II-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Alpha-decay correlated mass measurement of 206, 207 Ra using an
MRTOF-MS system equipped with an α-TOF detector
T. Niwase,∗1,∗2,∗3 M. Wada,∗3 P. Schury,∗3 P. Brionnet,∗2 S. D. Chen,∗4,∗3 T. Hashimoto,∗5 H. Haba,∗2Y. Hirayama,∗3 D. S. Hou,∗6,∗7,∗8 S. Iimura,∗9,∗2,∗3 H. Ishiyama,∗2 S. Ishizawa,∗10,∗2 Y. Ito,∗11 D. Kaji,∗2S. Kimura,∗2 J. Liu,∗4,∗3 H. Miyatake,∗3 J. Y. Moon,∗5 K. Morimoto,∗2 K. Morita,∗1,∗2 D. Nagae,∗1M. Rosenbusch,∗3 A. Takamine,∗2 T. Tanaka,∗12 Y. X. Watanabe,∗3 H. Wollnik,∗13 W. Xian,∗4,∗3 and S. X. Yan∗14∗1Department of Physics, Kyushu University
RIKEN Nishina Center∗3 Wako Nuclear Science Center (WNSC), KEK∗4 Department of Physics, The University of Hong Kong
∗5 Institute for Basic Science, Rare Isotope Science Project∗6 Institute of Modern Physics, Chinese Academy of Science∗7 University of Chinese Academy of Science∗8 School of Nuclear Science and Technology, Lanzhou University∗9 Department of Physics, Osaka University.
∗10 Graduate School of Science and Engineering, Yamagata University∗11 Japan Atomic Energy Agency∗12 Department of Nuclear Physics, The Australian National University∗13 Department of Chemistry and Biochemistry, New Mexico
State University∗14 Institute of Mass Spectrometer and Atmospheric Environment, Jinan University∗2207Ra-singles

207Fr-singles

210Counts / 3 nsToward the precise mass measurement of heavy andsuperheavy nuclides, the SHE-Mass-II facility1) was constructed with a multi-reflection time-of-flight mass spectrograph (MRTOF-MS)2) coupled with the gas-filled recoil ion separator GARIS-II.3) We installed an α-TOF4)
detector, which simultaneously records the time-of-flight(TOF) signal and subsequent α-decay. In order todemonstrate the α-TOF detector, an experiment wasperformed using the 51 V + 159 Tb reaction. A 51 V beam
was accelerated to 6.0 MeV/nucleon by the RIKEN Ring
Cyclotron (RRC). The beam energy on the target wasreduced by an aluminum degrader to 4.8 MeV/nucleon.
The beam impinged upon 460 µg/cm2 -thick 159 Tb targets with a 3 µm Ti backing, mounted in a rotatingtarget wheel.
The fusion evaporation residues (ERs) were separated
from the primary beam and transported using GARISII. After decelerating ERs using a Mylar foil, the ERs
were stopped in a cryogenic helium gas catcher, and thethermalized ions were extracted by a radio frequency(RF) carpet and transported to the MRTOF-MS via
multiple RF ion traps.
We observed ERs, 206, 207 Fr, and 206, 207 Ra as doubly
charged ions. The subsequent α-decays were additionally detected by the α-TOF detector. Using 206, 207 Fr
as the isobaric references, the masses of 206, 207 Ra weredirectly determined. The mass excess of 206 Ra was3540(54) keV, which agrees with the values reported inAME2016.5)
The TOF spectrum for the A/q = 103.5 region isshown in Fig. 1. The singles events and 207 Ra decaycorrelated events are plotted. In the case of the groundstate of 207 Ra, the correlated events of the TOF and theα-decay could not be observed, because the incoming

207Ra-correlated
E > 7.32 MeV
dt < 180 ms

1019410.5

9410.69410.79410.8Time-of-ﬂight [s]9410.9Fig. 1. Time-of-flight spectrum around the 207 Ra2+ region.
The red histogram indicates the decay-correlated events.
The green and blue lines show the fitting of the groundand isomeric states of the singles (solid lines) and decaycorrelated events (dot lines).

rate was higher than the decay rate, while the decaycorrelated events were observed in the isomeric state207m
Ra owing to its short half-life. The energies of theα-decay were selected as higher than 7.32 MeV, 2σ apartfrom the centroid of 207g Ra, to avoid contamination from207g
Ra.
The peaks of singles 207g/m Ra2+ and decay-correlated207m
Ra2+ were fitted. The shape of the peak was determined by 207 Fr2+ . The mass excess of 207g Ra was determined to be 3538(15) keV, and the excitation energy of207m
Ra was Eex = 552(42) keV from the α-decay correlated TOF spectrum. These values are consistent withthose evaluated by α-decay spectroscopy.5)
The alpha branching ratio of 207m Ra was determinedfrom the counting of TOF and α-decay events. The spinparity was expected to be 13/2+ based on its singleparticle level energy and the analogous reduced alphawidth to the neighboring nuclei.
References

1) M. Wada et al., RIKEN Accl. Prog. Rep. 52, 136 (2019).
2) P. Schury et al., Nucl. Instrum. Methods Phys. Res. B335, 39 (2014).
3) D. Kaji et al., Nucl. Instrum. Methods Phys. Res. B 317,311 (2013).
4) T. Niwase et al., Nucl. Instrum. Methods Phys. Res. A953, 163198 (2020).
5) M. Wang et al., Chin. Phys. C 41, 030003 (2017).

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