Abstract: The energy-dependent cross section of the (7)Bed(n,alpha)He-4 reaction, of interest for the so-called cosmological lithium problem in big bang nucleosynthesis, has been measured for the first time from 10 meV to 10 keV neutron energy. The challenges posed by the short half-life of Be-7 and by the low reaction cross section have been overcome at nTOF thanks to an unprecedented combination of the extremely high luminosity and good resolution of the neutron beam in the new experimental area (EAR2) of the nTOF facility at CERN, the availability of a sufficient amount of chemically pure Be-7, and a specifically designed experimental setup. Coincidences between the two alpha particles have been recorded in two Si-Be-7-Si arrays placed directly in the neutron beam. The present results are consistent, at thermal neutron energy, with the only previous measurement performed in the 1960s at a nuclear reactor. The energy dependence reported here clearly indicates the inadequacy of the cross section estimates currently used in BBN calculations. Although new measurements at higher neutron energy may still be needed, the n_TOF results hint at a minor role of this reaction in BBN, leaving the long-standing cosmological lithium problem unsolved.

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.

Abstract: In this work we report on the Monte Carlo study performed to understand and reproduce experimental measurements of a new plastic beta-detector with cylindrical geometry. Since energy deposition simulations differ from the experimental measurements for such a geometry, we show how the simulation of production and transport of optical photons does allow one to obtain the shapes of the experimental spectra. Moreover, taking into account the computational effort associated with this kind of simulation, we develop a method to convert the simulations of energy deposited into light collected, depending only on the interaction point in the detector. This method represents a useful solution when extensive simulations have to be done, as in the case of the calculation of the response function of the spectrometer in a total absorption gamma-ray spectroscopy analysis.

Abstract: Background: There have been measurements on roughly 230 nuclei that are beta-delayed neutron emitters. They range from He-8 up to La-150. Apart from 210Tl, with a branching ratio of only 0.007%, no other neutron emitter has been measured beyond A = 150. Therefore, new data are needed, particularly in the region of heavy nuclei around N = 126, in order to guide theoretical models and help understand the formation of the third r-process peak at A similar to 195. Purpose: To measure both beta-decay half-lives and neutron branching ratios of several neutron-rich Au, Hg, Tl, Pb, and Bi isotopes beyond N = 126. Method: Ions of interest were produced by fragmentation of a U-238 beam, selected and identified via the GSI-FRS fragment separator. A stack of segmented silicon detectors (SIMBA) was used to measure ion implants and beta decays. An array of 30 He-3 tubes embedded in a polyethylene matrix (BELEN) was used to detect neutrons with high efficiency and selectivity. A self-triggered digital system is employed to acquire data and to enable time correlations. The latter were analyzed with an analytical model and results for the half-lives and neutron-branching ratios were derived by using the binned maximum-likelihood method. Results: Twenty new beta-decay half-lives are reported for Au204-206, Hg208-211, Tl211-216, Pb215-218, and Bi218-220, nine of them for the first time. Neutron emission probabilities are reported for Hg-210,Hg-211 and Tl211-216. Conclusions: The new beta-decay half-lives are in good agreement with previous measurements on nuclei in this region. The measured neutron emission probabilities are comparable to or smaller than values predicted by global models such as relativistic Hartree Bogoliubov plus the relativistic quasi-particle random phase approximation (RHB + RQRPA).

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.