional (coincidence) spectra in the range from 4.5 to 8.8. MeV was done using thin 226Ra, 233U and 238, 239, 240Pu radioactive sources. Control measurement of the high energy alpha particles (in the range of 8 - 10 MeV) from the 149Sm(nth,alpha)146Nd nuclear reaction was also performed. 3. Results and discussion The single spectrum of protons from the 7Be(nth,p) 7Li nuclear reaction from the Be/Si target is shown in Fig.1 for illustration. The acquisition time was 65 hours and the total number of 7Be atoms in the target was found to be (4.0 ± 0.4) x 1013. In the spectrum, measured with the energy resolution of FWHM = 15 keV, two proton groups
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with energies 1438 keV (p0) and 1020 keV (p1) are clearly resolved. They correspond to the decay of 8Be to the ground and the first excited states of 7Li by protons. In Fig. 1 one can see that the main peak in the spectrum (p0) exhibits pronounced right hand side, which is due to accidental coincidence with photons of internal bremstrahlung from the 7Be radioactive decay. The right tail is about 2% of the total area of the p 0 peak. This ratio is in rough agreement with that estimated from the known intensity of the internal bremsstrahlung photons considering low photon detection efficiency of the PIN diode and time resolution of the spectrometric chain. The ratio of the σ(p 1) and σ(p0) cross section reactions is found to be σ(p 1)/σ(p0) = (1.58 ± 0.06) x 10-2 (the quoted uncertainty combines statistical error with the uncertainty of the underlying background shape). The present ratio is by about 30% higher than the ratio σ(p1)/σ(p0) = (1.09 ± 0.30) x 10-2 reported by Koehler et al. (1988). 10
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Fig. 1. Single spectrum of protons from 7Be(nth,p)7Li nuclear reaction (for details see the text).
The relevant part of the two-dimensional coincidence spectrum from the 7Be sample, prepared on thin Al backing, is shown in Fig. 2. For the sake of imaging the original 2048 channel spectra were summed over 8 channels 7
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in both axes. The energy calibration, equal in both axes, was accomplished using the set of the alpha radioactive sources specified above. The number of the 7Be atoms in the target was (2.0 ± 0.2) x 1013, measuring time was 485 hours. Only 15 alpha-alpha coincidence events were registered which are cumulated around a channel corresponding to the alpha particle energy Eα = 8775 ± 150 keV (the estimated error is rather conservative one). The total energy released, i.e., 2 x Eα = 17550 ± 200 keV, is significantly lower than the total reaction energy of 18991 keV, expected for the gamma-less 7Be(nth,4He)4He reaction. So, we obviously observe the reaction channel 7Be (nth,gamma,alpha)4He in which the emission of two alpha particles is preceded by gamma-ray emission, and transition to lower excited state in 8Be. From our more precise energy measurement it follows that the energy of the gamma transition is Eγ = 1750 ± 200 keV and the energy of intermediate excited state 17500 ± 200 keV. A possible candidate of the intermediate state at high excited energy in 8Be may be the state at 17640 keV (1+), see the level diagram in Fig. 3. This is in contrast to the previous works by Tilley et al. (2004) and Bassi et al. (1963), where the state at 16626 keV was considered as the intermediate one.
Fig. 3. Energy levels of the compound 8Be nucleus; Tilley et al. (2004).
Conclusion Preliminary results of the study of nuclear reactions induced by thermal neutrons on 7Be are reported. The reaction products were recorded in both single and coincidence modes using large area PIN detectors. The single proton spectrum was measured with very high counting statistics and decays to the 7Li ground (p0) and first (p1) excited states were clearly resolved. The ratio of the cross sections was determined to be σ(p 1)/σ(p0) = (1.58 ± 0.06) x 10-2. In the coincidence measurement, simultaneous emission of two alpha particles with the energy Eα = 8775 ± 150 keV was observed. This reaction channel corresponds to a two-stage process comprising transition from the neutron capture state (18991 keV) to an intermediate one by emission of gamma photon with the energy Eγ = 1750 ± 200 keV followed by the decay into two alphas. In the experiment no D particles with the energy 9495 keV (that would confirm parity violation in strong interaction) were observed. Acknowledgement: The experiments were carried out at the CANAM infrastructure of the NPI ASCR Rez and supported by the Grant Agency of the Czech Republic (Project No P108/12/G108). References Bassi, P., Campellani, F., Venturini, G., Bertolini, G.C., Cappellani, F., Mandl, V., Restelli, G.B., Rota, A., 1963. The reaction 7Be(n,α) 4He and parity conservation in strong interactions. Il Nuovo Cimento, XXVIII, No.5, 1049-1065. Coc, A., Vangioni-Flam, E., Descouvemont, P. Adahchour, A., Angulo, C., 2004. Updated Big Bang Nucleosynthesis confronted to WMAP observations and to the Abundance of Light Elements. Astrophysical Journal 600, 544-552. Koehler, P.E., Bowman, C.D., Steinkruger, F.J., Moody, D.C., Hale, G.M., Starner, J.W., Wender, S.A., Haight, R.C., Lisowski, P.W., Talbert, W.L., 1988. 7Be(n,p)7Li total cross section from 25 meV to 13.5 keV. Physical Review 37C, 917-926. Tilley, D.R., Kelley, J.H., Godwin, J.L., Millener, D.J., Purcell, J., Sheu, C.G., 2004. Energy levels of light nuclei A = 8, 9, 10. Nuclear Physics A745, 155-362.
