Ⅱ-9. Instrumentation
RIKEN
Accel. Prog. Rep. 56 (2023)

RIKEN Accel. Prog. Rep. 56 (2023)

Improvement of gas inlet system for FRAC at SCRIT facility
S. Iimura,∗1,∗2 T. Ohnishi,∗2 T. Yamano,∗1 Y. Abe,∗2 K. Kurita,∗1 R. Ogawara,∗3,∗2 M. Wakasugi,∗3,∗2M. Watanabe,∗2 and H. Wauke∗4,∗2
At the SCRIT facility,1) we have been studying thecharge density distribution of unstable nuclei using theelectron scattering method. To perform electron scattering with radioactive isotopes at a low production
rate, it is necessary to convert the ions generated continuously by the ISOL-type ion separator, the electronbeam-driven RI separator for SCRIT (ERIS),2) toa pulsed beam with an eﬃciency as high as possible. A fringing-RF-ﬁeld-activated dc-to-pulse converter (FRAC)3) is a gas-ﬁlled beam cooler buncher
developed for this purpose and achieves an accumulation of 107 ions with an eﬃciency of approximately
90%.4,5) However, a decrease was observed in the accumulation eﬃciency with a longer stacking time (seethe blue squares in Fig. 2), and there were concernsthat contaminants with small ionization potential in
the noble gas could neutralize and molecularize thetarget ions. Therefore, a buﬀer gas puriﬁcation system was introduced in the FRAC.
Figure 1 presents a schematic of the newly modiﬁed
gas inlet system. Neon gas was puriﬁed to the ppb
level or less of impurities using a ﬁlter (IG-105-200630,ARM Puriﬁcation) and ﬂowed into the FRAC, where
the gas pressure was controlled to ∼10−3 Pa throughdiﬀerential pumping. A further improvement was theinstallation of a variable leak valve (951-7170, CanonANELVA) instead of the needle valves. This valve enabled microﬂow control and signiﬁcantly contributed
to optimizing the gas pressure in the FRAC. Following
assembly, all the gas lines were evacuated and baked at110◦ C for nearly a day to eliminate as much residual

 

      0.9

∗2
∗3
∗4

without purifier  with purifier   110  100

 

1000

Fig. 2. Dependence of stacking time on FRAC eﬃciency
with (red solid circles) and without (blue solid squares)
puriﬁer.

This study has established the signiﬁcance of buﬀer
gas purity, and the possibility of longer accumulation
times and more eﬃcient cooling at higher pressures
has indicated the possibility of the electron scatteringof exotic nuclei with small production yields in thefuture.

Fig. 1. Schematic of the gas inlet system before (blueframe) and after (red frame) modiﬁcation. Neon gas
was regulated at 0.11 MPa before the variable leakvalve, and the microﬂow rate was controlled and introduced downstream of the valve to the FRAC.
∗10.70.5

    0.80.6

 1   impurity as possible.
Oﬄine experiments were performed to evaluate theimproved eﬃciency. The 132 Xe+ ion beam deliveredfrom ERIS was injected into the FRAC for 0.4 ms at
1 Hz (∼107 ions/cycle), trapped and cooled (for 1–1000 ms), and subsequently ejected into a Faraday cup
located downstream of the FRAC to measure the ioncurrent. The FRAC was operated with a 1.8 MHz,
1 kVpp RF ﬁeld and a DC potential of depth 30 V along
the beam axis at a neon gas pressure of 2.0 × 10−4 Pa.
Figure 2 presents the oﬄine test results of the stacking time dependence of the accumulation eﬃciency
with and without the puriﬁer. The eﬃciencies weredeﬁned as the ratio of the ion current to that without
stacking in the FRAC. Before the improvement, the efﬁciency started decreasing after 10 ms of stacking anddropped to approximately 66% at 500 ms, whereas theintroduction of gas puriﬁcation enabled the eﬃciency
to be maintained at over 90% for more than 1 s.

References
1) M. Wakasugi et al., Nucl. Instrum. Methods Phys. Res.
B 317, 668 (2013).
2) T. Ohnishi et al., Nucl. Instrum. Methods Phys. Res. B
317, 357 (2013).
3) M. Wakasugi et al., Rev. Sci. Instrum. 89, 095107(2018).
4) S. Sato et al., RIKEN Accel. Prog. Rep. 52, 144 (2019).
5) S. Sato et al., RIKEN Accel. Prog. Rep. 53, 111 (2020).

Department of Physics, Rikkyo University
RIKEN Nishina Center
Institute for Chemical Research, Kyoto University
Research Center for Electron-Photon Science, Tohoku University- 89 -