II-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 55 (2022)

RIKEN Accel. Prog. Rep. 55 (2022)

Status of the study of masses and half-lives of 252 Cf ﬁssion fragments
by the MRTOF-MS
S. Kimura,∗1 M. Wada,∗2 H. Haba,∗1 S. Ishizawa,∗3 T. Niwase,∗2 M. Rosenbusch,∗2 and P. Schury∗2
for the SHE-Mass Collaboration

Intensity ratio, 92Br+ / 92Rb+

The nuclear data of neutron-rich nuclei, e.g., mass,
lifetime, and neutron-capture cross section, are important for understanding the rapid neutron-capture (r -)process, because a balance of net neutron-capture rateand β − -decay rate is crucial to determine its path.
Thus, accurate and high-precision nuclear data aboutboth neutron-rich isotopes’ mass and half-life are required.1)
The multi-reﬂection time-of-ﬂight mass spectrograph (MRTOF-MS) has been developed for not only
mass measurement but also isobar separation. Here,
we brieﬂy report the present status of the mass measurement of neutron-rich isotopes and, in addition,
the half-life measurement as an application of theMRTOF-MS.
The mass measurement study of the neutron-rich
252
Cf-ﬁssion-fragments with the MRTOF-MS2) waspresented in the previous report.3) We had used the350 kBq 252 Cf source and newly installed a more intense source (9.25 MBq). More than 200 isotopes
have been conﬁrmed in the time-of-ﬂight spectra andare shown in Fig. 1. In addition, many long-lived
isomers were also observed during the measurements.
The mass resolving power of the MRTOF-MS reaches
Rm > 5 × 105 ; thus, the measurements were achieved
with precision better than 10−7 in most of the cases.
We have developed a decay-scheme-independent<410<510<63010<750607080
Neutron number90100110Fig. 1. Location of the observed isotopes on the nuclearchart. The observed isotopes are indicated with orangecolored squares. The black squares represent the stable
isotopes. The contour map shows the ﬁssion yield of252Cf.4)
∗1
∗2
∗3250

450

half-life measurement method with the MRTOF-MS.
The experimental setup consists of a cryogenic gascell, the MRTOF-MS, and a suite of ion traps witha quadrupole ion guide (QPIG), a linear trap, and a
ﬂat trap used as an injector for the MRTOF-MS. Bychanging the operation cycle of the linear trap, theion storage time of the linear trap can be changed,
making it possible to measure the lifetimes of shortlived isotopes. We brieﬂy discuss here the lifetime
measurement of 92 Br as an example of this “storagemethod.” Figure 2 shows the storage time dependenceof the yields of 92 Br+ ions normalized by the 92 Rb+
ion yields for canceling out the eﬃciency reduction dueto increasing of the storage time. A decreasing trendof 92 Br+ ion yields is clearly observed and the 92 Br+ ’s
half-life is measured to be 349(178) ms through a ﬁt
with the theoretical curve. This value is in agreementwith the value in the literature of 314(16) ms. In thisanalysis, we assumed that an inﬂuence of the decay
loss of the 92 Rb+ ion is negligibly small because its
half-life (4.48 s) is suﬃciently long compared to thestorage time.

4040

1.5×10-3Fig. 2. Intensity ratio of 92 Br+ /92 Rb+ as a function of storage time in the linear trap. A black line shows a ﬁt result. The measured half-life, 349(178) ms, is consistent
with the value in the literature of 314(16) ms.

Fission yield

Proton number

10-3

Storage time in the linear trap ( ms )60

50

2.0×10t1/2 = 349(178) ( ms )

50

10<2

<3-3

1.0×10-37010

2.5×10

References
1) M. R. Mumpower et al., Prog. Part. Nucl. Phys. 86, 86(2016).
2) P. Schury et al., Nucl. Instrum. Methods Phys. Res. B
335, 39 (2014).
3) S. Kimura et al., RIKEN Accel. Prog. Rep. 53, 52(2019).
4) J. Katakura, JAEA-Data/Code 2011-025.

RIKEN Nishina Center
Wako Nuclear Science Center (WNSC), IPNS, KEK
Institute for Materials Research, Tohoku University-8-1