Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 47 (2014)

GARIS-II commissioning #3 and #4
D. Kaji,∗1 K. Morimoto,∗1 Y. Wakabayashi,∗1 M. Takeyama,∗1,∗2 S. Yamaki,∗1,∗3 K. Tanaka,∗1,∗4 H. Haba,∗1
M. Huang,∗1 J. Kanaya,∗1 Y. Ito,∗1 H. Kikunaga,∗1,∗5 F. Tokanai,∗1,∗2 S. Goto,∗6 M. Asai,∗1,∗7and K. Morita∗1,∗8We developed a new gas-ﬁlled recoil ion separator(GARIS-II) to study asymmetric actinide-target based
fusion reactions1) . As the ﬁrst step, we measured thesolid angle of the separator oﬄine using a standard
α-source of 241 Am, and it was determined to be 18.2msr1) . As the second step, we performed online testing
to evaluate the separation capability of GARIS-II frombackground particles, and its transmission using 40 Arinduced fusion reactions. These reaction products werecollected onto a focal plane detection (FPD) systemwith high eﬃciency under extremely low backgroundconditions2) .
As the third step of commissioning #3, we performed online operating tests on GARIS-II using 22 Neinduced fusion reactions of 197 Au, 205 Tl, 208 Pb, 209 Bi,
232
Th, and 238 U. The reaction products were separated
in-ﬂight from projectiles and other by-products using
GARIS-II, and then they were guided into the FPD
system after passing through the time-of-ﬂight detector3) . The separator was ﬁlled with He gas at the pressure of 10, 33, 80, and 173 Pa. For further backgroundrejection using GARIS-II, we tested He-H2 mixture as
the ﬁlled gas at the same gas pressure. Figure 1(A)shows the intensity distribution of 215 Ac, which is produced via the 197 Au(22 Ne,4n) reaction, at FPD in thecase of ﬁlling at 33 Pa He gas and 33 Pa He-H2 mixture (He:H2 =2:1). The optimum Bρ was shifted up to11% and the transmission was increased from 11.4%
to 14.6%. The Bρ shift implies that the average equilibrium charge state of recoil ions moving in a ﬁlled
gas becomes small. The improvement of transmissionis due to a decrease in the multiple scattering between
the recoil ion and ﬁlled gas atom. Figure 1(B,C) shows
a comparison of background (BG) level at each peak ofintensity distribution between the He and the He-H2mixture. The BG level was signiﬁcantly changed, andthe beam-like particles were strongly suppressed.
As the fourth step of commissioning #4, we performed online tests on GARIS-II using 48 Ca-induced
fusion reactions of 208 Pb . We measured an excitation
function of 208 Pb(48 Ca,2n)254 No and the transmissionof GARIS-II for 254 No. The maximum transmission
was 73% assuming σ = 2.05 µb4) when the separator
∗1
∗2
∗3
∗4
∗5
∗6
∗7
∗8RIKEN Nishina Center
Department of Physics, Yamagata University
Department of Physics, Saitama University
Tokoy University of Science
Research Center for Electron Photon Science, Tohoku University
Department of Chemistry, Niigata University
Advanced Science Research Center, JAEA
Department of Physics, Kyushu University- 213 -was ﬁlled with He gas at a pressure of 73 Pa, and themagnetic rigidity Bρ was set to 2.064 Tm. The maximum transmission of GARIS-II is two times higher
than that of GARIS, which is 36%. Further, it is better
than design value of 61% for GARIS-II. Transmission
data are summarized in Fig. 2.

Fig. 1. (A) Intensity distribution of 215 Ac at FPD, (B, C)
Two-dimensional views of energy measured by Si detector vs. recoil velocity measured using the timing
counter.

Fig. 2. Transmission curve. Velocity regions of interest for
the reactions of both cold fusion and hot fusion aregiven by the blue and red stripes, respectively. ○, △:GARIS, ×: GARIS-II. Solid and dashed curves are estimated by considering multiple scattering with the filled
gas for GARIS and GARIS-II, respectively.

References
1) D. Kaji et al.: Nucl. Instrum. Methods B 317, p.311(2013).
2) D. Kaji et al.: RIKEN Accel. Prog. Rep 46, p.189(2013).
3) D. Kaji et al.: RIKEN Accel. Prog. Rep 46, p.190(2013).
4) Yu. Ts. Ogganessian et al.: Phys. Rev. C 64, p.054606(2001).