RIKEN Accel. Prog. Rep. 50 (2017)

Ⅱ-9. Instrumentation

First cooling test of rf ion guide gas cell at the SLOWRI facility
A. Takamine,∗1 M. Rosenbusch,∗1 P. Schury,∗2 J. Y. Moon,∗3 M. Wada,∗1,∗2 H. Miyatake,∗2 Y. Ito,∗1T. Sonoda,∗1 Y. X. Watanabe,∗2 Y. Hirayama,∗2 T. Sato,∗1 T. M. Kojima,∗1 I. Katayama,∗1 H. Wollnik,∗1,∗4
and H. Ueno∗1

downstream

upstream

N2 / LN2

PT100-2
suppoter

cryo1

cryo2

77K Cryocooler(Sumitomo CH-110)

Fig. 1. Schematic of the gas cell cooling scheme.
∗1∗2∗3∗4RIKEN Nishina Center
Wako Nuclear Science Center (WNSC), IPNS, KEK
Institute for Basic Science
Department of Chemistry & BioChemistry, New Mexico
State University- 194 -Temperature [K]0Gas cell

250-50200]

PT100-1
LN2300Temperature [Molecularization is a serious problem in gas catcher
cells around the world. It is not so signiﬁcant for nuclear mass measurements but crucial for laser spectroscopy. If a gas, typically a noble gas, contains a nonnegligible amount of impurities such as water, hydrocarbons, nitrogen, and alcohol, thermalized ions havea fair chance to form molecules with impurities. Even
though molecules can be broken up via collisions withresidual gases by acceleration into the high-density region immediately after the gas-cell exit hole, molecularions have a lower transportation eﬃciency on an rf carpet. This is because the repelling force produced by rf
ﬁelds is proportional to the square of ions’ mobilities1)
which are generally much smaller for molecular ionsthan for atomic ions. Gas puriﬁcation is, therefore, important to avoid the molecularization of injected ions.
We use the highest grade G1 He gas additionallypuriﬁed through a getter-based gas puriﬁer which provides 99.9999999% purity. Moreover it is desirable thatthe gas cell is pre-cooled to freeze out impurities onto
the inner wall before catching RI beams. In order tocool down the rf ion guide gas cell at the SLOWRI
facility, the chamber has a vacuum double-wall structure for vacuum thermal insulation. We prepared two
77-K cryocoolers (Sumitomo CH-110), “cryoU” and“cryoD,” as shown in Fig. 1. The gas cell will be cooledby the cryocoolers through aluminum supporters andby a copper pipe winding around the gas cell to carryliquid nitrogen (LN2 ). An indium sheet is placed between the supporters and the gas cell for heat conduction. Since the heat capacity of the gas cell is estimatedto be as large as ∼300 kJ/K, we conducted a coolingtest although only one cryocooler, “cryoD,” was readyto use in Sep. 2016. For temperature measurementswith the four-terminal resistance measuring method,Al supporter
15001020-1003040
Time [hrs]506070Fig. 2. Preliminary cooling test result.

thermo sensors of type PT100 have been attached attwo positions: the gas-cell vessel and the Al support(see Fig. 1).
Figure 2 shows the result of the preliminary cooling test. The gas cell was pre-cooled by cryoD andthen started to be cooled with LN2 at time = 0 in theplot. We continuously kept supplying LN2 while alternately using the two LN2 tanks of 120 L and 100 Lcapacity. It was cooled down to 237 K in one day
after the start of LN2 cooling and to 233 K in 2.5
days. After stopping the cooling, the heating rate
was 0.071 K/min. We estimated the conductive heattransfer from this value combined with the estimation of the radiation heat transfer. When object 1with an outer surface area of A1 is surrounded byobject 2 with an inner surface area of A2 , the radiation heat transfer Q from 2 to 1 is expressed by
Q = σA1 (T24 − T14 )/(1/ε1 + (A1 /A2 )(1/ε2 − 1)), where
σ is the Stephan-Boltzmann constant, εi the emissivity of object i, and Ti the temperature of object i.
Assuming ε1 = ε2 = 0.1 (typical value for stainlesssteel), T1 = 290 K, T2 = 240 K, A1 = 1.73 m2 , andA2 = 6.72 m2 , Q was calculated as 0.12 kW, which
resulted in a heating rate of 0.024 K/min.. Therefore
the heating rate of the conductive heat transfer was
estimated to be 0.047 K/min, corresponding to a heattransfer of 0.24 kW, which is thought to be mainly
responsible for the heat connection to the unused cryocooler.
Now both cryocoolers are ready to use. We continue the cooling test using both cryocoolers and will
introduce many superinsulator layers between the gas
cell and the outer vessel to suppress the radiation heattransfer.
Reference1) M. Wada et al., Nucl. Instr. Meth. B 204, 570 (2003).