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

RIKEN Accel. Prog. Rep. 56 (2023)

Development of cryogenic helium gas cell at KISS
Y. Hirayama,∗1 M. Mukai,∗2 P. Schury,∗1 Y. X. Watanabe,∗1 S. Iimura,∗2 H. Ishiyama,∗2 S. C. Jeong,∗1H. Miyatake,∗1 T. Niwase,∗1 M. Rosenbusch,∗1 A. Takamine,∗2 A. Taniguchi,∗3 and M. Wada∗1∗1∗2∗3

Wako Nuclear Science Center (WNSC), IPNS, KEK
RIKEN Nishina center
Institute for Integrated Radiation and Nuclear Science, Kyoto University (KURNS)- 103 -Straight-type
RF carpetRF wire
carpet+Ion
source++++Exit+

MNT products
Thin window for
implanting theMNT products
136Xebeam

198

Pt rotating target

Fig. 1. Cross-sectional view of the helium gas cell. MNTproducts (blue lines) are implanted into the gas cellthrough a thin window. The ﬂow of ions toward thegas cell exit by DC ﬁeld and RFTW is shown by yellow
and white arrows, respectively.

Pt counts (cps)

400

198

We have developed a cryogenic helium gas cell toeﬃciently extract unstable nuclei produced by multinucleon transfer (MNT) reactions of 136 Xe beam impinging a 198 Pt target at the KEK Isotope Separation
System (KISS).1) Unstable nuclei implanted in sucha helium (ionization potential (IP): 24.6 eV) gas cell
can survive as singly or doubly charged ions dependingon their ionization potential. In contrast to the laser
ionization used with the present argon (IP: 15.8 eV)ﬁlled KISS gas cell, a helium gas cell oﬀers faster, more
eﬃcient, and element-independent extraction of ions.
Delivering such wide isobaric cocktail beams to the existing multi-reﬂection time-of-ﬂight mass spectrograph(MRTOF-MS) will allow for high-eﬃcacy mass measurements as the device can simultaneously analyze numerous ion species. For example, in decay studies, theMRTOF-MS could be used to provide an isobarically(or even isomerically) pure sample. Therefore, the useof a helium gas cell could provide at least one order ofmagnitude improvement in experimental eﬃcacy overthe existing argon gas cell.
Interactions of target-like and projectile-like MNTproducts and scattered primary beams, with the helium gas result in a strong plasma of He2+ and e − . Toeﬃciently transport the desired radioactive ions in sucha plasma, we manufactured a radio-frequency (RF)wire-carpet unit for use with a 4-phase RF travelingwave (RFTW) technique.2) The RF wire carpet unit
consists of 224 beryllium-copper wires (100 µm in diameter) with 600 µm pitch arranged in a plane. A
negative DC bias applied to the wire carpet accumulates ions near the wires, while a superposed 4-phase
RFTW moves the ions across the structure. Figure 1
shows a cross-sectional view of the helium gas cell installed at KISS. In the helium gas cell, a system of two
RF wire carpet units accumulates and transports ionsto a RF carpet on a Kapton substrate from which theions are extracted from the gas cell. The gas cell wascooled to T ≈ 70 K to freeze out unwanted impurities
by combining a cryo-cooler and liquid nitrogen.
We performed initial oﬄine tests to optimize the DCand RF conditions using stable 85 Rb+ emitted from analkali ion source installed in the gas cell. The performance was then conﬁrmed in online tests by extracting the MNT products implanted in the gas cell under
the dense plasma condition. We measured the extraction yield as a function of the primary beam intensity,
as shown in Fig. 2, using the MRTOF-MS to identify

300
200
100
0020

40

60

80

100

Primary beam intensity [pnA]

Fig. 2. Measured extraction yield of 198 Pt+ as a functionof the primary beam intensity.

and count 198 Pt2+ ions extracted from the gas cell.
The extraction eﬃciency from the gas cell was determined to be approximately 1%, indicating more than
one order of magnitude improvement over the argon
gas cell. Futhermore, the extraction yield increased
linearly in proportion to the primary beam intensity
up to at least 100 particle nA, in contrast with theperformance of the argon gas cell wherein yields typically saturate above 50 particle nA. In total, we expect
more than a 20-fold gain in extraction yield by using
the helium gas cell. We plan to measure the massesof unstable nuclei in the vicinity of N = 126 with thissystem in the near future.
References1) Y. Hirayama et al., Nucl. Instrum. Methods Phys. Res.
B 463, 425 (2020).
2) K. R. Lund et al., Nucl. Instrum. Methods Phys. Res.
B 463, 378 (2020).