II-9. Instrumentation

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Temperature and pressure dependence of ion extraction
from RF gas cell
D. Hou,∗1,∗2,∗3 A. Takamine,∗4 S. Iimura,∗4,∗5 S. Chen,∗2,∗6 Y. Hirayama,∗2 H. Ishiyama,∗4 Y. Ito,∗7 S. Kimura,∗4J. Liu,∗1 H. Miyatake,∗2 S. Nishimura,∗4 M. Rosenbusch,∗2 P. Schury,∗2 M. Wada,∗2 Y. X. Watanabe,∗2H. Wollnik,∗8 W. Xian,∗2,∗6 and S. Yan∗2,∗9The SLOWRI facility relies on gas catcher cells toemploy the radio-frequency (RF) ion guide method forthermalizing and transporting radioactive ions.1) During an online experiment, we studied the pressure andtemperature dependence of ion extraction from the gascell. Molecular impurities in the gas cell can react andexchange charge2) with the ions of interest during theirtransport, and even after extraction, the ions can be neutralized and combined with molecular impurities. Because of the relatively low mobility of molecular impurities, they decrease the overall extraction eﬃciency of thegas cell. To reduce the amounts of molecular contaminants, we used high-purity helium and a gas puriﬁcation
system and operated the gas cell at very low temperatures. At temperatures below 50 K, most of these molecular impurities are absorbed at the wall of the gas cell.
In this online test setup, secondary beams passed
through the ZeroDegree spectrometer and stopped in thegas cell. Subsequently they were extracted from the gas
cell and transported to a multi-reﬂection time-of-ﬂight(MRTOF) mass spectrograph for mass measurements.
The setup, as implemented in the commissioning runs,allowed for cooling the gas cell to ≈180 K. To avoid convoluting the stopping and extraction eﬃciencies, a pressure regulation system maintains a constant gas densityof ≈33 µg/cm3 for all temperatures. As shown in Fig. 1,Fig. 1. Rates of 135 Sb+ at diﬀerent temperatures of the gascell chamber.
∗1
∗2
∗3
∗4
∗5
∗6
∗7
∗8
∗9

Institute of Modern Physics, Chinese Academy of Sciences
Wako Nuclear Science Center (WNSC), IPNS, KEK
School of Nuclear Science and Technology, Lanzhou University
RIKEN Nishina Center
Department of Physics, Osaka University
Department of Physics, The University of Hongkong
Advanced Science Research Center, JAEA Ibaraki
Dep. of Chem. and Biochem., New Mexico State University
Institute of Mass Spectrometer and Atmospheric Environment, Jinan University

Fig. 2. Rates of 135 Sb+ and 136 Te+ observed at MRTOF as
functions of RTE gas-cell pressure.

the extraction of 135 Sb+ increases dramatically as thetemperature falls below 210 K, at which some molecular
impurities, such as H2 O, SO2 , and Cl2 , start to decrease.
As the cryogenic cooling system reaches its lower limit
before the extraction eﬃciency saturates, we intend toimplement improvements to allow better cooling prior
to the next online campaign.
By varying the gas density, we can probe the stopping
eﬃciency. Figure 2 shows the results of such a measurement, performed with the gas cell at ≈180 K, with theobserved rate of 136 Te+ and 135 Sb+ plotted as functionsof room temperature-equivalent (RTE) helium pressure.
As one might expect, the stopping fraction increased as
the gas density increased. As such, we must consider
how to safely operate at the highest possible gas density.
We used lise++ to evaluate the stopping eﬃciency inthe gas cell. With a pressure of 220 mbar RTE, 6.4%of 136 Te and 6.5% of 135 Sb should be stopped in helium gas, while decreasing the pressure to 133 mbar RTE
would result in the reduction of stopping eﬃciencies to
2.3% and 2.4%, respectively. As shown in Fig. 2, therates improved by a factor of 3.4 for 136 Te and 4.3 for
135Sb, while lise++ indicated that the stopping eﬃciencies should only increase by a factor of 2.7 from 133 mbarto 220 mbar. The diﬀerence likely results from the pressure dependence of the RF ion guide eﬃciency and ionmobility. Detailed analysis is underway to ﬁnd an optimum operation condition ahead of the next online experiment.
References

1) M. Wada et al., Nucl. Instrum. Menthods Phys. Res. B
204, 570 (2003).
2) D. J. Morrissey et al., Nucl. Instrum. Menthods Phys. Res.
B 266, 4822 (2008).

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