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

Ⅱ-9. Instrumentation

Measurement of activation around the He gas stripper at RIBF
A. Akashio,＊1 K. Tanaka,＊1 and H. Imao＊1＊1

RIKEN Nishina Center

100

Count/second/efficiency

The activation of the He gas stripper setup caused by auranium beam was evaluated. There are several types of gas
and solid stripper setup in RIBF, and a stripper setup isselected for the beam nuclide, such as uranium, xenon, andcalcium. The He gas stripper setup has been developed foruranium beam acceleration at RIBF. 1) The stripper setups
are often exchanged with other types to change the beam
nuclide of the RIBF accelerator. The radiation exposure
caused by the residual dose during the exchange andmaintenance works is a serious issue. Thus, evaluating theresidual dose and studying the origin of the dose areimportant to improve future setups. This evaluation is also
applicable for the shielding design of future facilities.
In this study, the activation method was applied for theHe gas stripper irradiated by a uranium beam. During themachine operation in the autumn of 2014, the samples wereplaced both inside and outside the stripper chamber. Afteruranium beam irradiation, the  rays emitted from thesamples were measured using a Ge detector.
The energy of the uranium beam on the stripper was
10.75 A MeV and the beam intensity in operation was 1200
particle nA. The beam irradiated for one month. The He gaspressure was 7 kPa, and the gas thickness wasapproximately 50 cm which corresponds to 0.7 mg/cm2.
The sample materials were aluminum and bismuth. The
aluminum sample is made of the same material as the gasstripper chamber. Bismuth is a useful element for neutron
measurement. Radioactive isotopes of bismuth aregenerated by neutron irradiation, and the generated isotope
nuclides depend on the neutron energy because of thethreshold energy of the reactions, as listed in table 1.
Some of the aluminum samples were set in the gas
stripper chamber where the He gas pressure was highest.
The distance between the nearest sample and the uranium
beam was 47.25 mm. The samples placed inside the stripper
chamber were hit by the nuclei of the fission products of theuranium beam. The other aluminum samples set outside thestriper chamber are sensitive to the neutron only because
the fission products stop in the chamber wall and do notreach outside. Thus, the aluminum samples are thebenchmark of the neutron activation of aluminum chamber
itself. The bismuth samples are set with the aluminum
samples outside the chamber to obtain the neutron energy
on the aluminum samples.
Figure 1 shows the measured -ray spectrum obtained
from the aluminum sample set inside the chamber after
cooling for 13.5 days. Table 2 lists the identified nuclides
with short half lives. Typical nuclides of the fission
products from the 238U beam were observed.  ray peaks
from long-lived nuclides were not observed yet in thespectrum owing to the high background  rays from theAl sample

1010.1

0500

1000

1500

2000

Energy (keV)
Fig. 1. Typical  ray spectrum of the Al sample set in thegas stripper chamber. Many energy peaks caused byradioactive isotopes generated from the fission of 238U
were observed.

short-lived nuclides. -ray peaks of aluminum itself, such as
24
Na with a 15 hour half life, were not found.
Table 1 lists the observed bismuth isotopes in the natural209
Bi samples that were set outside the stripper chamber.
The result suggests that the energy of irradiated neutrons on
the bismuth samples was up to 22.55 MeV, since  raysfrom the 205Bi nucleus, which have a threshold energy of
28.6 MeV, were not detected.
After cooling the samples for a long time and backgroundlevels become low,  rays from the long-lived nuclides will
be detected. The result will be compared with a Monte
Carlo simulation result to evaluate radiation issues in thefuture.
Table 1. Threshold of neutron energy corresponding to theproduction of radioactive bismuth isotopes observed in thisstudy.
NuclideHalf life
Reaction
Threshold
207
209
Bi
31.6 year
Bi(n,3n)207Bi 14.12
206
209
Bi
6.4 dayBi(n,4n)206Bi 22.55
Table 2. Typical radioactive nuclei observed in thealuminum sample inside the stripper chamber.
NuclideHalf life
NuclideHalf life
95
136
Zr
64 dayCs
13 day95
140
Nb35 dayBa
13 day99140
Mo
2.8 dayLa
1.7 day99m
141
Tc
6.0 hourCe
32.6 day
103147Ru
39 dayNd
11 day131I
8.0 day
References1) H. Imao et al.: Proc. of IPAC2013, Shanghai, China, May 2013
THPWO038, p.3851 (2013).

- 238 -完全版2014_本文.indd 238

15/10/16 17:57