Measuring the ß-decay Properties of Neutron-rich Exotic Pm, Sm, Eu, and Gd Isotopes to Constrain the Nucleosynthesis Yields in the Rare-earth Region

The ß-delayed neutron-emission probabilities of 28 exotic neutron-rich isotopes of Pm, Sm, Eu, and Gd were measured for the first time at RIKEN Nishina Center using the Advanced Implantation Detector Array (AIDA) and the BRIKEN neutron detector array. The existing ß-decay half-life (T1/2) database was significantly increased toward more neutron-rich isotopes, and uncertainties for previously measured values were decreased. The new data not only constrain the theoretical predictions of half-lives and ß-delayed neutron-emission probabilities, but also allow for probing the mechanisms of formation of the high-mass wing of the rare-earth peak located at A ˜ 160 in the r-process abundance distribution through astrophysical reaction network calculations. An uncertainty quantification of the calculated abundance patterns with the new data shows a reduction of the uncertainty in the rare-earth peak region. The newly introduced variance-based sensitivity analysis method offers valuable insight into the influence of important nuclear physics inputs on the calculated abundance patterns. The analysis has identified the half-lives of 168Sm and of several gadolinium isotopes as some of the key variables among the current experimental data to understand the remaining abundance uncertainty at A = 167–172.Peer ReviewedPostprint (published version

The ß-decay of 71Kr: Precise measurement of the half-life

4 pags., 6 figs. --European Nuclear Physics Conference (EuNPC 2022), Section: P2 Nuclear Structure, Spectroscopy and DynamicsThe very proton-rich 71Kr isotope was produced through the in-flight fragmentation of 78Kr on a beryllium target at RIKEN ¿ Nishina Center in order to study its ß-decay properties. A stack of double-sided silicon strip detectors, called WAS3ABi, was used as the decay station, where the detection of ion implants, ß-decays and ß-delayed protons took place. Beta-delayed ¿-rays were measured using a system of 84 HPGe detectors, called EURICA, surrounding the decay station. The main goal of the present study was the precise measurement of the half-life of 71Kr, as in the literature there is an almost 10 ¿ difference between the most precise independent results. Implant¿ß time correlations, implant¿proton time correlations and implant¿ß¿¿ time correlations were all used to derive the half-life value, followed by a thorough investigation of systematic uncertainties for each method. As these values were found to be consistent, the weighted average t1/2 = 94.40+19ms is reported as a new half-life value in this work. Furthermore a total of 26 previously unreported ¿ following the ß-decay of 71Kr were also identified in the analysis.This work was carried out at the RIBF operated by RIKEN Nishina Center and CNS, University of Tokyo. We acknowledge the EUROBALL Owners Committee for the loan of germanium detectors and the PreSpec Collaboration for the readout electronics of the cluster detectors. This work was supported by the Spanish MICINN grants FPA2014-52823-C2-1-P, FPA2017-83946-C2-1-P (MCIU/AEI/FEDER); Ministerio de Ciencia e Innovacion grant PID2019-104714GB-C21; Centro de Excelencia Severo Ochoa del IFIC SEV - 2014 - 0398; Junta para la Ampliacion de E studios ´ Programme (CSIC JAEDoc contract) co-financed by FSE, by NKFIH (NN128072), the National Research, Development, and Innovation Fund of Hungary Project No. K 128729 the STFC (UK) through Grant No. ST/P005314/, the PROMETEO/2019/007 project and by the ÚNKP-20-5-DE-02 New National Excellence Program of the Ministry of Human Capacities of Hungary and by the JSPS KAKENHI of Japan (Grant No. 25247045). G. G. Kiss acknowledges support from the János Bolyai research fellowship of the Hungarian Academy of Sciences. A. A. acknowledges partial support of the JSPS Invitational Fellowships for Research in Japan (ID: L1955) P. S. acknowledges support from MCI/AEI/FEDER,UE (Spain) under grant PGC2018-093636-BI00. F. M. acknowledges support from ANID FONDECYT Regular Project 1221364 and ANID - Millennium Science Initiative Program - ICN2019-044. Supported by the ÚNKP-22-3 New National Excellence Program of the Ministry for Culture and Innovation from the source of the National Research, Development and Innovation Fund (Project No. ÚNKP-22-3-II-DE-132)

