Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 47 (2014)

RIKEN Accel. Prog. Rep. 47 (2014)

Progress of study of β-decay of neutron-rich nuclei with Z ∼ 60
J. Wu,∗1,∗2 S. Nishimura,∗1 G. Lorusso,∗1 Z.Y. Xu,∗3 H. Baba,∗1 F. Browne,∗1,∗4 R. Daido,∗5 P. Doornenbal,∗1Y.F. Fang,∗5 E. Ideguchi,∗6 T. Isobe,∗1 Z. Li,∗2 A. Odahara,∗5 Z. Patel,∗1,∗7 S. Rice,∗1,∗7 G. Simpson,∗8L. Sinclair,∗1,∗9 P.-A. Söderström,∗1 T. Sumikama,∗10 H. Watanabe,∗11 A. Yagi,∗5 R. Yokoyama,∗12 N. Aoi,∗6F.L. Bello Garrote,∗13 G. Benzoni,∗14 G. Gey,∗7 A. Gottardo,∗15 G.D. Kim,∗16 Y.K. Kim,∗16 K. Kobayashi,∗17I. Kojouharov,∗18 N. Kurz,∗18 H. Nishibata,∗5 H. Sakurai,∗1 H. Schaffner,∗18 M. Tanaka,∗6 J. Taprogge,∗19T. Yamamoto∗5 and the EURICA collaboration
Approximately half of the elements heavier than ironare formed by the rapid neutron-capture process (rprocess). In the solar r-process abundance distribution, the region of rare-earth elements forms a peakaround A = 160, which may have a different mechanism of formation compared with the other two distinct
peaks at A = 130 and A = 195 relating to neutronclosed shells at N = 82 and N = 126, respectively1) .
β-decay half-lives of the elements always play an important role at both the cold and hot r-process paths
and will be expected to constrain the conditions in understanding the r-process nucleosynthesis.
To study the rare-earth peak, a β-decay experimentwith Z ∼ 60 was performed at the RIBF facility inJune 2013. This experiment was carried out using
the in-flight fission of a 345 MeV/nucleon 238 U beamcolliding with a Be target. The secondary beam, including a cocktail of highly neutron-rich isotopes, wasimplanted in the β-decay counting system WAS3ABi2)(Wide-range Active Silicon-Strip Stopper Array for
Beta and ion detection), which consists of a stack offive highly segmented DSSSDs (Double-Sided Silicon
Strip Detectors). With the help of the high-purity
germanium detectors (EURICA)3) , γ rays with a highproduction rate emitted from implanted radioactive
isotopes or the daughters nuclei fed through the β decay can be measured. The β-decay half-lives could
be determined by fitting the distribution of the timedifference between the implantations in the WAS3ABi
and the following β-decay events.
In this experiment, approximately 35 half-lives were
measured, including approximately 25 new half-lives.

Counts/50 ms

Counts/50 ms

Figure 1 displays some preliminary results of four decay curves obtained in this experiment. Daughter halflives, granddaughter half-lives, as well as the constantbackground are taken into account by using the Likelihood fitting method. The β-decay half-lives can also
be obtained by using β-delayed γ rays detected by theEURICA detector, which can eliminate the uncertainties from the daughter and granddaughter half-lives.
Figure 2 shows the β-decay curve of 149 La gated theβ-delayed γ rays.
104

102

149

103

Ba

149

La

10102

1
2

4

6810Time(S)

5152

102

1015

10Ce

1020Time(s)154Counts/50 ms

Counts/50 ms

103

Pr

1

1
5101520Time(S)5101520Time(S)Counts/200 ms

Fig. 1. Decay curves of four kinds of isotopes (149 Ba,149 La,
152Ce, 154 Pr) are displayed. The red lines correspond toparent nuclei. The blue curves, black curves, and green
lines correspond to the daughter nuclei, granddaughternuclei, and a constant background.

1011

∗1

RIKEN Nishina Center
Department of Physics, Peking University∗3 Department of Physics, University of Tokyo∗4 University of Brighton∗5 Department of Physics, Osaka University∗6 Research Center for Nuclear Physics, Osaka University∗7 Department of Physics, University of Surrey∗8 LPSC, France∗9 Department of Physics, University of York
∗10 Department of Physics, Tohoku University∗11 Department of Physics, Beihang University∗12 Center for Nuclear Study, University of Tokyo∗13 University of Oslo
∗14 INFN,Milano∗15 INFN,Legnaro∗16 IBS∗17 Rikkyo University∗18 GSI∗19 Instituto de Estructura dela Materia∗223456789 10Time(S)149

La decay curve obtained gating on the β-delayed
Fig. 2.
γ-ray energy with 245.4keV.

In the latter phases of analysis, further new half-lives
will be obtained. Simulation work of r-process will beperformed by comparing the theoretical calculations
with our experimental results.
References
1) Matthew R. Mumpower et al, Phys. Rev. C 85, 045801(2012).
2) S.Nishimura, G.Lorusso, Z.Xu et al., RIKEN Accel.
Prog. Rep. 46, 182 (2013).
3) P.-A. Söderström et al.: JPS Conf. Proc. 1, 013046(2014).

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