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Valencia, E. et al, Tain, J. L., Algora, A., Agramunt, J., Estevez, E., Jordan, M. D., et al. (2017). Total absorption gamma-ray spectroscopy of the beta-delayed neutron emitters Br-87, Br-88, and Rb-94. Phys. Rev. C, 95(2), 024320–18pp.
Abstract: We investigate the decay of Br-87,Br-88 and Rb-94 using total absorption gamma-ray spectroscopy. These important fission products are beta-delayed neutron emitters. Our data show considerable beta gamma intensity, so far unobserved in high-resolution gamma-ray spectroscopy, from states at high excitation energy. We also find significant differences with the beta intensity that can be deduced from existing measurements of the beta spectrum. We evaluate the impact of the present data on reactor decay heat using summation calculations. Although the effect is relatively small it helps to reduce the discrepancy between calculations and integral measurements of the photon component for U-235 fission at cooling times in the range 1-100 s. We also use summation calculations to evaluate the impact of present data on reactor antineutrino spectra. We find a significant effect at antineutrino energies in the range of 5 to 9 MeV. In addition, we observe an unexpected strong probability for. emission from neutron unbound states populated in the daughter nucleus. The. branching is compared to Hauser-Feshbach calculations, which allow one to explain the large value for bromine isotopes as due to nuclear structure. However the branching for Rb-94, although much smaller, hints of the need to increase the radiative width gamma by one order of magnitude. This increase in gamma would lead to a similar increase in the calculated (n, gamma) cross section for this very neutron-rich nucleus with a potential impact on r process abundance calculations.
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BRIKEN Collaboration(Tarifeño-Saldivia, A. et al), Tain, J. L., Domingo-Pardo, C., Agramunt, J., Algora, A., Morales, A. I., et al. (2017). Conceptual design of a hybrid neutron-gamma detector for study of beta-delayed neutrons at the RIB facility of RIKEN. J. Instrum., 12, P04006–22pp.
Abstract: BRIKEN is a complex detection system to be installed at the RIB-facility of the RIKEN Nishina Center. It is aimed at the detection of heavy-ion implants, β-particles, γ-rays and β-delayed neutrons. The whole detection setup involves the Advanced Implantation Detection Array (AIDA), two HPGe Clover detectors and a large set of 166 counters of 3He embedded in a high-density polyethylene matrix. This article reports on a novel methodology developed for the conceptual design and optimisation of the 3He-tubes array, aiming at the best possible performance in terms of neutron detection. The algorithm is based on a geometric representation of two selected parameters of merit, namely, average neutron detection efficiency and efficiency flatness, as a function of a reduced number of geometric variables. The response of the detection system itself, for each configuration, is obtained from a systematic MC-simulation implemented realistically in Geant4. This approach has been found to be particularly useful. On the one hand, due to the different types and large number of 3He-tubes involved and, on the other hand, due to the additional constraints introduced by the ancillary detectors for charged particles and gamma-rays. Empowered by the robustness of the algorithm, we have been able to design a versatile detection system, which can be easily re-arranged into a compact mode in order to maximize the neutron detection performance, at the cost of the gamma-ray sensitivity. In summary, we have designed a system which shows, for neutron energies up to 1(5) MeV, a rather flat and high average efficiency of 68.6%(64%) and 75.7%(71%) for the hybrid and compact modes, respectively. The performance of the BRIKEN system has been also quantified realistically by means of MC-simulations made with different neutron energy distributions.
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Orrigo, S. E. A. et al, Rubio, B., Gelletly, W., Agramunt, J., Algora, A., & Molina, F. (2016). Observation of the 2(+) isomer in Co-52. Phys. Rev. C, 94(4), 044315–8pp.
Abstract: We report the first observation of the 2(+) isomer in Co-52, produced in the beta decay of the 0(+), Ni-52 ground state. We have observed three. rays at 849, 1910, and 5185 keV characterizing the beta de-excitation of the isomer. We have measured a half-life of 102(6) ms for the isomeric state. The Fermi and Gamow-Teller transition strengths for the beta decay of Co-52m to Fe-52 have been determined. We also add new information on the beta decay of the 6(+), Co-52 ground state, for which we have measured a half-life of 112(3) ms.
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Blank, B. et al, Agramunt, J., Algora, A., Guadilla, V., Montaner-Piza, A., Morales, A. I., et al. (2016). New neutron-deficient isotopes from Kr-78 fragmentation. Phys. Rev. C, 93(6), 061301–5pp.
Abstract: In an experiment with the RIKEN projectile fragment separator called BigRIPS at the RIKEN Nishina Center, the fragmentation of a Kr-78 beam allowed the observation of new neutron-deficient isotopes at the proton drip line. Clean identification spectra could be produced and Se-63, Kr-67, and Kr-68 were identified for the first time. In addition, Ge-59 was also observed. Three of these isotopes, Ge-59, Se-63, and Kr-67, are potential candidates for ground-state two-proton radioactivity. In addition, the isotopes Ge-58, Se-62, and Kr-66 were also sought but without success. The present experiment also allowed the determination of production cross sections for some of the most exotic isotopes. These measurements confirm the trend already observed that the empirical parametrization of fragmentation cross sections, EPAX, significantly overestimates experimental cross sections in this mass region.
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Orrigo, S. E. A. et al, Rubio, B., Gelletly, W., Agramunt, J., Algora, A., & Molina, F. (2016). beta decay of the exotic T-z =-2 nuclei Fe-48, Ni-52, and Zn-56. Phys. Rev. C, 93(4), 044336–18pp.
Abstract: The results of a study of the beta decays of three proton-rich nuclei with T-z = -2, namely Fe-48, Ni-52, and Zn-56, produced in an experiment carried out at GANIL, are reported. In all three cases we have extracted the half-lives and the total beta-delayed proton emission branching ratios. We have measured the individual beta-delayed protons and beta-delayed. rays and the branching ratios of the corresponding levels. Decay schemes have been determined for the three nuclei, and new energy levels are identified in the daughter nuclei. Competition between beta-delayed protons and. rays is observed in the de-excitation of the T = 2 isobaric analog states in all three cases. Absolute Fermi and Gamow-Teller transition strengths have been determined. The mass excesses of the nuclei under study have been deduced. In addition, we discuss in detail the data analysis taking as a test case Zn-56, where the exotic beta-delayed gamma-proton decay has been observed.
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