Ⅰ. HIGHLIGHTS OF THE YEARRIKEN Accel. Prog. Rep. 51 (2018)

Decay properties of neutron-rich nuclei around mass A = 100
S. Nishimura,∗1 A. Algora,∗2 V. Phong,∗3,∗1 S. Go,∗1 J. Liu,∗4,∗1 G. Lorusso,∗5,∗1 K. P. Rykaczewski,∗6J. L. Tain,∗2 R. K. Grzywacz,∗7 J. Agramunt,∗2,∗1 N. Brewer,∗6 C. Bruno,∗7 R. Caballero-Folch,∗9T. Davinson,∗8 I. Dillmann,∗9 A. Estrade,∗10 N. Fukuda,∗1 G. Kiss,∗11,∗1 K. Matsui,∗12,∗1 A. I. Morales,∗2N. Nepal,∗9 A. Tarifeño-Saldivia,∗13 A. Tolosa-Delgado,∗2 R. Yokoyama,∗7 D. S. Ahn,∗1 H. Baba,∗1F. Calvino,∗13 C. Domingo-Pardo,∗2 T. Isobe,∗1 S. Kubono,∗1 H. Sakurai,∗12,∗1 P.-A. Söderström,∗14,∗1T. Sumikama,∗1 H. Suzuki,∗1 H. Takeda,∗1 M. Wolińska-Cichocka,∗15 P. Woods,∗8 J. Wu,∗16,∗1
and the BRIKEN Collaboration
The neutron-rich nuclei around the mass A ∼ 100
are expected to have rapid changes in nuclear shapes
and possible shape coexistence. The decay properties
are expected to be sensitive to the structural changes
and are desired to be measured experimentally.1) Theβ-decay half-lives T1/2 and the delayed-neutron emission probabilities Pn in the mass A = 90 − 125 are considered as critical physics inputs in the astrophysicalrapid-neutron capture process (r process), where theunderestimation of r-process distribution in the massA = 110–125 as well as the origin of elemental deviation of Zr to Cd (Z = 40–48) in the extremely metalpoor stars are still open questions.2)
The BRIKEN project, which aims to survey several hundreds of decay properties toward the neutrondrip-line, has been launched at RIBF. The progenitors of the r-process elements in the mass A = 90–125 were produced following the projectile ﬁssion of a345 MeV/nucleon 238 U beam with primary beam intensity of above 60 pnA. Two data sets centered on
100
Br and 115 Nb isotopes in the BigRIPS separator
were collected with 2- and 2.5-days of measurement
times, respectively. The selected and identiﬁed isotopes were transported through the ZeroDegree spectrometer and implanted into two types of β-counting
systems, namely WAS3ABi Si array3) and segmentedYSO scintillation detector,4) which were positioned
in the center of the state-of-the-art neutron detector
BRIKEN at the F11 focal point. BRIKEN consistsof 140 3 He neutron proportional counters arranged ina high-density polyethylene (HDPE: 90×90×75 cm3 )matrix to achieve constant detection eﬃciency of above
65% up to neutron energies of 1 MeV.5) In addition,

Fig. 1. Particle-identification plot and the areas of isotopes
for unknown Pn and unknown T1/2 , respectively.

two high-purity Ge clover detectors were inserted fromside-holes of the HDPE and placed in closed geometry of the β-counters to maximize the γ-ray detection
eﬃciency. The experiment was conducted successfully
and the problematic backgrounds of neutrons and lightparticles were securely reduced by closing the stainlesssteel collimators prepared at the F2 focal plane.
The particle identiﬁcation plot of the accumulated
measurement is shown in Fig. 1. A careful analysisof β decay, γ rays, and neutrons in coincident measurement is in progress for one hundred isotopes to deduce T1/2 , Pn , the multi-neutron emitters (Pxn ), newisomers, and level schemes of low-lying states. Thesedecay properties are expected to give signiﬁcant feedback to nuclear theories and improve the reliability ofr-process abundance in the network calculations.

∗1RIKEN Nishina Center
Instituto de Fı́ı́ı́sica Corpuscular, CSIC-Univ. de Calencia∗3 Dept. of Nuclear Physics, VNU Hanoi Univ. of Science∗4 Dept. Physics, University of Hong Kong
∗5 National Physics Laboratory (NPL)∗6 Physics Div., Oak Ridge National Laboratory (ORNL)∗7 Dept. of Physics and Astronomy., Univ. of Tennessee∗8 School of Physics and Astronomy, Univ. of Edinburgh∗9 Dept. of Physical Sciences, TRIUMF∗10 Dept. Physics, Central Michigan Univ.
∗11 Institute for Nuclear Research, Atomki∗12 Dept. Physics, Univ. of Tokyo∗13 Univ. Politècnica de Catalunya (UPC)∗14 Technische Univ. Darmstadt∗15 Heavy Ion Lab., Univ. of Warsaw∗16 Physics Div., Argonne National Laboratory (ANL)∗2

References
1) P. Sarriguren, Phys. Rev. C 91, 044304, 1 (2015).
2) C. Sneden, J. J. Cowan, R. Gallino, Annu. Rev. Astro.
Astrophys. 46, 241 (2008).
3) S. Nishimura et al., RIKEN Accel. Prog. Rep. 46,182(2013).
4) R. Grzywacz et al., in this report.
5) A. Tarifeño-Saldivia et al., J. Instrum. 12, P04006(2017).

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