Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 53

Mass measurements of 252 Cf fission fragments via MRTOF-MS
S. Kimura,∗1 M. Wada,∗2 H. Haba,∗1 S. Ishizawa,∗1,∗3,∗2 T. Niwase,∗1,∗4,∗2 M. Rosenbusch,∗2 and P. Schury∗2for the SHE-Mass Collaboration

Z60

2 2 21 1 11 1155

22 222 21 11 1122 222 22 2122222222222222222 222 22 22 22 22 2 22 2 22 2 2250
80

8590
N9510010510610710810910101011∗2∗3∗4240 laps
152Pr++Counts / 3.2 ns bin

1401201008060152Ce++

07277400

7277600
TOF ( ns )Counts / 3.2 ns bin

7277800

241 laps

152Nd++

50

152Pr++

4030
20152Ce++

1007307200

7307400
TOF ( ns )Fig. 2. TOF spectra of doubly charged A = 152 series. The
red line indicates a fit obtained using an exponentialGaussian hybrid function.3)
1.5

Exp(AME16)
FRDM12KTUY05
HFB24
WS4
Present study

10.5
0-0.5
-1-1.5
848688909294
Neutron number

9698

100Fig. 3. Neutron-number dependence of cerium-isotope
mass excess. Each line indicates a theoretical prediction. In the region of N > 93, the zero line corresponds
to the extrapolated masses listed in AME16.

determined by a single reference method.3) A typicalmeasurement achieved a relative precision of δm/m ∼10−7 . Figure 2 shows TOF spectra of the doubly
charged A = 152 series with diﬀerent numbers of laps.
As the relative position and intensity remain constantwhen the number of laps is varied, we can infer that thethree peaks are isobaric. In this case, 152 Nd was used
as a mass reference, and we experimentally determine
the mass of 152 Ce for the ﬁrst time. The measured
mass excess of 152 Ce is M E = −58845(61) keV andindicated in Fig. 3 with several theoretical predictions.
This result shows that the ground state of 152 Ce is lessbound than all the predictions listed in Fig. 3.

Fig. 1. Location of the observed isotopes on the nuclear
chart. The numbers, 1 or 2, indicate the observed
charge state. The color bar represents the precisionof experimental mass data taken from the 2016 Atomic
Mass Evaluation (AME16).
∗1152Nd++

180
160

4020

Mass Precision

65

220
200

ME-MEAME16 ( MeV )The rapid neutron-capture (r-) process path is governed by the balance between the β − -decay rate andneutron-capture rate. Masses of nuclei involved in ther-process path play a crucial role in determining theﬁnal isotopic abundance. It is diﬃcult to access ther-process nuclei due to their locations far from stability; thus, theoretical predictions are needed to obtain the ﬁnal isotopic abundances. However, in theregion where there are no experimental data on nuclear masses, the predicted mass values show a largevariance.1) Thus, accurate and precise mass data ofneutron-rich nuclei are required.
Here, we report mass measurements of the neutronrich nuclei produced by the spontaneous ﬁssion 252 Cf
with multi-reﬂection time-of-ﬂight mass spectroscopy(MRTOF-MS).2) The MRTOF-MS system consistsof a cryogenic gas cell (GC) with a 0.5 µm Mylar
window, an ion transport system having two linear
RF quadrupole traps, and an ejection system to theMRTOF device. A 350 kBq 252 Cf source (SF: 3.09%)
with 6 µm Ti degrader was installed in front of the GC
window. Throughout the measurements, the GC wascryogenically cooled blow 70 K while the He gas pressure was regulated to maintain a density equivalent to150 mbar at room temperature.
The measurement results are summarized in Fig. 1.
In the present study, we observed 78 isotopes fromiodine (Z = 53) to europium (Z = 63) in the TOF
spectra. The masses of the observed isotopes wereReferences

1) M. R. Mumpower et al., Prog. Part. Nucl. Phys. 86, 86(2016).
2) P. Schury et al., Nucl. Instrum. Methods Phys. Res. B
335, 39 (2014).
3) Y. Ito et al., Phys. Rev. C 88, 011306(R) (2013).

RIKEN Nishina Center
WNSC, IPNS, KEK
Department of Physics, Yamagata University
Department of Physics, Kyushu University- 52 -