Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 53

In-gas-cell laser ionization spectroscopy of Os isotopes using
MRTOF-MS at KISS
Y. Hirayama,∗1 H. Choi,∗2 J. Y. Moon,∗3 T. Hashimoto,∗3 M. Mukai,∗4 T. Niwase,∗5 M. Oyaizu,∗1M. Rosenbusch,∗1 P. Schury,∗1 A. Taniguchi,∗6 M. Wada,∗1 Y. X. Watanabe,∗1 and H. Miyatake∗1
We developed the KEK Isotope Separation System(KISS)1) to study the nuclear properties of neutron-richisotopes with neutron numbers around N = 126. To
study the nuclear structures at KISS, we measured thehyperﬁne structure (HFS) of 194 Os (I π =0+ , T1/2 =6.0 y) and 196 Os (I π = 0+ , T1/2 = 34.9 m) to determine the change in charge radius using the in-gascell laser ionization spectroscopy technique2) assistedby the multi-reﬂection time-of-ﬂight mass spectrograph(MRTOF-MS). The MRTOF-MS installed at KISS cansuccessfully identify isotopes from mass-dependent timeof-ﬂight (TOF) spectra.3)
We measured the HFS of short-lived (T1/2 ∼ 30 min)isotopes by detecting the β- and γ-rays at KISS, and it isdiﬃcult to measure the HFS of isotopes with T1/2 > 1 hby detecting the decay radiations in a limited beam time.
However, we can eﬃciently measure the HFS of these isotopes through ion counting using the MRTOF-MS without waiting for the radiation decays. Here, we report theHFS measurement of 194 Os using the MRTOF-MS.
The 194 Os isotopes were produced in multi-nucleon
transfer reactions by using a stable 136 Xe beam (50 pnA)
with an energy of approximately 10 MeV/nucleon impinging on a 198 Pt target (12.5 mg/cm2 , enriched to 91%
with about 3% each of 194, 195, 196 Pt). Singly charged isotopes, produced by the in-gas-cell laser ionization technique, with an energy of 20 keV were extracted from theKISS gas cell for the HFS measurements. The extracted
isotopes were injected into a helium gas cell for thermalization. Doubly charged ions were primarily producedby the charge exchange reaction with helium atoms in1942+

Pt

1942+

Os

1942+AuFig. 1. Measured TOF spectrum of 194 Os2+ .
∗1

∗2
∗3
∗4
∗5∗6

Wako Nuclear Science Center (WNSC), Institute of Particle
and Nuclear Studies (IPNS), High Energy Accelerator Research Organization (KEK)
Seoul National University
Institute for Basic Science (IBS)
Department of Physics, University of Tsukuba
Department of Physics, Kyushu University
Institute for Integrated Radiation and Nuclear Science, Kyoto
University (KURNS)

yrain

iml
re

backgroundPFig. 2. Measured HFS spectrum of 194 Os (I π = 0+ ). Horizontal uncertainty estimated from the accuracy of awavemeter.

the stopping process, and they were extracted from thehelium gas cell. Subsequently, the bunched isotopes wereinjected into the MRTOF-MS for particle identiﬁcation.
The details of the MRTOF-MS system are reported inRef. 3).
Figure 1 shows the TOF spectrum of 194 Os2+ measured using the MRTOF-MS at KISS. We can clearly
identify the 194 Os2+ isotope with a contaminant peakof 194 Pt2+ ions, which were emitted through elastic
events from the production target and transported tothe MRTOF-MS as survived ions. By ﬁtting the TOF
spectrum, we can deduce the number of ions extracted
from the KISS gas cell. The HFS spectrum, as shown inFig. 2, was obtained by measuring the number of laserionized 194 Os isotopes as a function of the laser wavelength. There appears one resonance peak stemmingfrom one atomic transition of 194 Os due to I π = 0+ .
The constant background (dashed line) in Fig. 2 indicates the contaminant at the 194 Os2+ peak, which corresponds to 194 Au2+ transported as survived ions. Theexpected TOF peak position of 194 Au2+ indicated by theblack dotted line in Fig. 1 deviates by about 4 ns fromthe 194 Os2+ peak. To evaluate the extraction yields of194
Os2+ accurately, we ﬁt the spectrum by omitting the194
Au2+ peak because of the closeness of the peak positions. However, we could clearly deduce the resonancepeak of 194 Os and the amount of 194 Au as the constantbackground, as shown in Fig. 2. From the peak position, we can determine the isotope shift value of 194 Os
to deduce the change in the charge radius and discuss
the nuclear deformation. Further analysis is in progress.
References

1) Y. Hirayama et al., Nucl. Instrum. Methods Phys. Res.
B412, 11 (2017).
2) Y. Hirayama et al., Phys. Rev. C 96, 014307 (2017).
3) J. Y. Moon et al., in this report.

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