Commissioning of the BRIKEN beta-delayed neutron detector for the study of exotic neutron-rich nuclei

published_or_final_versio

Search for a decay of Te-104 with a novel recoil-decay scintillation detector

A search for superallowed α decay of N=Z nuclei Te104 and Xe108 was carried out using a novel recoil-decay scintillator detector at the tandem accelerator facility at the Japan Atomic Energy Agency (JAEA). Inorganic crystal scintillation material YAP:Ce (yttrium aluminum perovskite) coupled to a position-sensitive photomultiplier tube (PSPMT) was implemented for the first time in a radioactive decay experiment. Residues from the fusion-evaporation reaction Ni58+Fe54→Xe∗112 were separated by the JAEA Recoil Mass Separator (RMS) and implanted into the YAP:Ce crystal. α decays of neutron-deficient tellurium isotopes were identified and proton emission of I109 was observed. The α-decay chain Xe109→Te105→Sn101 was recorded with a time interval of 960 ns between two α pulses. Position localization in the crystal for decays and ions in the energy range from hundreds of keV to 60 MeV was achieved with an accuracy of 0.67 mm, proving that this detector is capable of making temporal and spatial correlations for fast decay events. No conclusive evidence was found for the decay chain Xe108→Te104→Sn100 within 3 days of experiment. However, two events were observed with properties consistent with the reported observation at the Fragment Mass Analyzer (FMA), but with a separation between signals of less than 4 ns. The cross section limit of 130 pb was obtained for production of two events of Xe108, about an order of magnitude below the expectation based on earlier cross section measurements and the hivap fusion-evaporation code

First beta-decay spectroscopy of In-135 and new beta-decay branches of In-134

The beta decay of the neutron-rich In-134 and In-135 was investigated experimentally in order to provide new insights into the nuclear structure of the tin isotopes with magic proton number Z = 50 above the N = 82 shell. The beta-delayed gamma-ray spectroscopy measurement was performed at the ISOLDE facility at CERN, where indium isotopes were selectively laser-ionized and on-line mass separated. Three beta-decay branches of In-134 were established, two of which were observed for the first time. Population of neutron-unbound states decaying via. rays was identified in the two daughter nuclei of In-134, Sn-134 and Sn-133, at excitation energies exceeding the neutron separation energy by 1 MeV. The beta-delayed one- and two-neutron emission branching ratios of In-134 were determined and compared with theoretical calculations. The beta-delayed one-neutron decay was observed to be dominant beta-decay branch of In-134 even though the Gamow-Teller resonance is located substantially above the two-neutron separation energy of Sn-134. Transitions following the beta decay of In-135 are reported for the first time, including. rays tentatively attributed to Sn-135. In total, six new levels were identified in Sn-134 on the basis of the beta.. coincidences observed in the In-134 and In-135 beta decays. A transition that might be a candidate for deexciting the missing neutron single-particle 13/2(+) state in Sn-133 was observed in both beta decays and its assignment is discussed. Experimental level schemes of Sn-134 and Sn-135 are compared with shell-model predictions. Using the fast timing technique, half-lives of the 2(+), 4(+), and 6(+) levels in Sn-134 were determined. From the lifetime of the 4(+) state measured for the first time, an unexpectedly large B(E2; 4(+)-> 2(+)) transition strength was deduced, which is not reproduced by the shell-model calculations.Peer reviewe

Conceptual design of a hybrid neutron-gamma detector for study of beta-delayed neutrons at the RIB facility of RIKEN

The conceptual design of the BRIKEN neutron detector at the radioactive ion beam factory (RIBF) of the RIKEN Nishina Center is reported. The BRIKEN setup is a complex system aimed at detecting heavy-ion implants, β particles, γ rays and β-delayed neutrons. The whole setup includes the Advanced Implantation Detection Array (AIDA), two HPGe Clover detectors and up to 166 3He-filled counters embedded in a high-density polyethylene moderator. The design is quite complex due to the large number and different types of 3He-tubes involved and the additional constraints introduced by the ancillary detectors for charged particles and γ rays. This article reports on a novel methodology developed for the conceptual design and optimisation of the 3He-counter array, aiming for the best possible performance in terms of neutron detection. The algorithm is based on a geometric representation of two selected detector parameters of merit, namely, the average neutron detection efficiency and the efficiency flatness as a function of a reduced number of geometric variables. The response of the neutron detector is obtained from a systematic Monte Carlo simulation implemented in Geant4. The robustness of the algorithm allowed us to design a versatile detection system, which operated in hybrid mode includes the full neutron counter and two clover detectors for high-precision gamma spectroscopy. In addition, the system can be reconfigured into a compact mode by removing the clover detectors and re-arranging the 3He tubes in order to maximize the neutron detection performance. Both operation modes shows a rather flat and high average efficiency. In summary, we have designed a system which shows an average efficiency for hybrid mode (3He tubes + clovers) of 68.6% and 64% for neutron energies up to 1 and 5 MeV, respectively. For compact mode (only 3He tubes), the average efficiency is 75.7% and 71% for neutron energies up to 1 and 5 MeV, respectively. The performance of the BRIKEN detection system has been also quantified by means of Monte Carlo simulations with different neutron energy distributions

