II-9. Instrumentation

RIKEN Accel. Prog. Rep. 55 (2022)

RIKEN Accel. Prog. Rep. 55 (2022)

Development of β-TOF detector for decay-correlated massmeasurement of β-decaying nuclides
T. Niwase,∗1 W. Xian,∗2,∗1 M. Wada,∗1 M. Rosenbusch,∗1 S. D. Chen,∗2,∗1 A. Takamine,∗3 J. Liu,∗2,∗1S. Iimura,∗4,∗3,∗1 D. S. Hou,∗5,∗6 S. X. Yan,∗7 H. Ishiyama,∗3 S. Nishimura,∗3 Y. Hirayama,∗1 S. Kimura,∗3D. Kaji,∗3 H. Miyatake,∗1 K. Morimoto,∗3 P. Schury,∗1 Y. X. Watanabe,∗1 and H. Wollnik∗8                   

Fig. 1. TOF spectrum of 81 Ga2+ . The TOF singles spectra
are drawn in black, while β-decay correlated events areshaded in red.

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A novel “α-TOF” detector,1) has been recently developed and installed on a multi-reﬂection time-of-ﬂight
mass spectrograph (MRTOF-MS). This detector canmeasure ion implantation to deduce the time-of-ﬂight(TOF), along with subsequent α-decay and spontaneousﬁssion (SF) events from the implanted ions. We recently
succeeded in the direct mass measurement of superheavy
nuclides, 257, 258 Db,2,3) wherein the count rate of 257 Db
was as low as two events per day.2) Furthermore, we haveinitiated a new ﬁeld of nuclear spectroscopy using decaycorrelated mass measurements.4) The potential of thistechnique was initially demonstrated using decay properties to discriminate between the two states of 207 Ra,
followed by mass analyses of each state; the two states
could not be independently resolved by the MRTOF.
While the α-TOF detector is limited to α-decay andSF nuclides, it would be valuable to expand its performance to encompass β-decays. Here, we report the development of a new “β-TOF” detector, an extension of
the α-TOF detector, that enables the decay-correlated
mass spectroscopy of α/β/SF decaying nuclides.
The β-TOF detector uses double-layered SSDs with athickness of 500 μm (Hamamatsu S-14605) to provide a
ΔE -ΔE telescope for β-decay. In the ﬁrst layer of theSSD, α-decay and SF are detected in the same manneras those in α-TOF. The basic characteristics—time resolution, α-particle energy resolution, implantation, andα-decay detection eﬃciency—of the β-TOF detector are
the same as those of α-TOF. Owing to the solid angle
coverage of the two SSDs, the telescope is geometricallylimited to a detection eﬃciency of 33% for β-rays.
Online commissioning for the β-TOF detector wasperformed via MRTOF,5) located at the end of the ZeroDegree spectrometer beamline.6) The so-called ZDMRTOF was coupled to a cryogenic helium gas cell that
thermalized the relativistic beam delivered by BigRIPS.
In the ﬁrst online test of this new detector, we succeeded
in measuring the correlation between the TOF of theneutron-rich 81 Ga isotope and subsequent β-decay. Figure 1 shows the histogram of TOF singles and decaycorrelated TOF events. Accordingly, the detection ef-#! Fig. 2. Measured decay time distribution of 81 Ga. The redsolid line and green dashed curve indicate the distribution
curve drawn with the experimental and literature (T1/2= 1.22 s) values, respectively.

ﬁciency for β-rays was found to be 27(7)%, which ingood agreement with the geometric estimation. The
half-life of 81 Ga was successfully extracted from the time
between individual implantations and subsequent decay
events. Figure 2 shows the decay time distribution withthe expected decay curve. The half-life of 81 Ga was determined to be 1.44(31) s, which is in agreement withthe value in literature, i.e., 1.217(5) s.7)
References∗1∗2∗3∗4∗5∗6∗7∗8

Wako Nuclear Science Center (WNSC), IPNS, KEK
Department of Physics, University of Hong Kong
RIKEN Nishina Center
Department of Physics, Osaka University
Institute of Modern Physics, Chinese Academy of Sciences
School of Nuclear Science and Technology, Lanzhou University
Institute of Mass Spectrometer and Atmospheric Environment, Jinan University
Department of Chemistry and Biochemistry, New Mexico
State University- 88 -1) T. Niwase et al., Nucl. Instrum. Methods Phys. Res. A,
953, 163198 (2020).
2) P. Schury et al., Phys. Rev. C 104, L021304 (2021).
3) P. Schury et al., in this report.
4) T. Niwase et al., Phys. Rev. C 104, 044617 (2021).
5) M. Rosenbusch et al., arXiv:2110.11507.
6) T. Kubo et al., Prog. Theo. Exp. Phys. 2012, 03C003(2012).
7) C. M. Baglin, Nucl. Data Sheets 109, 2257 (2008).

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