Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 52 (2019)

β-γ spectroscopy of 195 Os at KISS
M. Ahmed,∗1 Y. X. Watanabe,∗2 Y. Hirayama,∗2 M. Mukai,∗1 J. H. Park,∗3 P. Schury,∗2 Y. Kakiguchi,∗2S. Kimura,∗4 A. Ozawa,∗1 M. Oyaizu,∗2 M. Wada,∗2 and H. Miyatake∗2Almost a half of nuclei in nature heavier than iron are
considered to have been synthesized in the rapid neutron
capture process (r-process) under an explosive stellar environment with a temperature higher than 109 K and aneutron density higher than 1020 cm−3 . The r-process
goes through the region of extremely neutron-rich nuclei
under such an explosive condition. The peak at the mass
number of 195, called as the 3rd peak, on the observed
solar r-abundance distribution is considered to be originated from the waiting point nuclei with the neutron
number (N ) of 126 on the r-process path. The astrophysical environment such as the temperature and theneutron density is not known for the formation of the3rd peak. To investigate those conditions, the properties such as lifetimes and masses of the waiting pointnuclei are important. However, they are too far fromthe β-stability line to experimentally access. Therefore,
some theoretical prediction is employed for the physicalvalues of the nuclear properties concerning the r-process
in order to perform the nucleosynthesis simulation tosurvey the explosive stellar conditions. Thus a reliable
theoretical nuclear model is required to precisely predict those nuclear properties to elucidate the astrophysical environments of the r-process. The region of theneutron-rich nuclei around N = 126 is predicted as acompetitive region of the ﬁrst-forbidden (FF) and theallowed Gamow-Teller (GT) β-decays.1) Such competition makes it diﬃcult to theoretically predict their halflives, and those predicted values in various theoreticalmodels are deviated from each other. The systematic
experimental investigation of nuclear properties of theneutron-rich nuclei around N = 126 is important for theselection of reliable theoretical model and improvementof their predictions.
Neutron-rich osmium isotopes are candidates to study
their nuclear structures because they have the ﬁlled single particle orbits h11/2 for protons, which contribute tothe GT β-transitions for the nuclei around N = 126. We
have performed β-γ spectroscopy of 195 Os at KEK Isotope Separation System (KISS)2) to study its β-decay
scheme, which is unknown so far. The 195 Os isotopes
were produced by multi-nucleon transfer reactions between the 136 Xe beam (50 pnA on target) with energy ofaround 10 MeV/nucleon and a 198 Pt target. They werecollected, separated and extracted by the KISS, and ﬁnally the 195 Os ions of its ground state and isomericstate were transported with the rate of 14(2) cps to themeasurement area. They were implanted into an alu∗1
∗2

∗3
∗4

Department of Physics, University of Tsukuba
Wako Nuclear Science Center (WNSC), Institute of Particle
and Nuclear Studies (IPNS), High Energy Accelerator Research Organization (KEK)
Institute for Basic Science (IBS)RIKEN Nishina Center- 54 -Fig. 1. Meausred γ-ray energy spectra in coincidence with
the MSPGC detector hit patterns M = 1 (red line) andM = 2 (blue line).

minized Mylar tape, which was surrounded by the MultiSegmented Proportional Gas Counter (MSPGC)3) andfour High-Purity Germanium (HPGe) clover detectors.
The MSPGC was used to detect β-rays, X-rays and conversion electrons, and the HPGe clover detectors wereused to detect γ-rays. Figure 1 shows measured γ-ray
energy spectra in coincidence with the MSPGC detector
hit patterns M = 1 (red line) and M = 2 (blue line).
The hit patterns M = 1 and M = 2 are sensitive to Xrays and β-rays, respectively. We have found 28 γ-ray
peaks in total with some characteristic X-rays. 22 γ-ray
peaks denoted by black labels agree with energies in theliterature for γ-rays from 194 Ir(n, γ)195 Ir reactions andβ-delayed γ-rays of 195 Ir. We found 6 γ-ray peaks forthe ﬁrst time. The lifetime measurements and γ-γ coincidence analyses reveal that two of them with blue labels
are β-delayed γ-rays of 195 Os, and four of them with redlabels are γ-rays associated with a newly found isomeric
decay of 195 Os. Further analysis for the β-decay schemeand the isomeric decay of 195 Os is in progress.
References

1) I. N. Borzov, Phys. Rev. C 67, 025802 (2003).
2) Y. Hirayama et al., Nucl. Instrum. Methods Phys. Res. B
353, 4 (2015).
3) M. Mukai et al., Nucl. Instrum. Methods Phys. Res. A
884, 1 (2018).