Ⅳ. OPERATION RECORDS

RIKEN Accel. Prog. Rep. 47 (2014)

Radiation safety management at RIBF
K. Tanaka,*1 Y. Uwamino,*1 H. Sakamoto,*1 R. Hirunuma-Higurashi,*1 H. Mukai,*2 A. Akashio,*1 T. Okayasu,*1H. Fukuda,*1 S. Hashiguchi,*3 M. Takekoshi,*3 Y. Yamauchi,*3 S. Fujita,*1 K. Nakano,*1 H. Aiso,*1 K. Igarashi*1and S. Iizuka*1Since 1986, residual radioactivity at the deflectors ofcyclotrons has been measured regularly just before
maintenance work. The variations in the dose rates areshown in Fig. 1. New measurements in 2013 wereconducted only at AVF and RRC owing to theirmaintenance work. The beam intensity of AVF has been
increased since 2006 for the radioisotope production, andthe dose rate has also increased. Because the high intense
beam was not provided to RRC in 2013, there was a
reduction in the dose rates as shown in Fig. 1. The change
in the dose rate at RRC from 1990 to 2006 is not large, andthe value is typically around 20 mSv/h. After 2006, large
variations in the dose rate are observed. It depends on thebeam intensity of RIBF and the cooling time. The dose rateof SRC increased in 2011, and the value became similar to
those of AVF and RRC. The dose rates of IRC and fRC
were measured in 2012 only.
The residual radioactivity was measured along the beamlines after almost every experiment. Points 1–26, marked
with solid circles in Fig. 2, are the locations where
high-residual dose rates were usually observed. Table 1 lists
these dose rates and the measurement dates, beam

conditions, and the decay periods after the end of operation.
The maximum dose rate was found to be 7 mSv/h at point
25, which is below the beam dump chamber of BigRIPS.
The dose rates on the site boundary in 2013 have beenmonitored to prevent it be over legal limit of 1 mSv/y.

Fig. 1. Dose rates of residual radioactivity at thedeflectors of 5 cyclotrons.

Fig. 2. Layout of beam lines at RIBF. Locations where high dose rates were observed are indicated by solid circles
1–26.
*1 RIKEN Nishina center*2 Japan Environment Research Corporation*3 Daiwa Atomic Engineering Corporation- 307 -Ⅳ. OPERATION RECORDS

RIKEN Accel. Prog. Rep. 47 (2014)

Table 1. Dose rates measured at beam lines in 2013.
Points 1−26 indicate the measured locations shown in Fig.
2.
DoseDecay
Date Particle Energy Intensity period
Point rate (M/D)(MeV/u) (pnA)(μSv/h)
(h)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26

630
300
500
110
300
180
920
300
200
1300
1000
1300
80
80
160
143
260
230
6000
110
250
3000
400
1400
7000
250

8/22
8/22
7/24
12/24
7/1
12/26
12/26
7/17
7/4
7/4
7/4
7/4
7/4
7/4
7/4
4/19
7/4
4/197/4
7/4
7/4
7/4
7/4
7/4
7/4
7/4

dd
O-18
Xe-124
Xe-124
Xe-124
Xe-124
Rb-87
Xe-124
Xe-124
Xe-124
Xe-124
Xe-124
Xe-124
Xe-124O-18
Xe-124O-18
Xe-124
Xe-124
Xe-124
Xe-124
Xe-124
Xe-124
Xe-124
Xe-124

1212
6.28
10.75
10.75
10.75
10.75
66
50
50
50
50
114
345
345
250
345
250
345
345
345
345
345
345
345
345

10000
10000
1000
1210
1300
200
200
0.07
143
143
143
143
51
36
36
200
36
200
36
51
51
36
36
36
36
36

414
414
50
424
1300
466
466
31
74
74
74
74
74
75
75
275275747475757575
75

We continuously monitor the radiation in and around theRIBF facility using neutron and gamma area monitors. Thebackground dose rates were evaluated and the measured
values were corrected. The background data have beenacquired over a period of a month in August 2013 when all
the accelerators were not in operation. The background of
gamma-ray dose is currently about 2 times higher than thenatural dose rate because of the fallout due to the accident
at the Fukushima Dai-ichi power station. Before theaccident, the natural background of the gamma-ray dose at
the site boundary near the BSI East Bldg. was 0.039 µSv/hin January 2011. The background of gamma-ray dose in2013 was 0.062 µSv/h. Just like before, all of the corrected
dose rates monitored at 2013 were below the detection limit,
corresponding to 2 µSv/y for neutrons and 8 µSv/y for
gamma-rays. The total dose rate was less than 10 µSv/y andwas considerably lower than the legal limit.
The radiation dose on the boundary of theradiation-controlled area have also been monitored. The
monitors of gamma-rays and neutrons are placed at thethree points on the boundary. One is in the computer roomof the Nishina building, and the two other are on the roofsof the IRC and BigRIPS in the RIBF accelerator building.
The highest value was observed on the IRC roof as a resultof the beam loss at the transport line between SRC andBigRIPS. The annual neutron doses at these locations since
1999, which are sufficiently lower than the legal limit, areshown in Fig. 3. The value of the BigRIPS roof was similar
to the background level and is not shown in Fig. 3. Thelegal limit of a boundary of a radiation-controlled area is
1.3 mSv/3month, and all the measured doses were low
enough.

Fig. 3. Accumulated leakage radiation at theboundary of the radiation-controlled area.

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