Ⅳ. OPERATION RECORDS
RIKEN
Accel. Prog. Rep. 56 (2023)

RIKEN Accel. Prog. Rep. 56 (2023)

Radiation safety management at RIBF
K. Tanaka,∗1 H. Sakamoto,∗1 R. Hirunuma-Higurashi,∗1 H. Mukai,∗1 A. Akashio,∗1 T. Okayasu,∗1 R. Suzuki,∗2M. Takekoshi,∗2 T. Sato,∗2 K. Igarashi,∗1 S. Iizuka,∗1 N. Usudate,∗1 Y. Shioda,∗1 and M. Murata∗1
The results of radiation monitoring at RIBF, conducted at the border of the facility and the radiationcontrolled area are reported. The residual doses along
the accelerator setups are also presented. In 2022, 238 U
beam of approximately 345 MeV/nucleon was provided
at an intensity of 70 particle nA in April. Then 75 Kr
beam of about 345 MeV/nucleon of 600 particle nA
were used in May and June. Further, 70 Zn beam of approximately 345 MeV/nucleon of 400 particle nA wereused in December.
The dose rates at the boundary of the radiationcontrolled area were monitored. Neutron and γ-ray
monitors were used at three locations: roofs of theRRC, IRC, and BigRIPS. Figure 1 shows the annual
neutron dose at these positions. As the thickness of theradiation shield at IRC roof was relatively small, thedose rate was high. In 2017, additional local-concrete
shield of 1 meter thick was set on a beamline in IRCroom. The dose was successfully reduced. In 2022,
even the highest annual dose of 4.1 µSv/y at the IRC
roof was lower than the legal limit of 5.2 mSv/y. The
dose at IRC roof at 2022 is sensitive to IRC and SRC
operation time. In 2022, these were operated for only
four months. Therefore, the annual dose of IRC roofwas small.

level. Therefore, the radiation dose rate was considerably lower than the legal limit.
The residual radioactivity at the deﬂectors of the cyclotrons was measured immediately before the maintenance work. The residual dose is dependent on factors
such as the beam intensity, accelerator operation time,and cooling time. The data have been obtained at
the cyclotrons maintenance works, when the deﬂectors
were able to accessible. Therefore, the cooling times
were not constant. The dose rates from 1986 are shownin Fig. 2. The dose rates for FRC, IRC, and SRC areshown for the years after 2006, when the RIBF operation started. For AVF, the dose rate increased in 2006
because the radioisotope production was started, andthe beam intensity increased.

Fig. 2. Dose rates of residual radioactivity at the deﬂectorsof 5 cyclotrons.

Fig. 1. Radiation dose at the boundary of the radiationcontrolled area.

The dose rates at the site boundary with Wako city,where the legal limit is 1 mSv/y, were monitored byneutron and γ-ray monitors. The annual dose in 2022
was 9.0 µSv neutrons after the background correction.
The annual dose of the γ-ray was under the background∗1∗2

RIKEN Nishina Center
Daiwa Atomic Engineering Corporation- 213 -The residual radioactivity along the beam lines wasmeasured after almost every experiment. Figure 3
shows the locations of the measurement points wherehigh residual-doses rates greater than 100 µSv/h wereobserved. Table 1 lists the dose rates, beam conditions,and cooling time at the measurement points. The maximum dose was 28 mSv/h at point 15, which is near
the beam dump of BigRIPS.
The radioactivity in the closed cooling system at BigRIPS was measured. The water for the F0 target,
exit beam dump, and side-wall dump were sampled inMarch 2022. The water in the closed cooling systems
were partly replaced on October 2021. The results
are shown in Table 2. A liquid scintillation counter(LSC-7400, Hitachi Co. Ltd.) was used for the low
energy β ray of 18 keV from H-3 nuclide. A Ge detector (GC2019, Canbbera Co. Ltd.) was used for γ rays
emitted from other radionuclides. The radionuclides,

Ⅳ. OPERATION RECORDS

RIKEN Accel. Prog. Rep. 56 (2023)

Table 1. Dose rates measured at beam lines in 2021.
Points 1–17 indicate the locations where measurements
were taken as shown in Fig. 3.

Dose Date Particle Energy Intensity Decay
Poirate(M/(MeV/ (pnA) periodnt (μSv/hD)u)
(h)
α
7.3
2500
362
1 130 7/27
α
7.3
2500
362
2 260 7/27
343
3 300 7/27 U-238 10.8 300
940
269
4 2200 6/14 Kr-78 50940
269
5 1700 6/14 Kr-78 50
1306/14Kr-7850
940
269
6631
269
7 4400 6/14 Kr-78 345
631
269
8 6900 6/14 Kr-78 345631
269
9 140 6/14 Kr-78 345
2006/14Kr-78345
631
269
10631
269
11 100 6/14 Kr-78 345
631
269
12 730 6/14 Kr-78 345
631
269
13 2380 6/14 Kr-78 345
3650
6/14
Kr-78345
631
269
14
631
269
15 27500 6/14 Kr-78 345
631
269
16 910 6/14 Kr-78 345
631
269
17 470 6/14 Kr-78 345

Table 2. Concentrations of radionuclide in the cooling water at BigRIPS, allowable legal limits for drain water,and ratios of concentration to the allowable limit.

Fig. 3. Layout of the beam lines at RIBF. The measurement locations listed in Table 1 are indicated.

except for H-3, were already ﬁltered by an ion exchange
resin set in the closed cooling systems. Although theoverall value of contamination was less than the legal
limit for drain water, as shown in Table 2, the waterfrom the closed cooling system will be dumped into
the drain tank prior to the subsequent operation toprevent contamination in the room in case of a waterleakage.
The E-learning module, which can be accessedanytime and from anywhere (even from the outsideRIKEN), has been used for the re-training to the radiation workers at RIBF. Approximately 612 radiation
workers have completed the training in 2022.
As described above, radiation management to comply with lows and to maintain a radiation level as low
as usual has been conducted successfully.

2- 214 -Cooling Nuclide
waterBigRIPS H-3
F0 target
H-3
Be-7
BigRIPS Mn-54
Co-56
exit
Co-57
beam
Co-58
dumpCo-60
H-3
BigRIPS Be-7
side-wall Co-57
beam
Co-60
dump
1) read as 4.3× 10-3

Concentration[a] Limit[b] Ratio to(Bq/cm3)(Bq/cm3) limit [a/b]2.060
3.3e-2summation
3.3e-29.360
1.6e-14.3e-31)30
1.4e-45.0e-315.0e-38.2e-40.3
2.8e-37.4e-34
1.8e-33.9e-31
3.9e-26.7e-30.2
3.4e-2summation
0.20
15.8
60
4.3e-13.7e-330
1.2e-44.4e-44
1.1e-46.0e-40.2
3.0e-3summation
0.43