RIKEN Accel. Prog. Rep. 48 (2015)

Ⅱ-9. Instrumentation

Advanced development of GARIS-II using He-H2 mixture
as a filled gas toward the study of superheavy element
D. Kaji,∗1 K. Morimoto,∗1 H. Haba,∗1 Y. Wakabayashi,∗1 M. Takeyama,∗1,∗2 S. Yamaki,∗1,∗3 K. Tanaka,∗1,∗4and K. Morita∗1,∗5∗1
∗2
∗3
∗4
∗5

RIKEN Nishina Center
Department of Physics, Yamagata University
Graduate School of Science, Tokyo University of Science
Department of Physics, Saitama University
Department of Physics, Kyushu Universitylish a suitable condition to study SHE using the He-H2mixture, further investigation is in-progress.

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Fig. 1. Yield curve of 214 Th as a function of magnetic rigidity for various He-H2 mixture gases (○:pure He, ▽:10%H2 , □:20% H2 , △:36% H2 ). Each solid curve is a Gaussian function fitted to data points.

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Performance of a gas-filled recoil ion separator(GARIS-II) has been investigated using various asymmetric fusion reactions1–5) . The studies have indicatedthat the separator has a large transmission under alow-background condition in comparison with GARIS
and that the separation of unwanted particles is improved using He-H2 mixture as a filled gas. The lowbackground condition is very important for identifying
superheavy elements (SHE) produced with a low crosssection of pb-order. Therefore, the usefulness of He-H2
mixture as a filled gas toward the study of SHE was investigated further in this work. As a typical example,results for 214 Th, which was produced via the reactionof 197 Au(23 Na,6n), are given here.
The reaction products of 214 Th were separated inflight from projectiles and other by-products using
GARIS-II, and then they were guided into a double
sided silicon detector after passing through a time-offlight detector1–3) . The separator was filled with He-H2
mixture gases with various H2 mixing ratios (0, 10, 20,and 36%). The gas pressure was maintained 47 Pa.
The yields of 214 Th, which was assigned from an
α-transition of 7.678 MeV, were measured by varying
the fraction of H2 composition from 0 to 36% as shownin Fig. 1. Each yield is plotted against the magnetic
rigidity Bρ. The optimum Bρ value was determinedby fitting to the data points using a Gaussian function.
The optimum Bρ value was shifted from 1.711 to 1.821
Tm by increasing the mixing ratio of H2 from 0 to 36%,
and the yields of 214 Th were enhanced 1.43 times.
The shift of the optimum Bρ value implies that theequilibrium charge state q of recoil ions moving in afilled gas becomes small. The q, which was deduced
from the optimum Bρ values, are plotted against themixing ratio of H2 in Fig. 2. The q was decreased
with increasing H2 composition. The q
in pure H2
can be estimated to be 3.58 using empirical systematics, which was obtained using a Dubna gas-filled recoil
separator DGFRS6) . Interpolated values of q between
4.28 and 3.58 in the case of pure He and H2 are indicated as a broken line in Fig. 2. The interpolation wellagrees with the obtained q values using various mixing ratios within an error bar. On the other hand, thetransmission is improved with increasing the mixing
ratio of H2 , although the width parameter ∆Bρ/Bρ
becomes slightly worse from 8.4% to 9.4%. To estab-
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References
1) D. Kaji et al.: J. Radioanal. Nucl. Chem. 303, p.1523(2015).
2) D. Kaji et al.: JPS Conf. Proc. 1, p.013051 (2014).
3) D. Kaji et al.: Nucl. Instrum. Methods B 317, p.311(2013).
4) D. Kaji et al.: RIKEN Accel. Prog. Rep 47, (2014) [Inprint].
5) D. Kaji et al.: RIKEN Accel. Prog. Rep 46, p.189(2013).
6) Yu. Ts. Ogganessian et al.: Phys. Rev. C64, p.064309(2001).

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