VIII-Ⅱ-1. Project Research
Project 11

PR11 Behavior of Radioactive Nuclides in Intense Radiation Fields in Accelerator Facilities
Y. Oki(Y. Oki, K. Yamashita1, Y. Ishi, T. Uesugi and Y.
Kuriyama)

Research Reactor Institute, Kyoto University
OBJECTIVES AND RESEARCH SUBJECTS: Recently high-energy and high-intensity accelerators havebeen developed. They are widely used for many fields
such as physics, chemistry and cancer therapy. The
high-energy proton accelerators produce high-energy
primary and secondary hadron particles, and a variety ofradionuclides are formed through nuclear reactions including spallation reactions in high-energy accelerator
facilities. The activation takes place in accelerator hardware, air, cooling water and other materials such as soil
outside of shielding concrete walls.
From the viewpoints of radiation safety and safe accelerator operation, the management of the formed radionuclides is very important. The behavior and property of
the radionuclides is the most important information toestimate radiation exposure and to ensure the accelerator
safety.
In this work, the behavior and property was investigated in the air of the accelerator rooms and the coolingwater. In addition, problems concerning activation in accelerator facilities were also investigated to obtain correct
estimation of amount of the radionuclides formed in airand accelerator hardware.
The following problems are studied in this work.
1. The property of radioactive airborne species (radioactive aerosols and radioactive gases) in the air.
2. The behavior of dissolved metals from cooling watertubes and radioactive nuclides formed in the water.
3. Problems related to activation in accelerator facilities
such as development of new radiation detectors forhigh energy accelerators and measurement of formation cross section of the principal radionuclides fordose estimation.
The participating research subjects (PRS) in Fiscal
2012 and co-workers in each subject are listed below.
PRS-1 (24P11-1) Difference of number and radioactivity
size distribution of airborne aerosols formed in target
room depending on beam currents(S. Yokoyama, Y. Oki and S. Sekimoto)PRS-3 (24P11-3) Corrosion of Metals and Colloid
Formation in Water under Intense Raiation Field(K. Bessho, H. Matsumura, Y. Oki1, N. Akimune2, S.
Sekimoto, M. Hagiwara, K. Iijima, T. Sanami, K. Masumoto, J. Hori, N. Abe)

MAIN RESULTS:
PRS-1
Radioactive size distribution and radioactivity of airborne aerosols formed on different beam current intensity
in the target room of the electron linear accelerator in
KURRI were estimated. The size distribution was measured with a scanning mobility particle sizer (SMPS) and alow pressure impactor. The size dependence on beam
current was discussed for both of 13N-bearing aerosol
particles and non-radioactive aerosol particles.
PRS-2
An air irradiation experiment was performed for the
first time in an FFAG proton accelerator in order to study
the property of the radiation-induced radioactive airborne
nuclides. The radioactivity and the number of the particles were measured. Preliminary results obtained in thefirst irradiation experiment were reported.
PRS-3
Metallic copper (Cu) contacted with water were irradiated with γ-rays in the 60Co irradiation facility inKURRI, bremsstrahlung, or neutrons in the electron linac,
as a model environment inside accelerator facilities. Radiation effects on corrosion of Cu in water and formationof colloidal Cu species were observed.
In addition, a new radiation monitor for burst neutron
field was tested under a burst neutron field that was generated using a Ta target bombarded with electrons fromthe linac. The monitor has been developed to measuredose rate from beam loss of a pulse-mode accelerator toprevent activation accelerator hardware.

PRS-2 (24P11-2) Measurement of Radioactive Airborne
Nuclides in an FFAG Accelerator

採択課題番号 24P11 加速器施設の高線量場における放射性核種の挙動に関する研究 プロジェクト（京大・原子炉）沖 雄一

Different of Number and Radioactivity Size Distribution of Airborne Aerosols
Formed in Target Room Depending on Beam Currents

