Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 50 (2017)

Measurement of the activation of helium gas stripper by 238U beam irradiation
at 11 A MeV
A. Akashio,䠆1 K. Tanaka,䠆1 H. Imao,䠆1 and Y. Uwamino䠆1
The activation of the helium gas stripper setup1) for
uranium beam was evaluated. At RIBF, a charge stripper ofhelium gas has been applied for the uranium beam as a
substitute for one of solid material to avoid the limited
lifetime of solid material charge stripper1). However, theresidual dose around the setup is a serious problem for
maintenance. Radiation properties such as the strong source
point and lifetime of radioactive nuclei were obtained inthis study. The activation around the gas stripper setup wasmeasured using activation samples. The activation wasevaluated using the PHITS Monte-Carlo simulation code2)
and the result was compared with the measured one.
A uranium beam with an energy of 10.75 MeV/u andintensity of 0.86 particle PA irradiated and passed throughthe helium gas cell. The helium-gas pressure in the regionof highest pressure was 7 kPa, and the thickness of thehelium layer was about 500 mm, which corresponds to 0.7
mg/cm2. The helium gas was irradiated with 5.4 × 1018
uranium ions (20% accuracy) during about 18 days of theoperation of the accelerator in 2015.
The activation samples were located inside the gas
stripper chamber. The aluminum and bismuth samples were
10 × 10 mm in size and 1 mm thick. Figure 1 shows thesetup of the helium stripper by PHITS. The distance of thesamples from the beam axis was 47.25 mm. After the beamirradiation on the helium gas, the J-rays from the samples
were measured using a Ge detector after about three monthsof cooling. In addition, the several source RIs in thesamples were identified and the number of RI nuclei werecounted. The identified nuclei are listed in Table 1.

Fig.1 Flux of the nuclei (upper) and neutrons (lower)
calculated by PHITS.

䠆1

RIKEN Nishina Center

The calculation was performed using PHITS ver. 2.761)
for the geometry shown in Fig. 1, which shows the flux ofthe generated nuclei and neutrons. The fission products
were generated from uranium beam nuclei. Part of thefission products reached and were stopped at the samples.
The number of generated RIs at the samples were counted
and compared with the observed number of RIs. Figure 2
shows the preliminary ratios of the number of nuclei
calculated by PHITS to the observed number of nuclei atthe samples. The errors in the plots arise mainly from thestatistics of the J-ray and PHITS calculations and the 20%
accuracy of the beam intensity. The PHITS result for thefission products of uranium beam irradiation on helium gas
reproduced well the observed nuclei. These nuclides mainly
concentrate the downward interior of the helium stripperchamber.
Table 1. Identified nuclei of fission products in thealuminum sample after about three months of cooling.
NuclideHalf-life
NuclideHalf-life
91
134
Y
58.5 dayCs
2.06 year
95
137
Zr
64.0 dayCs
30.1 year
103
141
Ru
39.3 dayCe
32.5 day106
144
Ru
371 dayCe
285 day124
155
Sb
60.2 dayEu
4.75 year125
Sb
2.76 year

Fig. 2. Ratios of calculated fission products to theobserved fission products.
References
1) H. Imao et al. Proc. of IPAC2013, Shanghai, China, May 2013
THPWO038, p.3851 (2013).
2) D. Sato et al., Nucl. Inst. Meth. A, 583, 507 (2007).

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