Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 53

β-delayed neutron emission probabilities for understanding theformation of the r-process rare-earth abundance peak (REP)
G. G. Kiss,∗1 A. Tarifeño-Saldivia,∗2 J. L. Tain,∗3 A. Estrade,∗4 S. Nishimura,∗5 J. Agramunt,∗3 A. Algora,∗3N. T. Brewer,∗6 R. Caballero-Folch,∗7 F. Calvino,∗2 T. Davinson,∗8 I. Dillmann,∗7 N. Fukuda,∗5R. K. Grzywacz,∗9 O. Hall,∗6 A. I. Morales,∗3 A. Navarro,∗2 N. Nepal,∗5 B. C. Rasco,∗6 K. P. Rykaczewski,∗6N. T. Szegedi,∗1 A. Vitéz-Sveiczer,∗1 A. Tolosa-Delgado,∗3 P. Vi,∗10 R. Yokoyama,∗9 M. Wolinska-Cichocka,∗11and P. Woods∗8 for the BRIKEN collaboration∗12
The abundance distribution of the rapid-neutron
capture (so-called r-) process is characterized by twolarge maxima at masses of A ∼ 130 and A ∼ 195,
which are related to the ﬂow of matter through theneutron shell closures at N = 82 and N = 126.
However, there is an additional, relatively small—butdistinct—peak around A ∼ 160, which corresponds
to the region of the rare-earth elements. In contrast to the main abundance maxima that form during
the (n, γ) ↔ (γ, n) equilibrium, the rare-earth abundance peak (REP) originates later, after neutron exhaustion, thus representing a unique opportunity to
study the late-time environmental conditions of the rprocess.1–3) Several diﬀerent peak-production mechanisms were suggested, but experimental data—masses,β-decay parameters, and neutron capture rates—areclearly needed to evaluate the diﬀerent astrophysicalscenarios. The most inﬂuential nuclei to the REP formation, located in the A ∼ 160, 55 ≤ Z ≤ 64 neutronrich region, have been identiﬁed by sensitivity studies.3)
The aim of the NP1612-RIBF148 experimental program is to measure the β-decay parameters, halflives, and delayed-neutron-emission probabilities (Pnvalues) of these species using the BRIKEN array,
which is the largest and most eﬃcient β-delayed neutron detector built.4,5) It consists of 140 3 He gasﬁlled proportional counters embedded in a high-density
polyethylene moderator. The neutron detector andtwo CLARION-type clover high purity germanium detectors are placed around the AIDA DSSSD array6)
which contains six layers of highly segmented Si detectors for the detection of implantations and β electrons.
The study was conducted at the Radioactive Isotope Beam Factory. A 60-pnA intensity 238 U beam

Fig. 1. Preliminary half-life analysis for 163 Pm isotopes,
showing the contributions to the total ﬁt from the parent, daughter, and granddaughter isotopes. Our T1/2
result is compared to the existing data7) (bold andslanted characters, respectively).

was accelerated up to an energy of 345 MeV/nucleon
before incidence on a 4-mm thick Be target to produce
radioactive secondary beams by in-ﬂight ﬁssion. The
nuclei of interest were separated and identiﬁed in theBigRIPS spectrometer, transported through the ZeroDregree spectrometer, and implanted in the AIDA
array. Figure 1 shows the results of the preliminary
half-life analysis of the 163 Pm isotope. Although somehalf-lives in this region have already been already measured7) our experiment will not only provide a large
number of new Pn values8) and half-lives but also considerably improve the precision of the available data.
References

1) R. Surman et al., Phys. Rev. Lett. 79, 1809 (1997).
2) A. Arcones, G. Martinez Pinedo, Phys. Rev. C 83,045809 (2011).
3) R. M. Mumpower et al., Prog. Part. Nucl. Phys. 86, 86(2016).
4) A. Tarifeno-Saldivia et al., J. Instrum. 12, P04006 (2016).
5) A. Tolosa-Delgado et al., Nucl. Instrum. Methods Phys.
Res. 925, 133 (2019).
6) C. Griﬃn et al., Proc. Sci. NIC-XIII, 097 (2014).
7) J. Wu et al., Phys. Rev. Lett. 118, 072701 (2017).
8) A. Tarifeño-Saldivia et al., RIKEN Accel. Prog. Rep.
52, 39 (2019).

∗1Institute for Nuclear Research (Atomki)∗2 Universitat Politècnica de Catalunya (UPC)∗3 Instituto de Física Corpuscular (IFIC)∗4 University College of Science and Engineering, CentralMichigan University (CMU)∗5 RIKEN Nishina Center∗6 Oak Ridge National Laboratory∗7 Physical Sciences Division, TRIUMF∗8 School of Physics and Astronomy, University of Edinburgh∗9 Department of Physics and Astronomy, University of Tennessee∗10 Faculty of Physics, VNU University of Science∗11 HIL, University of Warsaw∗12 www.wiki.edu.ac.uk/display/BRIKEN/Home- 33 -