Gamow–Teller Beta Decay and Pseudo-SU(4) Symmetry

We report on recent experimental results on (Formula presented.) decay into self-conjugate ((Formula presented.)) nuclei with mass number (Formula presented.). Super-allowed (Formula presented.) decays from the (Formula presented.) ground state of a (Formula presented.) parent nucleus are to the isobaric analogue state through so-called Fermi transitions and to (Formula presented.) states by way of Gamow–Teller (GT) transitions. The operator of the latter decay is a generator of Wigner’s SU(4) algebra and as a consequence GT transitions obey selection rules associated with this symmetry. Since SU(4) is progressively broken with increasing A, mainly as a consequence of the spin–orbit interaction, this symmetry is not relevant for the nuclei considered here. We argue, however, that the pseudo-spin–orbit splitting can be small in nuclei with (Formula presented.), in which case nuclear states exhibit an approximate pseudo-SU(4) symmetry. To test this conjecture, GT decay strength is calculated with use of a schematic Hamiltonian with pseudo-SU(4) symmetry. Some generic features of the GT (Formula presented.) decay due to pseudo-SU(4) symmetry are pointed out. The experimentally observed GT strength indicates a restoration of pseudo-SU(4) symmetry for (Formula presented.)

Configuration mixing investigation in germanium isotopes through measurement of E0 transition strengths

Experimental and theoretical studies of the germanium isotopes on the neutron-rich side of the stability valley point increasingly toward the emergence of triaxiality, configuration mixing, and shape coexistence phenomena. Studies of the E0 strengths, which can provide a direct measure of the amount of configuration mixing, are lacking. Thus, determining E0 transition strengths is essential for an understanding of the evolution of structures in the Ge isotopes. Beta-decay experiments populating excited states in the 72,74,76,78Ge isotopes were performed at the TRIUMF-ISAC radioactive beam facility. Gamma-ray and electron spectroscopic investigations have been performed, to measure E0 strengths between states of J > 0, exploiting the GRIFFIN spectrometer combined with the PACES silicon detector array. Preliminary results from this study are presented

Studying the Exotic Decay 70^{70}Kr →70\rightarrow {}^{70}Br

Beta-decay of the very neutron-deficient Kr isotope, 70Kr, was studied at RIKEN-RIBF using the EURICA cluster array. The experiment significantly increased our knowledge of the beta-decay of this isotope. Namely, 16 new γ-ray transitions were identified and the half-life was derived from time correlations of the beta particles (t iβ 1/2 = (44.99 ± 0.16) ms) and from the decay curves of the observed γ-ray transitions (t iβγ 1/2 = (45.16±0.71) ms), respectively

The β-decay of 70^{70}Kr into 70^{70}Br: Restoration of the pseudo-SU(4) symmetry

International audienceThe β-decay of the even-even nucleus 70Kr with Z=N+2, has been investigated at the Radioactive Ion Beam Factory (RIBF) of the RIKEN Nishina Center using the BigRIPS fragment separator, the ZeroDegree Spectrometer, the WAS3ABI implantation station and the EURICA HPGe cluster array. Fifteen γ-rays associated with the β-decay of 70Kr into 70Br have been identified for the first time, defining ten populated states below Eexc=3300 keV. The half-life of 70Kr was derived with increased precision and found to be t1/2=45.19±0.14 ms. The β-delayed proton emission probability has also been determined as εp=0.545(23)%. An increase in the β-strength to the yrast 1+ state in comparison with the heaviest Z=N+2 system studied so far (62Ge decay) is observed that may indicate increased np correlations in the T=0 channel. The β-decay strength deduced from the results is interpreted in terms of the proton-neutron quasiparticle random-phase approximation (pnQRPA) and also with a schematic model that includes isoscalar and isovector pairing in addition to quadrupole deformation. The application of this last model indicates an approximate realization of pseudo-SU(4) symmetry in this system