Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 51 (2018)

β-delayed neutron emission probabilities for understanding theformation of the r-process rare-earth peakA. Tarifeño-Saldivia,∗1 G. G. Kiss,∗2,∗3 A. I. Morales,∗4 A. Estrade,∗5 S. Nishimura,∗2 C. Domingo-Pardo,∗4A. Agramunt,∗4 D. S. Ahn,∗2 A. Algora,∗3,∗4 H. Baba,∗2 N. T. Brewer,∗6 R. Caballero Folch,∗7 F. Calvino,∗1T. Davinson,∗8 I. Dillmann,∗7 N. Fukuda,∗2 S. Go,∗2 R. K. Grzywacz,∗9 O. Hall,∗8 L. Harkness-Brennan,∗8T. Isobe,∗2 C. J. Griﬃn,∗8 D. Kahl,∗8 A. Korgul,∗10 S. Kubono,∗2 M. Labiche,∗11 J. Liu,∗2 K. Matsui,∗2K. Miernik,∗10 N. Nepal,∗5 B. C. Rasco,∗6 K. P. Rykaczewski,∗6 Y. Saito,∗7 H. Sakurai,∗2 Y. Shimizu,∗2D.W. Stracener,∗6 T. Sumikama,∗2 H. Suzuki,∗2 J. L. Tain,∗4 H. Takeda,∗2 A. Tolosa,∗4 P. Vi,∗2 P. J. Woods,∗8and R. Yokoyama∗9The main signatures of the rapid-neutron capture process (so-called r-process) are the three large abundance
maxima in the solar-system composition at masses ofA∼80, 130, and 195, which are related to the ﬂow ofmatter through the three neutron shell closures at N=50,82, and 126. In contrast to these three characteristic peaks, there is a small—but distinct—peak aroundA∼160, that corresponds to the region of rare-earth elements. From the astrophysics point of view, the mostinteresting feature of the rare-earth peak (REP) is that,
contrary to the three main maxima that form during (n,γ) ↔ (γ, n) equilibrium, the REP originates later, afterneutron exhaustion, thus representing a unique opportunity to study the late-time environmental conditions.
Although several diﬀerent production mechanisms
were suggested, most models agree on the fact thatβ-delayed neutron emission plays a crucial role in theREP formation.1–3) Recently, the largest and most efﬁcient β-delayed neutron detector4) was built by theBRIKEN collaboration at the RIKEN Nishina Center
to study systematically the decay of very neutron-richnuclei across the nuclear chart. The so-called BRIKEN
neutron detector consists of 140 3 He gas-ﬁlled proportional counters embedded in a high-density polyethylene
moderator. The neutron detector and two CLARIONtype clover HPGe detectors are placed around the AIDADSSSD array,5) which contains six layers of highly segmented Si detectors for the detection of implantations
and beta electrons.
The NP1612-RIBF148 experiment is focused on themeasurement of the β-delayed neutron emitters, which
are critical for the formation of the REP in the solar
system abundances. In an exploratory run (∼25% total beamtime), performed in June 2017, the β decays of
exotic Pr-Gd species were studied at the Radioactive Isotope Beam Factory. A 50-pnA-intensity 238 U beam was∗1Universitat Politècnica de Catalunya (UPC)
RIKEN Nishina Center∗3 Institute for Nuclear Research (Atomki)∗4 Instituto de Fı́sica Corpuscular∗5 College of Science and Engineering, Central Michigan University∗6 Oak Ridge National Laboratory∗7 TRIUMF∗8 School of Physics and Astronomy, University of Edinburgh∗9 Department of Physics and Astronomy, University of Tennessee∗10 Department of Physics, University of Warsaw∗11 STFC Daresbury Laboratory∗2- 55 -＊12Department of Physics, University of Hong KongFig. 1. PID of the exploratory run for the NP1612-RIBF148
experiment.

accelerated up to an energy of 345 MeV/nucleon before
it was incident 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 (top) shows the particle identiﬁcation matrix
for the statistics accumulated during the ﬁrst 24 h. The
bottom panel of Fig. 1 shows the projection of the PID
matrix on the A/Q axis for the Nd isotopes; the A/Q
resolution achieved—less than 0.05%—ensures a good
separation of fully- stripped and H-like ions. The results of the exploratory run are promising; for the ﬁrst
time ever, key REP progenitors3) have become accessible and the study of their beta decay properties will bepossible thanks to the use of the state-of-the-art detectors BRIKEN and AIDA. Our intention is to continue
the experiment as soon as the 238 U beam is available
again.
References
1) R. M. Mumpower, R. Surman, G. C. McLaughlin,
A. Aprahamian, Prog. Part. Nucl. Phys. 86, 86 (2016).
2) A. Arcones, G. Martinez Pinedo, Phys. Rev. C 83, 045809(2011).
3) M. Mumpower et al., Prog. Part. Nucl. Phys. 86, 86(2016).
4) A. Tarifeno-Saldivia et al., Jour. of Instr. 12, P04006(2016).
5) C. Griﬃn et al., POS (NIC-XIII) 097 (2014).