II-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 55 (2022)

RIKEN Accel. Prog. Rep. 55 (2022)

Highly precise mass measurements of neutron-rich nuclei at
A = 82–92 at ZeroDegree-MRTOF(ZD-MRTOF)
W. Xian,∗1,∗2 M. Rosenbusch,∗2 S. Chen,∗1,∗2 D. Hou,∗3,∗2 S. Iimura,∗5,∗4 H. Ishiyama,∗4 Y. Ito,∗6 S. Kimura,∗4T. M. Kojima,∗4 J. Liu,∗3 J. Lee,∗1 H. Miyatake,∗2 S. Michimasa,∗10 S. Nishimura,∗4 T. Niwase,∗7,∗4,∗2 S. Naimi,∗4P. Schury,∗2 T. Sonoda,∗4 A. Takamine,∗4 M. Wada,∗2 Y. X. Watanabe,∗2 H. Wollnik,∗8 and S. Yan∗9Counts / 3.6 [ns]

104

8384

Se

Kr 1H+

1384++Ge

C 1H 35Cl2+



C21H619F 35Cl+32 +C4 H4 S

12C 1H216O 19F 35Cl+

84

As+

10
1
15.87215.87415.87615.878

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be measured simultaneously. Stable molecules provided
isobaric references for extracting masses of ion of interest. On the other hand, the stable molecules also served
as benchmarks for the evaluation of the systematic uncertainties.
In total, the HiCARI campaign 2020 had ﬁve separate experiments, covering four diﬀerent regions. Theresults introduced in this report were from the data acquired during NP1912-RIBF196 and NP1912-RIBF190.
Among other mass regions, abundant species of isotopes
have been observed in the mass range A = 82–92. In
total, 35 nuclides were measured in a neutron-rich region of Ga, Ge, As, Se, Br, Kr, and Rb in which 84 Ga,86
Ge, 89 As, 91 Se, and 92 Br are the most exotic isotopes(see Fig. 2). Among those isotopes, mass uncertainties
of 86 Ge, 90 Se, and 91 Se, could be reduced signiﬁcantly toless than 5 keV. Moreover, for the ﬁrst time, the massesof 88, 89 As were unveiled. In addition to radioactive ions,at least one stable atomic or molecular ion was found foreach mass number, e.g., 82 Kr1 H+ , 83 Kr1 H+ , 12 C37 Cl+2,etc. The mass deviations of these ions from AME2020
enable an evaluation of system accuracy of the new ZDMRTOF system. The weighted mean deviation of themeasured masses from the precisely known values wasfound to be approximately only 2 keV, which proves avery high accuracy for future measurements.

C 35Cl 37Cl+12





Fig. 2. Meassured isotopes (A = 82–92) in on-line tests.

121   Kr+12     



84

103
102



A state-of-art multi-reﬂection time-of-ﬂight (TOF)
mass spectrograph (MRTOF-MS) coupling to a cryogenic helium ﬁlled gas catcher (gas cell) was developed
and prepared for on-line tests prior to the launch of theHiCARI campaign 20201) for γ-ray spectroscopy. For
the ﬁrst time, the ZD-MRTOF system was operated
on-line running as parasitic experiment downstream of
the ZeroDegree (ZD) spectrometer. Produced by inﬂight ﬁssion of 238 U primary beam with 9 Be target, radioactive ions were selected, identiﬁed, and guided by
BigRIPS to the focal plane F8, where the exotic ionsreacted with a secondary target for in-beam γ-ray research. Reaction products were then analyzed using theZD spectrometer before they were ﬁnally entrapped by
the gas cell and measured by the MRTOF-MS.
Abundant stable molecules contribute to strong peaks
with long tails in a TOF spectrum, which causes diﬃculties identifying the much weaker peaks of the ionsof interest. Beneﬁting from the introduction of a new
mass ﬁlter method,2) we obtained low-contaminantbackground with high resolution isobaric TOF spectrums, as shown in Fig. 1. In this spectrum, in addition
to isobaric ions from the on-line beam, stable moleculesfrom background contaminations in the gas were also observed in the same measurement as several species can

15.880

TOF [ms]

Fig. 1. TOF spectrum at A = 84 with radioactive ions (red)and stable molecules (blue).
∗1

University of Hong Kong
Wako Nuclear Science Center (WNSC), IPNS, KEK∗3 Institute of Modern Physics, Chinese Academy of Sciences∗4 RIKEN Nishina Center∗5 Osaka University∗6 Advanced Science Research Center, Japan Atomic EnergyAgency∗7 Department of Physics, Kyushu University∗8 Department of Chemistry and Biochemistry, New Mexico
State University∗9 Jinan University∗10 University of Tokyo∗2References
1) M. Rosenbusch et al., RIKEN Accel. Prog. Rep. 54, S18(2021).
2) S. X. Yan et al., RIKEN Accel. Prog. Rep. 54, S28 (2021).

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