II-9. Instrumentation

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Improvement of evacuation time of RI from argon gas cellT. Sonoda,∗1 M. Wada,∗2 K. Hattori,∗3 H. Iimura,∗4 S. Iimura,∗1 H. Ishiyama,∗1 I. Katayama,∗1 T. M. Kojima,∗1M. Rosenbusch,∗2 V. Sonnenschein,∗3 R. Terabayashi,∗3 A. Takamine,∗1 and H. Tomita∗42e-12

An argon gas cell coupled to a resonant laser ionization is a powerful tool for the production of high-purity
low-energy RI-beam. This method utilizes neutral RI
transportation using simply a gas ﬂow, until it reaches
the location for laser ionization which is typically close
to the exit of the gas cell. A feasible RI is restricted byits half-life, which should be longer than the evacuation
time of the gas cell. The evacuation time can be determined from the conductance of a small exit aperture andgas cell volume. Under adiabatic expansion, the conductance depends only on the diameter of the exit aperture1)
as follows:
C = 0.14 × ϕ2 .

V.
C(2)

For example, when the gas cell volume is 100 cm3 andthe exit diameter is 1 mm, the evacuation time is 714 ms.
This value makes it diﬃcult to aim for very rare RI regions, where the half-lives are typically less than 100 ms.
In order to address a fast evacuation, two solutions
can be considered: (1) using a small volume for the gas
cell and (2) using a large exit aperture. However, theseimprovements are limited by the stopping eﬃciency for

Gas cell
Ar 50kPa

Lasercompressor

exit hole(>2mm )

~500Pa1Pa

10-3Pagas jetrf-carpet(exit:0.4mm

spig/-qpig

skimmer

Dry
pump1
Dry
pump2
T.M.P

Fig. 1. Proposed gas cell and diﬀerential pumping layout incombination with rf-carpet.
∗1∗2∗3∗4Ion current (A)1.6e-12RIKEN Nishina Center
Wako Nuclear Science Center (WNSC), IPNS, KEK
Faculty of Engineering, Nagoya University
Advanced Science Research Center, JAEA

I.P.=
58762.31cm-1

6p27S4P1/2

32588.17cm-1

1.4e-121.2e-12

F.C @rf-carpet

83Bi
306.86 nm

6p34S3/20cm-1

1e-12
8e-13

F.C @spig

6e-13
4e-13
2e-13
0-2e-13306.84(1)

where the units are L/s for C and mm for ϕ. The gasis argon. The evacuation time of a gas cell of volume V
can be written as:
t=355 nm (YAG355)

1.8e-12306.845306.85306.855

306.86306.865306.87306.875

306.88

Wavelength(nm)Fig. 2. Ionization spectrum by scanning ﬁrst step excitation
when rf-carpet/spig is used as Faraday cup.

high-energy RIs, which require a large volume acceptance. They are also limited by the capability of differential pumping. Here we propose installation of anrf-carpet with a very small exit aperture in the initialdiﬀerential pumping room while using a large aperture
for the gas cell exit. Figure 1 shows a schematic layout. The rf-carpet is placed after the gas cell exit. The
pressure in this room can be increased up to 1 kPa atwhich the ion extraction by the rf-carpet is still practical without a discharge problem. The ions move witha gas jet toward the rf-carpet after exiting the gas cell;subsequently they are guided by the rf- and dc-electricﬁelds to the exit of the rf-carpet. Owing to the small
exit aperture of the rf-carpet, the loads from the following diﬀerential pumping are suppressed. When the exit
aperture of the gas cell is large, the total gas throughput becomes very high, leading to an extremely high gas
consumption cost. However, by using a gas circulation
system,2) the gas consumption rate can be drasticallysaved.
A preliminary oﬀ-line experiment was conducted with
a 0.4 mm rf-carpet aperture for photo-ionized Bi ions.
The pressure at the rf-carpet was approximately 500 Pa.
Bi atoms were produced inside the gas cell, and subsequently they were ionized by the laser and extracted
from the gas cell exit (2 mmϕ). Figure 2 shows the resultof the comparison when the ions are collected on the rfcarpet and on the spig rods placed next to the rf-carpet.
We conﬁrmed a reasonable extraction eﬃciency for therf-carpet. The feasibility study is in progress.
References

1) A. Roth, Vacuum Technology (North-Holland, Amsterdam 1982).
2) T. Sonoda, et al., Rev. Sci. Instrum. 87, 065104 (2016).
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