Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 53

Yield development with in-gas-jet laser ionization at KISS
Y. X. Watanabe,∗1 Y. Hirayama,∗1 H. S. Choi,∗2 M. Mukai,∗3 H. Miyatake,∗1 S. Iimura,∗4,∗5 M. Oyaizu,∗1Y. Kakiguchi,∗1 N. Ishiyama,∗5 M. Wada,∗1 P. Schury,∗1 M. Rosenbusch,∗1 A. Taniguchi,∗6 J. H. Park,∗7 andT. T. Yeung∗8,∗5
We are developing the KEK Isotope Separation System (KISS)1) to perform experimental studies suchas lifetime measurements, mass measurements, β-γ
spectroscopy, and laser spectroscopy for neutron-richnuclei around the closed neutron shell N = 126,
which is relevant to r-process nucleosynthesis. KISS
is an argon-gas-cell-based laser ion source, in whichthese neutron-rich nuclei are produced by multinucleon
transfer (MNT) reactions between a 136 Xe beam and198
Pt target.2) The beam is provided by RILAC2 +RRC with an energy of 9.4 MeV/nucleon on the target, which had a thickness of 12.5 mg/cm2 . The reaction products ejected from the target enter the gas
cell through a 5-µm polyimide window placed 10 mm
downstream from the target. The gas cell is ﬁlled with
1-atm argon gas, which thermalizes and neutralizes injected reaction products. They are transported to theexit of the gas cell by laminar gas ﬂow. In or out of thegas cell, these neutral atoms are irradiated by lasers forlaser resonance ionization. Two lasers are used: one isfor excitation, while the other is for ionization. One
speciﬁc element can be selected for ionization by tuning the wavelength of the excitation laser. These ions
are transported by radio frequency (RF) ion guides andaccelerated by a high voltage of 20 kV followed by adipole electromagnet for mass separation.
Since the neutron-rich nuclei of interest around N =126 are produced in rare channels of MNT reactions,
their eﬃcient extraction is essential for their measurements. Thus far, in-gas-cell laser ionization has been
used, in which two lasers are irradiated into the gascell. Recently, KU Leuven group demonstrated in-gasjet laser ionization with eﬃciency more than 10 times
that of in-gas-cell ionization.3) In-gas-jet laser ionization uses two lasers, which are irradiated on a gas jet
formed by a de Laval nozzle attached at the exit of
the gas cell. Low-temperature and low-pressure conditions in the supersonic jet could reduce the loss of ionscaused by their collisions with dense gas atoms. For
yield development at KISS, we introduced an in-gas-jet
∗1

∗2
∗3
∗4
∗5
∗6
∗7
∗8Wako Nuclear Science Center (WNSC), Institute of Particle and Nuclear Studies (IPNS), High Energy Accelerator
Research Organization (KEK)
Department of Physics and Astronomy, Seoul National University
Department of Physics, University of Tsukuba
Department of Physics, Osaka University
RIKEN Nishina Center
Institute for Integrated Radiation and Nuclear Science, Kyoto University (KURNS)
Rare Isotope Science Project (RISP), Institute for Basic Science (IBS)
Department of Physics, The University of Hong Kong

- 114 -Fig. 1. KISS setup for in-gas-jet laser ionization.

laser ionization setup, as shown in Fig. 1. In order tosuﬃciently irradiate two lasers on the neutral atoms
of interest in the supersonic jet, the overlap among
them are optimized by arranging the excitation laser
collinearly with the gas jet, and the ionization laser
is perpendicularly reﬂected several times by two mirrors. An S-shaped RF quadrupole ion guide (SRFQ)
was introduced to enable the collinear injection of theexcitation laser. The SRFQ was segmented along theion transportation to apply an increasing electric potential from the upstream to the downstream because
a simulation with the SIMION code for the ion transportation indicated that ions stopped in the absenceof such an electric-potential gradient. The on-line experiment did not show increased extraction yields within-gas-jet laser ionization. When the same SRFQ wasused with in-gas-cell ionization, the count rates of extracted ions were the same between extraction withand without the electric-potential gradient. This result indicates that the initial ion velocities obtained
from the gas jet are not correctly evaluated in the simulation. We will further investigate the SRFQ designfor yield development with in-gas-jet laser ionization
through a more realistic simulation.
References
1) Y. Hirayama et al., Nucl. Instrum. Methods Phys. Res.
B 412, 11 (2017).
2) Y. X. Watanabe et al., Phys. Rev. Lett. 115, 172503(2015).
3) R. Ferrer et al., Nat. Commun. 8, 14520 (2017).