RIKEN Accel. Prog. Rep. 50 (2017)

Ⅰ. HIGHLIGHTS OF THE YEAR

94 β-decay half-lives of neutron-rich nuclei from 55 Cs to 67 Ho†
J. Wu,∗1,∗2 S. Nishimura,∗2 G. Lorusso,∗2,∗3,∗4 P. Möller,∗5 E. Ideguchi,∗6 P.-H. Regan,∗3,∗4G.S. Simpson,∗7,∗8,∗9 P.-A. Söderström,∗2 P.M. Walker,∗4 H. Watanabe,∗10,∗2 Z.Y. Xu,∗11,∗12 D.S. Ahn,∗2H. Baba,∗2 F. Browne,∗13,∗2 R. Daido,∗14 P. Doornenbal,∗2 Y.F. Fang,∗14 N. Fukuda,∗2 G. Gey,∗7,∗15,∗2N. Inabe,∗2 T. Isobe,∗2 D. Kameda,∗2 K. Korkulu,∗17 T. Kudo,∗2 P.S. Lee,∗16 J.J. Liu,∗11 Z. Li,∗1 D. Murai,∗2Z. Patel,∗4,∗2 V. Phong,∗18,∗2 S. Rice,∗4,∗2 H. Sakurai,∗2,∗12 Y. Shimizu,∗2 L. Sinclair,∗19,∗2 T. Sumikama,∗2H. Suzuki,∗2 H. Takeda,∗2 M. Tanaka,∗6 A. Yagi,∗14 Y.L. Ye,∗1 R. Yokoyama,∗20 G.X. Zhang,∗10 on behalf of theRIBF-86 and RIBF-88 collaborations
The two most prominent features of the r-process
abundances in the solar system are the large abundance of 52 Te, 54 Xe (mass number A∼130) and 78 Pt,
79 Au (A∼195), which are understood in terms of theenhanced stability of nuclei with ﬁlled major neutronshells (of neutron number N =82 and N =126). However, the production mechanism of the peak of rareearth elements (REE) (A∼165) is still a controversial
topic.1)
To address this problem, two β-decay spectroscopy
experiments optimized for the transmission of 158 Nd
and 170 Dy were performed at the Radioactive Isotope
Beam Factory (RIBF) by using the in-ﬂight ﬁssion ofa 345 MeV/A 238 U primary beam with an average intensity of 7 and 12 pnA, respectively. The secondary
beam selected and identiﬁed by BigRIPS separator wasimplanted to the beta-counting system of the Widerange Active Silicon-Strip Stopper Array for Beta andion detection (WAS3ABi), which is surrounded by thehigh purity germanium cluster detectors of the Euroball RIken Cluster Array (EURICA). The β-decay
half-life of an isotope of interest was extracted fromthe ﬁt of the time distribution of electrons detected after the implantation of an ion and correlated to themin position and time2) .
In this measurement, 57 β-decay half-lives ofneutron-rich nuclei from 55 Cs to 67 Ho were measured

for the ﬁrst time with 94 half-lives measured in total. The impact of newly measured β-decay halflives on the shape of the REE peak is illustrated inFig. 1. The ﬁgure shows the theoretical uncertainty
estimated for each model, determined by varying theoretical half-lives within a factor of two. The study
indicates that the new measurements remove a significant uncertainty in the calculations associated withtheoretical half-lives.

Fig. 1. (color online). The r-process abundance patternobserved in the solar system (open circles)3) and calculated using the experimental and theoretical half-lives.
KTUY054) mass and the ReaclibV1.05) Database of nuclear reaction rates were employed for the baseline calculation. Experimental data and three theoretical predictions replaced the half-lives of nuclei whose values
were measured for the ﬁrst time.

†Condensed from the article in J. Wu et al, Phys. Rev. Lett.
118, 072701 (2017).
∗1 School of Physics, Peking University∗2 RIKEN Nishina Center∗3 National Physical Laboratory∗4 Department of Physics, University of Surrey∗5 Los Alamos National Laboratory∗6 RCNP, Osaka University∗7 LPSC, Universite Joseph Fourier Grenoble 1∗8 School of Engineering, University of the West of Scotland∗9 Physics Alliance, University of Glasgow∗10 IRCNPC, School of Physics and Nuclear Energy Engineering, Beihang University∗11 Department of Physics, the University of Hong Kong
∗12 Department of Physics, University of Tokyo∗13 School of Computing Engineering and Mathematics, University of Brighton∗14 Department of Physics, Osaka University∗15 Institut Laue-Langevin∗16 Department of Physics, Chung-Ang University∗17 Hungarian Academy of Sciences∗18 Faculty of Physics, VNU Hanoi University of Science∗19 Department of Physics, University of York
∗20 CNS, University of Tokyo

In summary, the newly measured half-lives have a direct impact in r-process abundance calculations aﬀecting almost all mass numbers in the range A=150−170.
This is an important step towards the long-term goalof removing nuclear-physics uncertainties so that theREE peak can be used as a unique probe of the rprocess freeze-out conditions and eventually reveal thecurrently unknown r-process site.
References1) G.J. Mathews, J.J. Cowan, Nature 345, 7 (1990).
2) Z.Y. Xu, PhD Thesis, University of Tokyo (2011).
3) S. Goriely, Astron. Astrophys. 342, 881 (1999).
4) http://wwwndc.jaea.go.jp/nucldata/mass/KTUY04 E.html
5) http://groups.nscl.msu.edu/jina/reaclib/db-2-