Ⅰ. HIGHLIGHTS OF THE YEARRIKEN Accel. Prog. Rep. 50 (2017)

First direct mass measurements of mendelevium and einsteiniumwith an MRTOF mass spectrograph
Y. Ito,∗1 M. Wada,∗1,∗2 P. Schury,∗2 M. Rosenbusch,∗1 D. Kaji,∗1 K. Morimoto,∗1 H. Haba,∗1 Y. Hirayama,∗2S. Kimura,∗3 H. Koura,∗1,∗4 M. MacCormick,∗5 H. Miyatake,∗2 J.Y. Moon,∗2,∗6 K. Morita,∗1,∗7 M. Mukai,∗3I. Murray,∗1,∗5 T. Niwase,∗7 K. Okada,∗8 A. Ozawa,∗1,∗3 M. Reponen,∗1 A. Takamine,∗1 T. Tanaka,∗1,∗7H. Wollnik,∗9 S. Yamaki,∗1,∗10 and Y.X. Watanabe∗2
trapolated masses. By this work, the masses of 246 Es
and 249,250,252 Md were directly determined for the ﬁrst
time, and the uncertainties of 246 Es and 250 Md werereduced by 60%. Combining the direct masses of249,250
Md with α decay Q-values of decay parents, wecould determine masses of 7 heavier nuclei, up to meitnerium, for the ﬁrst time.
251Md++

5

10Counts / 4 ns

15

20

Precision mass measurements of trans-uranium nuclei, which provide a direct measure of the nuclear
binding energy, are invaluable for nuclear structure
study in heavy nuclear systems and have the potential
for unique identiﬁcation of atomic nuclides. Thus far,masses of only 6 nuclei of nobelium and lawrencium
were directly measured with the Penning trap mass
spectrometer1,2) . Recently, we implemented a multireﬂection time-of-ﬂight mass spectrograph (MRTOF)
located after a cryogenic helium gas cell coupled withthe gas-ﬁlled recoil ion separator GARIS-II3) and performed direct mass measurements of mendelevium andeinsteinium isotopes for the ﬁrst time.
We produced fusion-evaporation residues (ER) of254
No/249−251 Md via 208 Pb/nat Tl(48 Ca, xn) reactions
and 252 Md/246 Es via 238 U/232 Th(19 F, 5n) reactions.
The ER were transported into the gas cell throughGARIS-II and converted to low-energy radioactive ion(RI) beams. All isotopes were extracted as doubly
charged atomic ions from the cold gas cell at 130150 K. The extracted ions were transferred to theMRTOF, which can analyze the ions with a mass resolving power of Rm ∼ 140, 000. The MRTOF measurements were performed with a concomitant referencing method4) , wherein RI measurements and reference measurements were made sequentially in eachcycle of 15 ms. The reference ions of 133 Cs+ provided from a thermal ion source were used for bothtime-of-ﬂight drift compensation and mass reference.
Drift-compensated spectra were ﬁtted by an exponentially modiﬁed Gaussian function5) (Fig. 1). Usingtimes-of-ﬂight for RI and reference ions, we determined
the masses using a single-reference analysis method6)
and computed mass excesses and mass deviations fromAME20127) (Fig. 2). All masses are in good agreement with AME2012 within uncertainty even for ex-0red- 2: 0.9
56638155663855

566389556639355663975

5664015

Time of Flight (ns)Fig. 1. Time-of-ﬂight spectrum of 251 Md++ and exponentially modiﬁed Gaussian ﬁt

600

AME2012MRTOF

MMRTOF - MAME2012 (keV)

500
400
300
200
100
0-100-200-300-400Isotope:Counts:2461
Es#2492
Md#250Md
3 #32

8292514Md# 125252Md
5 #2546
No33313

Fig. 2. Preliminary evaluation of mass deviations from theAME2012. The solid lines mark the uncertainty of theAME2012 values and # on element symbols indicate
extrapolated values.

∗1RIKEN Nishina Center
Wako Nuclear Science Center (WNSC), Institute of Particle and Nuclear Studies (IPNS), High Energy Accelerator
Research Organization (KEK)∗3 Department of Physics, University of Tsukuba∗4 Advanced Science Research Center, Japan Atomic Energy
Research Institute (JAEA)∗5 Institut de Physique Nucléaire, Orsay∗6 Institute for Basic Science∗7 Department of Physics, Kyushu University∗8 Department of Physics, Sophia University∗9 Department of Chemistry and Biochemistry, New Mexico
State University∗10 Department of Physics, Saitama University∗2References
1) M. Block et al., Nature 463, 785 (2010).
2) E.M. Ramirez et al., Science 337, 1207 (2012).
3) P. Schury et al., Phys. Rev. C 95, 011305(R) (2017).
4) M. Wada et al., Patent No. 016-037679 (29th Feb. 2016).
5) M.J. Koskelo et al., Comp. Phys. Commun. 24, 11 (1981).
6) Y. Ito et al., Phys. Rev. C 88, 011306 (2013).
7) G. Audi et al., Chin. Phys. C 36, 1287 (2012).

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