RIKEN Accel. Prog. Rep. 50 (2017)

Ⅱ-1. Nuclear Physics

Direct high-precision mass measurements of octupole-deformednuclei with 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,∗2
S. 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∗2uated by α decay Q-values along the fast multiple α
decay to 207 Pb. We guess that some α decay goes
through an unknown excited state or mis-identiﬁcationof α peak. Using the measured masses, the S2n for
223,224Th become larger than AME2012 and enhanced
a kink at N = 133 (Fig. 2). This may indicate anenhancement of octupole deformation in this region.
104103Counts / 2 ns

Atomic mass is a global property that reﬂects the net
result of all interactions in the atom. Information concerning nuclear structure can be derived by examining
the binding energies and is particularly important for
nuclear structure studies. The evolution as a functionof proton or neutron number, often used the trend of
two neutron separation energy (S2n ), provides information about even microscopic eﬀects such as nuclear
deformation. Theoretically, Möller et al.1) showed that
octupole contributions to the binding energy and region of octupole deformation in heavy nuclei are distributed around Z = 85–94 and N = 130–137.
We recently performed direct mass measurementsof 223,224 Th and 224 Pa, which are located in the region. The radioactive ions (RI) were produced via afusion-evaporation reaction of 208 Pb/209 Bi(18 O, xn) attargeta bombarding energy of Elab= 86 MeV and transported by the gas-ﬁlled recoil ion separator GARISII. The RI were then transported to a multi-reﬂection
time-of-ﬂight mass spectrograph (MRTOF) after being thermalized in a cryogenic helium gas cell at 90 K.
The MRTOF measurements were performed with aconcomitant referencing method2) and the reference
ions of 133 Cs+ were used for both time-of-ﬂight driftcompensation and mass reference. By cooling thegas cell at 90 K, contaminant molecular ions weregreatly suppressed; therefore, we could obtain quite aclean spectrum with 223,224 Th16 O++ and 224 Pa16 O++(Fig. 1). Using times-of-ﬂight for the RI and reference
ions, we determined the masses using a single-reference
analysis method3) and computed mass excesses andmass deviations from AME2012.4) The mass deviations
were computed to be Δm(223 Th) = -124(24) keV/c2 ,Δm(224 Th) = -130(100) keV/c2 , and Δm(224 Pa) = 36(35) keV/c2 , respectively. We found a signiﬁcantly
large deviation for 223 Th. The mass of 223 Th was eval-

102101

100

N = 223 laps, T GC = 90 K
m/q = 119.5m/q = 120

133Cs+ (ref.)(switching noise)(detector B.G.)
8.4

8.5

8.6

8.7

8.8

8.9

9.0

9.1

9.2

Time-of-Flight (ms)
160

50
45140

223Th16O++

4035

Counts / 2 ns

Counts / 2 ns

120
100
8060

224Pa16O++

30
25
20
1540

10

224Th16O++

20

50-1500-150-100-50050

Time-of-Flight − 8497423 (ns)100

150-100-50C3H6NO2S+
050

100

150

Time-of-Flight − 8515298 (ns)Fig. 1. (top) Full-range time-of-ﬂight spectrum at 223 laps.
RI and reference peaks are only seen on a low-rate detector background. (bottom) Enlarged spectra in them/q = 119.5 and 120 regions with a Gaussian ﬁt.

∗1

RIKEN 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∗2Fig. 2. S2n plot in the region of this work. Black squares
show AME2012 and red circles show MRTOF data.

References
1) P. Moller et al., Atom. Data Nucl. Data Tab. 109, 1 (2016).
2) M. Wada, P. Schury, Y. Ito, and H. Wollnik, Japanese PatentNo. 2016-037679 (29th February 2016).
3) Y. Ito et al., Phys. Rev. C 88, 011306 (2013).
4) G. Audi et al., Chin. Phys. C 36, 1287 (2012).

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