Ⅰ. HIGHLIGHTS OF THE YEARRIKEN Accel. Prog. Rep. 51 (2018)

Doughnut-shaped gas cell for KEK Isotope Separation System†Y. Hirayama,∗1 Y.X. Watanabe,∗1 M. Mukai,∗2,∗3,∗1 M. Oyaizu,∗1 M. Ahmed,∗1,∗3 H. Ishiyama,∗4 S.C. Jeong,∗4Y. Kakiguchi,∗1 S. Kimura,∗2,∗3 J.Y. Moon,∗4 J.H. Park,∗4 P. Schury,∗1 M. Wada,∗1,∗2 and H. Miyatake∗1
We have developed the KEK Isotope Separation
System (KISS)1) to study the β-decay properties ofneutron-rich isotopes with neutron numbers around N= 126 for astrophysics research.2) KISS uses a laser
ion source to produce pure low-energy ion beams ofneutron-rich isotopes in the region around N = 126,
which are produced in multi-nucleon transfer reactions
by impinging a stable 136 Xe beam with an energy of approximately 10 MeV/nucleon on a 198 Pt target.3) The
extraction eﬃciency from the laser ion source, namely
an argon gas cell, was as low as 0.01%. The low extraction eﬃciency stems from the plasma induced by theprimary beam injection into the gas cell, which is believed to reduce the ionization eﬃciency and selectivityof the laser ionized atoms.
It is straightforward to increase the production
yields of unstable nuclei by increasing the primarybeam intensity. However, the plasma eﬀect obstructs
the increase of beam intensity. To overcome the difﬁculty, we have developed a doughnut-shaped argon
gas cell with a rotating target. Figure 1 shows aschematic 3D view of the doughnut-shaped gas cell.
The doughnut-shaped gas cell has an aperture fortransporting the primary beam without entering thegas cell and a large window (polyimide foil 5 µmin thickness) for implanting the target-like fragments(TLFs) recoiling out of a rotating 198 Pt target. Owing to the characteristic large emission angles of TLFs,
the TLFs could be injected into the gas cell with higheﬃciency.
To study the performance of the doughnut-shaped
gas cell with a rotating target, we performed
on-line experiments using a 136 Xe20+ beam with10.75 MeV/nucleon and a maximum intensity of
60 pnA. Figure 2 shows the measured extraction yieldof 199 Pt+ as a function of primary beam intensity. Theblack square and red circles in Fig. 2 indicate the measured extraction yields with the use of the old and newgas cells, respectively. The extraction yields were approximately 20 pps at 20 pnA with the use of the oldgas cell and approximately 260 pps at 50 pnA withthe use of the new doughnut-shaped gas cell. Theseresults clearly indicate that the doughnut-shaped gas
cell increased the extraction eﬃciency by suppressing
†∗1

∗2

∗3
∗4

Gas inlet

136

Xe beam

0.50
0.44
0.33

φ15 mm aperturefor primary beam

0.22
0.11

Rotating

1980.00
Flow velocity(m/s)

Pt target

TLFs

Polyimide foil
φ90 in diameter and 5µm in thickness

Ionization
lasersGas outlet
φ1 mmFig. 1. Schematic view of the doughnut-shaped gas cell.
The colored lines indicate the calculated argon gas flow
trajectories connected to the gas outlet, based on theexact geometry. The color code indicates the calculated
velocity (m/s) of the argon gas flow. Here, the pressureof the argon gas was 88 kPa.

the plasma eﬀect. Moreover, we could increase the extraction yields by increasing the primary beam intensity up to 50 pnA owing to the new gas cell. However,
we observed a decrease of the extraction yield as theprimary beam intensity was increased beyond 60 pnA.
This result indicates that we could not completely suppress the plasma eﬀect, and further improvements arerequired to the KISS gas cell system. This will be thefocus of our future research.

Fig. 2. Measured extraction yields as a function of primarybeam intensity with the use of the old and new gas cells.
Statistical error bars are smaller than the symbols.

Condensed from the article in Nucl. Instr. Meth. B412, 11(2017)
Wako Nuclear Science Center (WNSC), Institute of Particle and Nuclear Studies (IPNS), High Energy Accelerator
Research Organization (KEK)
RIKEN Nishina Center
Department of Physics, University of Tsukuba
Institute for Basic Science, Rare Isotope Science Project

References
1) Y. Hirayama et al., Nucl. Instrum. Methods Phys. Res.
B 353, 4 (2015).
2) S.C. Jeong et al., KEK Report 2010-2, (2010).
3) Y.X. Watanabe et al., Phys. Rev. Lett. 115, 172503(2015).

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