INTRODUCTION: Radioactive aerosols are formed bybeam irradiation around a beam line and in a target roomof accelerator facilities. The inhalation of the radioactive
aerosols may results in internal exposure for accelerator
maintenance workers. The radioactive size distribution ofaerosols is important factor to estimate internal dose. The
time variations of number size distribution and formation
mechanism of airborne aerosols formed in the accelerator
facilities were investigated in the past studies [1-3]. Inthis study, radioactive size distribution and radioactivityof airborne aerosols formed on different beam current
intensity in the target room were estimated.
EXPERIMENTS: In the target room of KUR-LINAC, awater cooled Ta target was irradiated with 30MeV electrons beam with currents of 20, 50, 80, 100 and 120μA
for about 2 hours per experiment and neutrons were generated. The number-size distribution of airborne aerosols
formed in the target room was measured continuously bySMPS (Model Series 3936, Tokyo Dylec corp.) during
the experiments. The radioactivity-size distribution ofaerosols was collected by a low pressure cascade impactor (Tokyo Dylec corp.) from 60 to 90 minutes afterirradiation start. The radioactivity of aerosols collected oneach stage of the cascade impactor were measured by anautomatically α and β particles detector (JDC-5300, Hitachi Aloka Medical, Ltd). The aerosols were collected byfilter and detected by BGO scintillation counter. As thisresult, it was found that 13N made a large contribution tototal activity. The radioactivity-size distribution of aerosols was also estimated by using the measured number-size distribution of aerosols and the attachment coefficient of 13N.
RESULTS: As shown in Fig. 1, the number-size distribution of aerosols had two peaks. The second large peak
was observed at 110-140nm diameters independently of
the beam currents. The peak of measured activity size
distribution of aerosol was also appeared at 230-260nm
diameters regardless of the beam currents although thedistribution had only a single peak (Fig. 2). Total measured radioactivity of aerosols was linearly proportional tobeam current. It is considered that the growth of radioactive aerosols is avoided even if the amount of generated
aerosols is large because the formed radioactive aerosols
diffuse sufficiently in the target room. Fig. 3 shows
measured and calculated activity size distribution of aerosols on beam currents of 20 and 100μA. The calculated
distribution on the beam current of 20 and 100μA had
peaks at 133 and 152 nm diameters, respectively. The

採択課題番号 24P11-1（藤田保衛大・医科）横山ACKNOWLEDGMENTS: In these experiments, measurement of the number-size distribution of aerosol using
SMPS system was supported by Tokyo Dylec corp.
5x105
20mA
50mA
80mA
100mA
120mA

4dN/dlogDp

School of Health Science, Fujita Health University
1Graduate School of Engineering, Kyoto University

diameter that the peak activity of calculated distribution
appeared is smaller than the measured one.

32
1
0

10

100

Diameter (nm)Fig. 1. Number size concentration distribution ofairborne particles at beam currents of 20-120μA at
60minitues after the beam irradiation started.
7000
6000
dA/dlogDp

S. Yokoyama, Y. Oki1and S. Sekimoto1

Beam current
20mA
50mA
100mA

5000
4000300020001000010100Diameter (nm)1000

Fig. 2. Measured activity size distribution at beamcurrents of 20, 50 and 100μA from 60 to 90 minutes.
1.0Norµalized dA/dlogDp

PR11-10.820µA(Meas.)
20µA(Cal.)
100µA(Meas.)
100µA(Cal.)

0.6
0.4
0.2
0.0
10100
Diaµeter (nµ)

1000Fig. 3. Activity size distribution of 13N aerosol
measured and estimated at current in the case of thebeam current of 20and 100μA.
REFERENCES:[1] S. YOKOYAMA et al., Jpn. J. Health. Phys., 43(2008) 333-340.
[2] S. YOKOYAMA et al., Rad. Prot. Dosi., 127 (2007)
392-397.
[3] N. Osada, Behavior and formation mechanism of radioactive aerosol in accelerator facilities (Kyoto University s) (2012).

