Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 49 (2016)

Study on background suppression of charged particles
using GARIS-II ﬁlled with He-H2 mixture
D. Kaji,∗1 K. Morimoto,∗1 S. Yamaki,∗1,∗2 H. Haba,∗1 Y. Komori,∗1 S. Yano,∗1
R. Aono,∗1,∗3 Y. Namba,∗1,∗3 and S. Goto∗1,∗3
are clearly identiﬁable with an increasing mixing ratio.
The values of the equilibrium charge state q, which are
deduced from the optimum magnetic rigidity Bρ values, are plotted against the H2 composition in Fig. 3.
The q in pure H2 is estimated to be 3.80 using empirical systematics, obtained using a Dubna gas-ﬁlled
recoil separator DGFRS4) . Interpolated values of q between 4.47 and 3.80 in the case of pure He and H2 areindicated as a broken line in Fig. 3. It seems that themeasured q values agree well with the linear interpolation of the DGFRS.

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Fig. 1. New gas control system for GARIS-II ﬁlled with
He-H2 mixture.
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Fig. 3. Equilibrium charge state of 217 Pa ions moving in a
He-H2 mixture. The broken line is the linear interpolation between the experimentally obtained q of 4.47 andthe estimated q of 3.80 from the DGFRS’s work4) .

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Fig. 2. Two-dimensional scatter plots, obtained by a timeposition correlation analysis, of decay time against decay energy. Fractions of H2 composition are (a) 0%, (b)10%, and (c) 30%.

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The performance of a gas-ﬁlled recoil ion separator(GARIS-II) has been evaluated using various asymmetric fusion reactions1–3) . The feasibility of a hightransmission under a low-background condition is akey issue for superheavy elements (SHEs) producedwith a low cross section of pb-order. In previouswork1) , it was found that GARIS-II ﬁlled with a He-H2
mixture as a ﬁlled gas is promising to suppress background particles. To aid future study of SHEs, theusefulness of a He-H2 mixture was investigated further in this work. As a typical example, the resultsfor 218,217 Pa, which were produced via the reaction of197
Au(24 Mg,xn) [x=3,4], are given here.
The products of 218,217 Pa were separated in-ﬂight
from projectiles and other by-products using theGARIS-II, and then they were guided into a double
sided silicon detector after passing through a time-ofﬂight detector. The separator was ﬁlled with a He-H2
mixture with various H2 mixing ratios (0, 10, 20, and30%). The gas pressure was maintained at 53 Pa. Recently, a new gas-mixing system, shown in Fig. 1, wasdeveloped as the previous system used a commercial
gas with ﬁxed mixing ratio. This system enables themixing ratio to be tuned under constant pressure. Thesystem was well calibrated by a gas analyzer.
The reaction products of 218,217 Pa, which were assigned to α-transitions of 9.616 and 8.337 MeV withhalf-lives of 113 μs and 3.8 ms respectively as shownin Fig. 2, were measured by varying the fraction ofH2 composition from 0 to 30%. The reaction productsof 217,218 Pa including long-lived isotopes of 215,214 Ac

RIKEN Nishina Center
Department of Physics, Saitama University
Graduate school of Science and Technology, Niigata University- 182 -1) D. Kaji et al., RIKEN Accel. Prog. Rep 48, 214 (2015).
2) D. Kaji et al., RIKEN Accel. Prog. Rep 47, 213 (2014).
3) D. Kaji et al., RIKEN Accel. Prog. Rep 46, 189 (2013).
4) Y.T. Ogganessian et al., Phys. Rev. C64, 064309 (2001).