Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 56 (2023)
RIKEN
Accel. Prog. Rep. 56 (2023)

Oﬄine measurement of 248 Cm ﬁssion products at ZD-MRTOF
J. M. Yap,∗1,∗2 A. Takamine,∗2 M. Rosenbusch,∗3 S. Chen,∗1,∗4 T. Gao,∗1,∗2 D. Hou,∗5,∗6 S. Iimura,∗2,∗7
H. Ishiyama,∗2 S. Kimura,∗2 T. M. Kojima,∗2 J. Lee,∗1 S. Nishimura,∗2 T. Niwase,∗3 P. Schury,∗3 T. Sonoda,∗2P. Vi,∗2 M. Wada,∗3 Y. X. Watanabe,∗3 and W. Xian∗1A 10-kBq 248 Cm (SF: 8.39%) ﬁssion source was installed in the gas catcher1) downstream from the ZeroDegree spectrometer to study the ion transport fromthe gas cell to the ZeroDegree Multi-reﬂection Time-ofﬂight Mass Spectrograph (ZD-MRTOF)2) in oﬄine tests,
providing pseudo conditions similar to online beam experiments. The ﬁssion source is placed between the inner gas cell and outer vessel chambers such that it canbe removed for beam-time experiments. A 2-µm Ti degrader is attached to the source to reduce the kinetic
energy of ﬁssion products and match the stopping powerof the gas cell. The degrader can also be rotated toincrease eﬀective thickness. The transport test of theﬁssion products, 104–106 Nb, Mo, Tc, and Ru, was performed shortly after installation. The stopped ﬁssion
products were extracted mostly as doubly charged ions,and high-precision mass measurements were made after
transportation to ZD-MRTOF. This measurement also
serves as an ion counter for transport eﬃciency measurements from the gas cell to ZD-MRTOF.
A degrader thickness scan was performed by measuring the doubly charged ion count rate at various degrader
angles to search for the condition of maximum stopping
eﬃciency. Figure 1 depicts the ion count rate of nine ﬁssion products. A similar peak count rate was determinedat the approximate angles of 45° and −60°. This diﬀerence in the positive and negative peak angular positions
is likely caused by the asymmetric structure of the gas
cell, such as the DC gradient ﬁeld applied to collect ionsat the RF carpet located on one side of the gas cell.
The maximum count rate of ﬁssion products was foundat a degrader rotation of approximately 45°, with an effective thickness of 2.83 µm. The stopping eﬃciency of
the gas cell in this condition was 4.9%, roughly estimatedby simulation, considering the solid angle, energy, energyspread,3) energy losses, and range in the material of theﬁssion products. Table 1 lists the observed count rates
and transport eﬃciencies of doubly charged ions. The
transport eﬃciency is obtained after correcting for thestopping eﬃciency from the total eﬃciency based on theﬁssion yields of 248 Cm provided by JAEA4) and the observed count rate. In the mass region of A = 104 − 106,
the Mo and Tc isotopes have a transport eﬃciency of approximately 12%, whereas the Nb isotopes have a relatively low eﬃciency of approximately 5%. The low trans-Fig. 1. Count rate (cph) of 104–106 Nb, Mo, and Tc within thedegrader angles of −70° to 70°. The positive angles denotecounter-clockwise rotation, towards the RF carpet.

Table 1. Observed count rate (cph) and transport eﬃciencyof doubly charged 104–106 Nb, Mo, and Tc ions at a degrader angle of 45°. The uncertainty is speciﬁed in parentheses.
Isotope
104
Nb105
Nb106
Nb104
Mo105
Mo106
Mo104
Tc
105
Tc
106
Tc

Count rate (cph)214 (3)139 (3)38 (1)
550 (5)658 (6)712 (6)34 (1)
155 (3)296 (4)

Transport eﬀ. (%)
4.7 (24)
5.3 (17)4.0 (25)
14.0 (45)11.7 (37)13.5 (43)11.1 (71)13.5 (87)10.6 (34)port eﬃciency of the Nb isotopes could be attributed tothe diﬀerent charge distribution from the slightly lower
second ionization potential.5) Another possible factor ismolecular formation of Nb isotopes with contaminantmolecules. Further investigations on Nb isotopes will
be carried out. The oﬄine transport eﬃciency measurement forms a baseline for future improvements. Twosuch upgrade plans, the addition of an RF-wire curtain
and modiﬁcation of the RF carpet scheme from the RF+ 4-phase AF to 4-phase RF, will be tested with theﬁssion source.
References

∗1
∗2
∗3
∗4
∗5
∗6

∗7

Department of Physics, University of Hong Kong
RIKEN Nishina Center
Wako Nuclear Science Center (WNSC), IPNS, KEK
School of Physics, Engineering and Technology, University ofYork
Institute of Modern Physics, Chinese Academy of Sciences
School of Nuclear Science and Technology, Lanzhou University
Department of Physics, College of Science, Rikkyo University

1) A. Takamine et al., RIKEN Accel. Prog. Rep. 53, 108(2019).
2) M. Rosenbusch et al., Nucl. Instrum. Methods Phys. Res.
A 1047, 167824 (2023).
3) A. Benoufella et al., Nucl. Phys. A 565, 563 (1993).
4) J. Katakura et al., EPJ Web Conf. 111, 08004 (2016).
5) A. Takamine et al., in this progress report.

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