II-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 55 (2022)

RIKEN Accel. Prog. Rep. 55 (2022)

Initial mass measurement of 258 Db by decay-correlated mass
spectroscopy
P. Schury,∗1 T. Niwase,∗1,∗2,∗3 M. Wada,∗1 P. Brionnet,∗2 S. Chen,∗4 T. Hashimoto,∗5 H. Haba,∗2 Y. Hirayama,∗1D. S. Hou,∗6,∗7,∗8 S. Iimura,∗9,∗2,∗1 H. Ishiyama,∗2 S. Ishizawa,∗10,∗2 Y. Ito,,∗11,∗2,∗1 D. Kaji,∗2 S. Kimura,∗2H. Koura,∗11 J. J. Liu,∗4,∗1 H. Miyatake,∗1 J. -Y. Moon,∗5 K. Morimoto,∗2 K. Morita,∗12,∗13 D. Nagae,∗13M. Rosenbusch,∗1 A. Takamine,∗2 Y. X. Watanabe,∗1 H. Wollnik,∗14 W. Xian,∗4,∗1 and S. X. Yan∗15
We present the ﬁrst direct measurement of the atomic
mass of the superheavy nuclide 258 Db. Atoms of 257 Db(Z = 105) were produced online at the RIKEN Nishina
Center for Accelerator-Based Science using the fusionevaporation reaction 208 Pb(51 V, 1n)258 Db. The gasﬁlled recoil ion separator GARIS-II was used to suppress
both the unreacted primary beam and some transferproducts, prior to delivering the energetic beam of 258 Dbions to a helium gas-ﬁlled ion stopping cell wherein they
were thermalized. Thermalized 258 Db2+ ions were thentransferred to a multi-reﬂection time-of-ﬂight mass spectrograph (MRTOF) for mass analysis. An alpha particle
detector embedded in the ion time-of-ﬂight detector allowed disambiguation of the rare 258 Db2+ time-of-ﬂight
detection events from background by means of correlation with characteristic α-decays. The extreme sensitivity of this technique1) allowed a precision atomic mass
determination from 22 decay-correlated events.
This measurement was made simultaneously with257
Db while testing PbS targets produced via sputtering. The targets were capable of withstanding 2 particle
μA primary beam without degradation in their performance. The pulsed deﬂector2) inside the MRTOF wasoperated to allow passage of A/q = 128.5 and A/q = 129
while rejecting transfer products such as 211 Po that had
previously produced spurious decay correlations. The
observed rate of 258 Db relative to 257 Db is within expectations based on the excitation functions.3)
Using a similar correlation method as employed for
257Db,4) extended to encompass spontaneous ﬁssion
events, a mass was determined for the detected ion ineach decay-correlated event. A histogram of the masses
is shown in Fig. 1. Based on the mass resolving power
seen for 257 Db2+ ,5) there appears to be an excess of         ! 

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Fig. 1. Histograms of evaluated atomic masses for ions detected with subsequent α-decays consistent with 258 Db,254
Lr, or 250 Md, 258 Rf, and spontaneous ﬁssion. The twopeak structure could indicate the isomeric excitation exceeds NUBASE estimates.6)

events in the low-mass tail. Applying two-peak ﬁttingwith ﬁxed resolution results in a good reproduction ofthe observed distribution. Presuming this is not a statistical anomaly, it would indicate the isomeric excitation
to be 300 (150) keV, a signiﬁcant deviation from theα-decay derived NUBASE value6) of 53 (14) keV.
A further followup eﬀort to gather more data in order
to better determine the isomeric excitation is planned
for FY2022. However, due to the small diﬀerences between α-particle energies emitted from the two states,
determination of the state ordering via precision decaycorrelated mass spectroscopy will require a considerably
improved detector station. Such a detector station ispresently in the initial design phase.

∗1

Wako Nuclear Science Center (WNSC), IPNS, KEK
RIKEN Nishina Center∗3 Department of Physics, Kyushu University∗4 Department of Physics, University of Hong Kong
∗5 Institute for Basic Science∗6 Institute of Modern Physics, Chinese Academy of Sciences∗7 University of Chinese Academy of Sciences∗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 Advanced Science Research Center, Japan Atomic EnergyAgency∗12 Department of Physics, Kyushu University∗13 Research Center for SuperHeavy Elements, Kyushu University∗14 New Mexico State University∗15 Institute of Mass Spectrometer and Atmospheric Environment, Jinan University∗2References
1) T. Niwase et al., Nucl. Instrum. Methods Phys. Res. A953, 163198 (2020).
2) M. Rosenbusch et al., submitted to Nucl. Instrum. Methods Phys. Res. A.
3) J. M. Gates et al., Phys. Rev. C 78, 034604 (2008).
4) P. Schury et al., Phys. Rev. C 104, L021304 (2021).
5) P. Schury et al., in this report.
6) M. Wang et al., Chin. J. Phys. C, 45 030001 (2021).

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