II-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

First high-precision direct determination of the atomic mass of asuperheavy nuclide
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
decay-correlated ToF events
Po coincidental decay-correlated ToF events
Uncorrelated ToF singles

257

2A/q - 85 702.006 [mu/e]

We present the ﬁrst direct measurement of theatomic mass of a superheavy nuclide. 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, 2n)257 Db. The gasﬁlled recoil ion separator GARIS-II was used to suppress both the unreacted primary beam and sometransfer products, prior to delivering the energetic
beam of 257 Db ions to a helium gas-ﬁlled ion stopping cell wherein they were thermalized. Thermalized 257 Db3+ ions were then transferred to a multireﬂection time-of-ﬂight mass spectrograph for mass
analysis. An alpha particle detector embedded in
the ion time-of-ﬂight detector allowed disambiguationof the rare 257 Db3+ time-of-ﬂight detection eventsfrom background by means of correlation with characteristic α-decays (see also T. Niwase in this issue). The extreme sensitivity of this technique allowed a precision atomic mass determination from11 events. The mass excess was determined to be
100 063(231)stat (72)sys keV/c2 .
In recent experience, elements with a second ionization potential below 24 eV have been near uniformly
extracted from the gas cell as doubly charged ions.
Surprisingly, no counts were seen for 257 Db2+ , while257
Db3+ was observed with the rate which, based oncross-section, target thickness, and primary beam intensity, indicated it constituted the preponderance of257
Db ions. The rate of 257 Db3+ extracted from thegas cell was roughly 4 per day. These ions were analyzed by the MRTOF-MS with the α-TOF detector,1)
which allowed us to correlate ToF events with subse-211

Db 3+

1E8E10E11E3E2E6E5E9E12E13E14-1-20

1234

Time of observation [Days]

56Fig. 1. Apparent A/q evaluated for each ToF single near
the expected position of 257 Db3+ . The data are plottedin terms of deviation from the A/q for 257 Db3+ as determined from AME16.2) Statistical uncertainties are
only evaluated for α-decay correlated ToF events.

quent α-decays. Such correlations allowed removal ofeven low-intensity backgrounds and conﬁrm that wewere truly measuring 257 Db3+ .
Figure 1 shows the data measured over 105 hours ofbeam on target. As the MRTOF was operated with aresolving power of Rm ≈ 300 000, the A/q range presented is about 10-FWHM. The red points represent
events where the ToF signal was followed within 120 sby an α decay event with energy Eα ≥ 7.0 MeV to encompass all possible α-decays from 257 Db and its decay
products; some α-decay-correlated ToF events within
this gate were attributed to the α-decay of 211 Po.
Based on the 11 alpha decay correlated ToF events,
the mass of 257 Db could be determined with a precision of 231 keV/c2 (82 µu/e). The value is in agreement with indirect measurements.2,3) Unfortunately,
the mass resolution of the MRTOF and the energy resolution of the α-TOF were insuﬃcient to resolve isomeric states. With recent improvements in the mass
resolving power (See S. Yan in this issue) we should be
able to resolve the isomer and ground state provided
the isomer has excitation energy above 250 keV/c2 .
A followup eﬀort to determine the state order via
MRTOF mass analysis is planned for FY2021.

∗1

Wako Nuclear Science Center (WNSC), IPNS, KEK
RIKEN Nishina Center∗3 Department of Physics, Kyushu University∗4 Department of Physics, The University of Hong Kong
∗5 Institute for Basic Science∗6 Institute of Modern Physics∗7 University of Chinese Academy of Sciences∗8 School of Nuclear Science and Technology∗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. A,953, 163198 (2020).
2) M. Wang et al., Chin. Phys. C, 41 030003 (2017).
3) Y. Ito et al., Phys. Rev. Lett. 120, 152501 (2020).

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