Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 51 (2018)

Measurement of β-delayed neutron emission probabilities forprogenitors of the A = 130 r-process abundance peak
J. Liu,∗1 V. H. Phong,∗2,∗3 A. Estrade,∗4 G. Lorusso,∗2,∗5 F. Montes,∗6 T. Davinson,∗7 O. Hall,∗7 K. Matsui,∗2N. Nepal,∗4 S. Nishimura,∗2 J. Agramunt,∗8 D. S. Ahn,∗2 A. Algora,∗8 H. Baba,∗2 N. T. Brewer,∗9 C. Bruno,∗7
R. Caballero-Folch,∗10 F. Calvino,∗11 I. Dillmann,∗10 C. Domingo-Pardo,∗8 S. Go,∗12 C. J. Griﬃn,∗7R. Grzywacz,∗9,∗12 T. Isobe,∗2 D. Kahl,∗7 G. Kiss,∗2 S. Kubono,∗2 A. I. Morales,∗8 B. C. Rasco,∗9K. P. Rykaczewski,∗9 H. Sakurai,∗2 Y. Shimizu,∗2 T. Sumikama,∗2 H. Suzuki,∗2 H. Takeda,∗2 J. L. Tain,∗8A. Tarifeno-Saldivia,∗11 A. Tolosa-Delgado,∗8 P. Woods,∗7 and R. Yokoyama∗12 for the BRIKEN collaboration
The ﬁrst observation of a merger of two neutronstars,1) with both gravitational and electromagnetic
wave signals, oﬀers tantalizing opportunities to ﬁnallyidentify the astrophysical site of the r-process. The
new observations will increase the demand for precise
nuclear data necessary to reach a detailed understanding of the r-process mechanism. The r-process abundance peak around A = 130 is of particular interest
because its shape and position is very sensitive to thethe neutron-richness of the astrophysical environment,
as its formation reﬂects the break-out of the reaction
ﬂow from the N = 82 classical waiting point isotopes.
However, the eﬀect on the ﬁnal r-process abundance isobscured by a numbers of β-delayed neutron emittersalong the decay path back to stability. In fact, the βdelayed neutron emission probabilities (Pn values) in
the region south-east of 132 Sn have a most pronounced
eﬀect on the ﬁnal r-process abundance, according to
the recent sensitivity study in Ref. 2).
In June 2017, we have performed an experiment tostudy the decay properties of the β-delayed neutron
emitters in the mass region A = 130 near the doubly magic nucleus 132 Sn. These neutron-rich isotopes
were produced by the projectile fragmentation of a345 MeV/nucleon 238 U beam on a Be target, before
being puriﬁed and identiﬁed by the BigRIPS spectrometer. They were then transported through the Zerodegree spectrometer to reach the decay station locatedat the F11 focal plane. In the decay station, the activestopper array AIDA3) was placed at a central position
for the implantation of nuclei of interested, and it detected their subsequent β decay. The AIDA detector
is a stack of six 8×8 cm2 DSSDs with 128×128 pixels
each. Neutrons emitted from the β decay of ions implanted in AIDA were detected by the BRIKEN neu-Fig. 1. Particle identiﬁcation plot for isotopes transported
to the decay station at F11, indicating the mostneutron-rich isotopes for each element.

tron detector array4) consisting of 140 gas-ﬁlled 3 Hecounters, which were inside a large moderation block
made of high-density polyethylene. In addition, twoclover-type high-purity Germanium detectors were employed to measure β-delayed and isomeric γ rays.
The particle identiﬁcation plot combining data ofthe two settings of the experiment, centered at 130 Ag
and 140 Xe, is shown in Fig. 1. The data analysis isongoing. Preliminary results indicate that new or improved measurements of Pn values will be obtained forover 40 isotopes, and for 11 isotopes in the case of halflives. These new measurements would make a significant contribution to the available experimental datafor r-process models.
References∗1Department of Physics, The University of Hong Kong
RIKEN Nishina Center∗3 Faculty of Physics, VNU Hanoi University of Science∗4 Department of Physics, Central Michigan Univeristy∗5 National Physics Laboratory(NPL)∗6 National Superconducting Cyclotron Laboratory∗7 School of Physics and Astronomy, University of Edinburgh∗8 Instituto de Fisica Corpuscular, Universidad de Valencia∗9 Oak Ridge National Laboratory∗10 TRIUMF∗14 Universitat Politecnica de Catalunya∗12 Department of Physics and Astronomy, University of Tennessee1) Abbott et al., Phys. Rev. Lett. 119, 161101 (2017).
2) M. R. Mumpower, R. Surman, G. C. McLaughlin,
A. Aprahamian, Prog. Part. Nucl. Phys. 86, 86 (2016).
3) C. J. Griﬃn et al., Proc. XIII Nuclei in the Cosmos 1,97 (2014).
4) A. Tarifeño-Saldivia et al., J. Instrum. 12, 04006 (2017).

∗2- 54 -