Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 50 (2017)

Initial eﬀort to resolve isomers in MRTOF-MS via in-trap decay∗1∗1,∗2,∗3
∗1
∗2
∗2
∗2
∗2P.
M.
D.
A.
H.
Y.
Kimura,∗1,∗2,∗3
P. Schury,Schury,∗1
M. Wada,Wada,∗1,∗2 Y. Ito,∗2
D. Kaji,Kaji,∗2
A. Takamine,Takamine,∗2
H. Haba,
Haba,∗2
Y. Hirayama,
Hirayama∗1
S. Kimura,
∗7,∗1
∗2
∗2,∗5
∗2,∗3
∗2,∗4
∗1∗2
∗2,∗5
∗2,∗3
∗2
H. Koura,
K. Morimoto, K.
K. Morita,
Morita,A. Ozawa,Ozawa,
M. Reponen,
Reponen,∗2H. Miyatake, J.Y. Moon,K.
A.
M.
∗2,∗5
∗1
∗6
∗2,∗5
∗1
∗
T. Tanaka,Tanaka,
Y. X.
X. Watanabe,Watanabe, andand H.
H. Wollnik
Wollnik 6T.
Y.

The SHE-mass project is a joint eﬀort between KEKand RIKEN with a long-term goal of identifying new
superheavy element (SHE) isotopes produced via hotfusion. It makes use of cryogenic-capable, high-purity
helium gas cell to convert the energetic (5∼50 MeV)
evaporation products of fusion reactions into thermalions. The evaporation products are separated fromprojectile-like fragments by use of the gas-ﬁlled recoil ion separator GARIS-II1) . The thermalized ions
are transferred to a multi-reﬂection time-of-ﬂight massspectrograph2) (MRTOF-MS) which can analyze theions with a mass resolving power of Rm > 100 000.
The SHE-mass system is described in some detail in3) .
In addition to SHE studies, it has been foreseen touse the system to study neutron-deﬁcient isotopes fromPo through Pa, where the N =126 shell-closure approaches the proton drip-line . Investigations of these
nuclei have mostly been limited to α-decay studies up
to now. However, the vast amount of known isomericstates among nuclei in this region casts some doubt on
the veracity of α-decay studies and indicates a need for
direct mass measurements.
Most isomeric states in this region have half-lives
shorter than 1 ms, signiﬁcantly shorter than the measurement cycle for MRTOF-MS. Unfortunately, many
known isomers in this region have suﬃciently long halflives as to impact MRTOF-MS. Additionally, distinguishing most of these isomers from their ground statesrequires higher mass resolving power than is currently
available from MRTOF-MS.
In the majority of cases, however, the ground state
and/or the isomeric state(s) exhibit T1/2 10 s. Thus,by storing the ions in the preparation trap for an extended period before analysis in the MRTOF-MS, theisomer can decay away. We have developed a mathematical framework to use mass measurements at multiple storage times to determine the isomeric ratio andallow for precise determination of ground state masses.
As a ﬁrst test of this, we used the 165 Ho(48 Ca, 7n) toproduce 206 Fr2+ ions4) . The duration of accumulationin the preparation trap was then varied up to 2.64 sprior to mass analysis. Unfortunately, the chargeexchange lifetime of the Fr2+ was short compared to
the ground state decay lifetime (see Fig. 1), resulting
∗1
∗2
∗3
∗4
∗5
∗6
∗7in a complicated and ongoing analysis to determine theisomeric ratio.

Fig. 1. ToF spectra observed for A/q=103 ions for various
accumulation durations. Reductions are presumed tobe caused by charge-exchange in the preparation trap.

We can use the relative intensities of observed ions asa function of accumulation time to estimate the charge
exchange lifetimes in the preparation trap for doublycharged ions of a variety of elements, listed in Table
1.
The short
short lifetimes
lifetimes cancan be
be expected, as the trap isI. The
not UHV clean and operates at room temperature. In
the future, we will try to repeat such studies with thetraps being cryogenically cooled.
Table 1. Loss half-lives in the preparation trap. IP1 andIP2 are the ﬁrst and second ionization potentials.
Ion
Ra2+
206
Fr2+
207
Fr2+
206
Rn2+
207
Rn2+
206
At2+
207
At2+211

IP1 [eV]
5.3
4.1
4.1
10.7
10.7
9.3
9.3

IP2 [eV]
10.14
22.4
22.4
21.4
21.4
17.9
17.9

decay
T1/2
13.2 s16 s
14.8 s
5.67 m
9.25 m30.6 m1.81 hloss
T1/2
>2 s
422(16) ms414(58) ms
183(14) ms
216(16) ms
134(32) ms
157(16) ms

References
1) D. Kaji et al., Nucl. Instrum. and Methods Phys. Res.
B, 317 (2013) 311-314.
2) P. Schury et al., Nuclear Inst. and Methods in Physics
Research B 335 (2014) 39-53.
3) P. Schury et al., Phys. Rev. C 95 (2017) 011305(R).
4) ”Observation of the extraction of Fr2+ from a cryogenic
gas cell”, this volume.

KEK Wako Nuclear Science Center (WNSC)
RIKEN Nishina Center
Institute of Physics, University of Tsukuba
Advanced Science Research Center, JAEA
Dept. Physics, Kyushu University
Dept. Chem. & BioChem., New Mexico State University
Institute for Basic Science- 65 -