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Jungclaus, A., Doornenbal, P., Acosta, J., Vaquero, V., Browne, F., Cortes, M. L., et al. (2024). Position of the single-particle 3/2- state in 135Sn and the N = 90 subshell closure. Phys. Lett. B, 851, 138561–5pp.
Abstract: The decay of excited states of the nucleus Sn-135, with three neutrons outside the doubly-magic Sn-132 core, was studied in an experiment performed at the Radioactive Isotope Beam Factory at RIKEN. Several gamma rays emitted from excited Sn-135 ions were observed following one-neutron and one-neutron-one-proton removal from Sn-136 and Sb-137 beams, respectively, on a beryllium target at relativistic energies. Based on the analogy to 133Sn populated via one-neutron removal from Sn-134, an excitation energy of 695(15) keV is assigned to the 3/2(-) state with strongest single-particle character in 135Sn. This result provides the first direct information about the evolution of the neutron shell structure beyond N = 82 and thus allows for a crucial test of shellmodel calculations in this region. The experimental findings are in full agreement with calculations performed employing microscopic effective two-body interactions derived from CD-Bonn and N3LO nucleon-nucleon potentials, which do not predict a pronounced subshell gap at neutron number N=90. The occurrence of such a gap in Sn-140, i.e., when the 1f(7/2) orbital is completely filled, had been proposed in the past, in analogy to the magicity of Ca-48, featuring a completely filled 0f(7/2) orbital one harmonic oscillator shell below.
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Vitez-Sveiczer, A. et al, Algora, A., Morales, A. I., Rubio, B., Agramunt, J., Guadilla, V., et al. (2022). The beta-decay of Kr-70 into Br-70: Restoration of the pseudo-SU(4) symmetry. Phys. Lett. B, 830, 137123–8pp.
Abstract: The beta-decay of the even-even nucleus Kr-70 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 gamma-rays associated with the beta-decay of( 70)Kr into Br-70 have been identified for the first time, defining ten populated states below E-exc=3300 keV. The half-life of Kr-70 was derived with increased precision and found to be t(1/2)=45.19 +/- 0.14 ms. The beta-delayed proton emission probability has also been determined as epsilon(p)=0.545(23)%. An increase in the beta-strength to the yrast 1(+) state in comparison with the heaviest Z=N+2 system studied so far (Ge-62 decay) is observed that may indicate increased np correlations in the T=0 channel. The beta-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.
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Algora, A., Ganioglu, E., Sarriguren, P., Guadilla, V., Fraile, L. M., Nacher, E., et al. (2021). Total absorption gamma-ray spectroscopy study of the beta-decay of Hg-186. Phys. Lett. B, 819, 136438–7pp.
Abstract: The Gamow-Teller strength distribution of the decay of Hg-186 into Au-186 has been determined for the first time using the total absorption gamma spectroscopy technique and has been compared with theoretical QRPA calculations using the SLy4 Skyrme force. The measured Gamow-Teller strength distribution and the half-life are described by mixing oblate and prolate configurations independently in the parent and daughter nuclei. In this theoretical framework the best description of the experimental beta strength is obtained with dominantly prolate components for both parent Hg-186 and daughter Au-186. The approach also allowed us to determine an upper limit of the oblate component in the parent state. The complexity of the analysis required the development of a new approach in the analysis of the X-ray gated total absorption spectrum.
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Hall, O. et al, Agramunt, J., Algora, A., Domingo-Pardo, C., Morales, A. I., Rubio, B., et al. (2021). beta-delayed neutron emission of r-process nuclei at the N=82 shell closure. Phys. Lett. B, 816, 136266–7pp.
Abstract: Theoretical models of beta-delayed neutron emission are used as crucial inputs in r-process calculations. Benchmarking the predictions of these models is a challenge due to a lack of currently available experimental data. In this work the beta-delayed neutron emission probabilities of 33 nuclides in the important mass regions south and south-west of Sn-132 are presented, 16 for the first time. The measurements were performed at RIKEN using the Advanced Implantation Detector Array (AIDA) and the BRIKEN neutron detector array. The P-1n values presented constrain the predictions of theoretical models in the region, affecting the final abundance distribution of the second r-process peak at A approximate to 130.
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Agramunt, J. et al, Tain, J. L., Albiol, F., Algora, A., Domingo-Pardo, C., Jordan, M. D., et al. (2016). Characterization of a neutron-beta counting system with beta-delayed neutron emitters. Nucl. Instrum. Methods Phys. Res. A, 807, 69–78.
Abstract: A new detection system for the measurement of beta-delayed neutron emission probabilities has been characterized using fission products with well known beta-delayed neutron emission properties. The setup consists of BELEN-20, a 4 pi-neutron counter with twenty He-3 proportional tubes arranged inside a large polyethylene neutron moderator, a thin Si detector for beta counting and a self-triggering digital data acquisition system. The use of delayed-neutron precursors with different neutron emission windows allowed the study of the effect of energy dependency on neutron, beta and beta-neutron rates. The observed effect is well reproduced by Monte Carlo simulations. The impact of this dependency on the accuracy of neutron emission probabilities is discussed. A new accurate value of the neutron emission probability for the important delayed-neutron precursor I-137 was obtained, P-n = 7.76(14)%.
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