II-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

BRIKEN measurements of Pn -values and half-lives for understanding
the formation of the r-process rare-earth peak: progress on the Ce to
Nd region
A. Tarifeño-Saldivia,∗1 G. G. Kiss,∗2 J. L. Tain,∗3 A. Estrade,∗4 S. Nishimura,∗5 J. Agramunt,∗3 A. Algora,∗3
N. T. Brewer,∗6 R. Caballero-Folch,∗7 F. Calvino,∗1 T. Davinson,∗8 I. Dillmann,∗7 N. Fukuda,∗5R. K. Grzywacz,∗9 O. Hall,∗6 N. Mont-Geli,∗1 A. I. Morales,∗3 A. Navarro,∗1 N. Nepal,∗5 M. Pallàs,∗1B. C. Rasco,∗6 K. P. Rykaczewski,∗6 N. T. Szegedi,∗2 A. Vitz-Sveiczer,∗2 A. Tolosa-Delgado,∗3 P. Vi,∗10
R. Yokoyama,∗9 M. Wolinska-Cichocka,∗11 and P. Woods∗8 for the BRIKEN Collaboration∗12
The Rare-Earth Peak (REP) is a distinctive local
maximum observed around mass A ∼ 160 in the elemental abundance distribution of the rapid-neutron capture process (r-process). Because the REP is formed after neutron exhaustion,1) it provides a unique probe forstudying the late-time environmental conditions of the rprocess site.2) According to theoretical models, β-decayrates (T1/2 ) and delayed neutron emission probabilities(Pn -values) play important roles in the formation of theREP.3) The region of nuclei with the most significant
impact on the formation of the REP has been determined via sensitivity studies.4) Most of the T1/2 on thisregion have already been measured by the EURICA collaboration.7) However, the experimental determinationof Pn -values is yet to be achieved.
The NP1612-RIBF148 experiment exploits the uniquecapabities of the BRIKEN setup5,6) for the measurement of β-delayed neutrons. This experiment attemps tostudy Pn -values and T1/2 for the nuclei which are important to REP formation.4) In the 2018 experimental run,a 60-particle-nA 238 U beam, with 345 MeV/nucleon, hitting a 4 mm thick Be target was used to produce the secondary radioactive beam. The neutron-rich fragments
were filtered out by the BigRIPS fragment separator
and the ZeroDegree spectrometer. The beam setting
was centered on 165 Pm.
Here, we report preliminary results from the 2018 experimental run. Figure 1 shows the measured beta-decay
half-lives in the region from Ce to Nd isotopes. The experimental results are compared with previous measurements performed by the EURICA collaboration,7) andwith FRDM + QRPA theoretical calculations.8,9) TheBRIKEN experimental data agree well with the previous measurements. In addition, our results exhibit animproved precision for the heavier nuclei region. Basedon the current status of the data analysis for this experiment, we expect to obtain at least one new T1/2 per
atomic number on the heavier isotopes from the Ce toNd region. These results will be reported in the future.

∗1References

Half-life (s)

158Ce

EL0.1

PR

Half-life (s)1IM0.1

0.01

59Pr

AR

BRIKEN / this work
EURICA / Wu et al 2017
FRDM+QRPA (2003)
FRDM+QRPA (2019)

Y

Half-life (s)10.1

BRIKEN / this work
EURICA / Wu et al 2017
FRDM+QRPA (2003)
FRDM+QRPA (2019)

IN

Universitat Politècnica de Catalunya (UPC)
Institute for Nuclear Research (Atomki)∗3 Instituto de Física Corpuscular (IFIC)∗4 University College of Science and Engineering, CMU∗5 RIKEN Nishina Center∗6 Physics Division, 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∗2

BRIKEN / this work
EURICA / Wu et al 2017
FRDM+QRPA (2003)
FRDM+QRPA (2019)

60Nd9496

98
100
Neutron number N

102

104

Fig. 1. Systematic trends in BRIKEN results of β-decay
half-lifes (blue) compared with previous measurements(red),7) and theoretical calculations.8,9)1) R. Surman et al., Phys. Rev. Lett. 79, 1809 (1997).
2) M. R. Mumpower et al., Astrophys. J. 752, 117 (2012).
3) A. Arcones et al., Phys. Rev. C 83, 045809 (2011).
4) M. R. Mumpower et al., Phys. Rev. C 85, 04580 (2012).
5) I. Dillmann et al., Nucl. Phys. News 28, 28 (2018).
6) A. Tolosa-Delgado et al., Nucl. Instrum. Methods Phys.
Res. A 925, 133 (2019).
7) J. Wu et al., Phys. Rev. Lett. 118, 072701 (2017).
8) P. Möller et al., Phys. Rev. C 67, 055802 (2003).
9) P. Möller et al., At. Data Nucl. Data Tables 125, 1 (2019).

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