Observation of a mu s isomer in In-134(49)85: Proton-neutron coupling "southeast" of Sn-132(50)82

We report on the observation of a microsecond isomeric state in the single-proton-hole, three-neutron-particle nucleus ¹³⁴In. The nuclei of interest were produced by in-flight fission of a ²³⁸U beam at the Radioactive Isotope Beam Factory at RIKEN. The isomer depopulates through a γ ray of energy 56.7(1) keV and with a half-life of T1/2=3.5(4)μs. Based on the comparison with shell-model calculations, we interpret the isomer as the Iπ=5− member of the π0g−19/2⊗ν1f37/2 multiplet, decaying to the Iπ=7− ground state with a reduced-transition probability of B(E2;5−→7−)=0.53(6)W.u.Observation of this isomer, and lack of evidence in the current work for a Iπ=5− isomer decay in ¹³²In, provides a benchmark of the proton-neutron interaction in the region of the nuclear chart “southeast” of ¹³²Sn, where experimental information on excited states is sparse

THE BRIKEN PROJECT: EXTENSIVE MEASUREMENTS OF beta-DELAYED NEUTRON EMITTERS FOR THE ASTROPHYSICAL r PROCESS

An ambitious program to measure decay properties, primarily β-delayed neutron emission probabilities and half-lives, for a significant number of nuclei near or on the path of the rapid neutron capture process, has been launched at the RIKEN Nishina Center. We give here an overview of the status of the project

β-Delayed One and Two Neutron Emission Probabilities Southeast of ^{132}Sn and the Odd-Even Systematics in r-Process Nuclide Abundances.

The β-delayed one- and two-neutron emission probabilities (P_{1n} and P_{2n}) of 20 neutron-rich nuclei with N≥82 have been measured at the RIBF facility of the RIKEN Nishina Center. P_{1n} of ^{130,131}Ag, ^{133,134}Cd, ^{135,136}In, and ^{138,139}Sn were determined for the first time, and stringent upper limits were placed on P_{2n} for nearly all cases. β-delayed two-neutron emission (β2n) was unambiguously identified in ^{133}Cd and ^{135,136}In, and their P_{2n} were measured. Weak β2n was also detected from ^{137,138}Sn. Our results highlight the effect of the N=82 and Z=50 shell closures on β-delayed neutron emission probability and provide stringent benchmarks for newly developed macroscopic-microscopic and self-consistent global models with the inclusion of a statistical treatment of neutron and γ emission. The impact of our measurements on r-process nucleosynthesis was studied in a neutron star merger scenario. Our P_{1n} and P_{2n} have a direct impact on the odd-even staggering of the final abundance, improving the agreement between calculated and observed Solar System abundances. The odd isotope fraction of Ba in r-process-enhanced (r-II) stars is also better reproduced using our new data

<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>β</mml:mi></mml:math> -delayed neutron emissions from <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>N</mml:mi><mml:mo>&gt;</mml:mo><mml:mn>50</mml:mn></mml:mrow></mml:math> gallium isotopes

β-delayed γ-neutron spectroscopy has been performed on the decay of A=84 to 87 gallium isotopes at the RI-beam Factory at the RIKEN Nishina Center using a high-efficiency array of He3 neutron counters (BRIKEN). β-2n-γ events were measured in the decays of all of the four isotopes for the first time, which is direct evidence for populating the excited states of two-neutron daughter nuclei. Detailed decay schemes with the γ branching ratios were obtained for these isotopes, and the neutron emission probabilities (Pxn) were updated from the previous study. Hauser-Feshbach statistical model calculations were performed to understand the experimental branching ratios. We found that the P1n and P2n values are sensitive to the nuclear level densities of 1n daughter nuclei and showed that the statistical model reproduced the P2n/P1n ratio better when experimental levels plus shell-model level densities fit by the Gilbert-Cameron formula were used as the level-density input. We also showed the neutron and γ branching ratios are sensitive to the ground-state spin of the parent nucleus. Our statistical model analysis suggested J≤3 for the unknown ground-state spin of the odd-odd nucleus Ga86, from the Iγ(4+→2+)/Iγ(2+→0+) ratio of Ga84 and the P2n/P1n ratio. These results show the necessity of detailed understanding of the decay scheme, including data from neutron spectroscopy, in addition to γ measurements of the multineutron emitters