Ⅰ. HIGHLIGHTS OF THE YEARRIKEN Accel. Prog. Rep. 53

Oﬄine measurement of mass and correlated decay properties using
radioactive 224 Ra source via MRTOF+α-TOF detector

Recently, we newly developed a detector called the “αTOF” detector1) for the measurement of mass and correlated decay properties by using a multi-reﬂection timeof-ﬂight mass spectrograph (MRTOF-MS).2) We performed realistic performance tests using a 224 Ra source.
The source, produced by a chemical separation from a228
Th source,3) was placed in a cryogenic gas cell.
Ions of 220 Rn and 216 Po produced by the decay of224
Ra were thermalized in the gas cell and extractedusing an RF carpet, transported to the MRTOF-MS
preparation traps, and injected into the MRTOF-MS forthe correlated measurement of the time of ﬂight (ToF)and α-decay. 220 Rn and 216 Po have half-lives of 55.6 s
and 145 ms with characteristic α-particle energies of6.29 MeV and 6.78 MeV, respectively. During this measurement, the MRTOF was operated for only 2 laps inside the MRTOF reﬂection chamber. This was suﬃcientto unambiguously determine the A/q value with a mass
resolving power of Rm ≈ 1500 with a wide mass bandwidth.

Counts / 12.8 ns

117
117

50
45
40
35
30
25
20
15
105050
1110
45
40

35
30
25
20
15
10501110
50

45
40
35
30
25
20
15
10
50
111.0
1110

118
118

119
119

A/q
120
120

singles ToF

216PoCO++

1120

1125

1130

1135

1115

1120

1125

1130

1135

111.5
1115

112.0
1120

112.5
1125

113.0
1130

113.5
1135

Po gated ToF

123
123

(a)

1115

Rn gated ToF

216

122
122

220RnHF++

220RnO++

220

121
121

CountsT. Niwase,∗1,∗2,∗3 M. Wada,∗3 P. Schury,∗3 H. Haba,∗2 S. Ishizawa,∗5,∗2,∗3 Y. Ito,∗4 D. Kaji,∗2 S. Kimura,∗2H. Miyatake,∗3 K. Morimoto,∗2 K. Morita,∗1,∗2 M Rosenbusch,∗2 H. Wollnik,∗6 T. Shanley,∗7 and Y. Benari∗7(b)×101140

1140

310(c)

×10114.01140

3Time-of-flight [μs]Fig. 1. (a) Singles ToF spectrum. (b) ToF spectrum in coincidence with 220 Rn α-decays obtained using a time gateof Tc = 180 s. (c) ToF spectrum in coincidence with 216 Podecays with Tc = 450 ms.
∗1∗2∗3∗4∗5∗6∗7decay time [s]Fig. 2. Distribution of decay time of 216 PoCO++ . The solidline indicates a distribution curve drawn with values obtained from the literature.

Figure 1 shows the ToF spectrum of 220 Rn and 216 Po.
ToF and α-decay signals were recorded event by eventwith absolute time stamps. Figure 1(a) shows the singles ToF spectrum. By using a coincidence time gate(Tc ) prior to the detection of an α-decay signal, it is possible to discriminate between ToF events corresponding
to 220 Rn and 216 Po, as shown in Fig. 1(b) and (c), respectively. To identify the peak of 216 Po, a ±150 keVenergy gate was used for the α-decay signal at approximately 6.78 MeV with Tc = 450 ms corresponding tothree half-life periods. To identify the peak of 220 Rn, asimilar gate was made on the 6.29 MeV α-decay signal
with Tc = 180 s.
Using the α-decay correlated ToF event, we determined that the ToF peak occurs at A/q = 118 and 120
from 220 Rn events and A/q = 122 from 216 Po events.
The limited mass resolution of the wide mass bandwidth measurement precluded precise molecular identiﬁcation. However, based on past experience, we havetentatively assigned the A/q = 118, 120 and 122 peaks to220
RnO++ , 220 RnHF++ and 216 PoCO++ , respectively.
In addition, by deﬁning the decay time as the time interval between the α-signal and the ToF event, we demonstrated the ability to measure half-lives with the α-TOF
detector. Figure 2 shows a plot of the measured decay
time of 216 PoCO++ ions along with the expected decay
time distribution function based on the value reported
in the literature. We evaluated the half-life of 216 Po as
T1/2 = 123(22) ms, which is in good agreement with theliterature value of 145(2) ms.
References

Department of Physics, Kyushu University
RIKEN Nishina Center
Wako Nuclear Science Center (WNSC), KEK
Japan Atomic Energy Agency
Yamagata University
New Mexico State University
ETP Ion Detect

1) T. Niwase et al., Nucl. Instrum. Methods Phys. Res. A
953, 163198 (2020).
2) P. Schury et al., Nucl. Instrum. Methods Phys. Res. B
335, 39(2014).
3) J. Narbutt et al., Appl. Radiat. Isot. 49, 89–91(1998).

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