加速器施設内生成粒子の粒径分布に関する研究
須美（京大・原子炉）沖 雄一、関本 俊

プロジェクトPR11-2

Measurement of Radioactive Airborne Nuclides in an FFAG Accelerator

Y. Oki, K. Yamashita1, Y. Ishi, T. Uesugi andY. Kuriyama
Research Reactor Institute, Kyoto University
Faculty of Engineering, Kyoto University1

INTRODUCTION: In accelerator facilities, radioactive
airborne species are formed in air of the accelerator
rooms during machine operation. Oxygene-15 (half life:2 min), 13N (10 min) and 11C (20 min) are principal radionuclides in the air [1]. In addition, 7Be (half life: 53 d)
is also formed through nuclear spallation reactions from
air in high-energy proton accelerators. The 7Be atoms in
air in the accelerator rooms are observed as aerosol particles; however, 15O, 13N and 11C exist in both states of gas
molecules and the aerosol particles. Their states are very
important information on accelerator radiation safetymanagement.
In this work, an air irradiation experiment was performed for the first time in an FFAG proton accelerator inorder to study the property of the radiation-induced radioactive airborne nuclides. The radioactivity and thenumber of the particles were measured. Preliminary results obtained in the first irradiation experiment are described below.
EXPERIMENTS: The irradiation experiment was carried out in the 150-MeV FFAG (Fixed Field AlternatingGradient) Proton Accelerator of Research Reactor Institute, Kyoto University (KURRI). In the FFAG accelerator
system, an 11-MeV H– ion beam is injected to the FFAG
main ring by the H– linear accelerator. Protons are accelerated to 150 MeV after conversion of H– ions to protons.
The specimen irradiation beam line, one of the extraction
beam lines of the main ring was used for air-irradiation.
The air-irradiation chamber was installed at the end of
the specimen irradiation beam line before the beam dump.
The chamber was a 1-m long stainless steel cylinder with
the internal diameter of 15 cm. The beam pipe and thechamber were separated with a 1-mm thick Al plate. The
aerosol-free air was introduced to the chamber at the flowrate of 20 L/min, and the irradiated air was sampled fromthe chamber. Fig. 1 shows the irradiation chamber together with the specimen irradiation beam line and theFFAG main ring.
The activity of the sampled air was measured with a
1.5-L ionization chamber after removing aerosol particles
by filtration. The fluctuation of the beam intensity wasmonitored using this ionization chamber measurement.
The radiation-induced aerosol particles in the air werecollected on a PTFE membrane filter (pore size:0.45-µm). The γ-spectra of the filters were measured withGe detectors. In addition, the 511-keV annihilation γ-rays

from the positron emitters were measured with a coincidence system of two bismuth germanate (BGO) scintillators.
The number concentration of the aerosol particles
formed in the irradiated air was counted with a Condensation Particle Counter (CPC) (TSI Inc., Model 3025A).

Fig. 1 Irradiation Chamber
RESULTS AND DISCUSSION: Dense aerosols areobserved in the irradiated air. The number concentration
exceeded the upper detection limit of the CPC (105 particles/cm3). The 511-keV annihilation peak due to the positron emitters and the 7Be peak of 478 keV were observedin the γ-spectrum of the collected aerosols. The decay
curve of the annihilation peak was fitted to the principal
positron emitters by a least-squares method. The radionuclide formed in air in the bremsstrahlung irradiation in
the electron linac of KURRI was almost 13N at the timeof measurement [2]; however, 11C was observed in addition to 15O and 13N in the proton irradiation. From theviewpoint of radiation safety, attention should be paid tothe formation of 11C whose half life is longer than otherpositron emitters.
Dense aerosol particles supplied at the constant flowrate are useful for calibration of size-measuring instruments for aerosol particles. Because stable production offine particles (nano- and sub-micron particles) is moredifficult than micron-order particles, devices that easily
produce the fine particles can be employed for calibrationof aerosol-measuring instruments for the fine particles.
This air-irradiation system in the proton accelerator canbe applied to the particle production device for the fineparticle calibration by combining with a Differential Mobility Analyzer (DMA) that separates particles of desired
size range from the radiation-induced particles.

REFERENCES:[1] A. Endo, Y. Oki, Y. Kanda, T. Oishi, and K. Kondo,Radiat. Prot. Dosim., 93 (2001) 223-230.
[2] N. Osada, Y. Oki Y, K. Yamasaki and S. Shibata, Progress in Nuclear Science and Technology, 1 (2011)483-486.

採択課題番号 24P11-2
加速器室の空気に生成する放射性化学種の生成挙動（京大・原子炉）沖 雄一、関本俊（京大院・工）秋宗尚弥、鈴木啓仁（藤田保健衛生大医療科学部）横山須美

プロジェクトPR11-3 Corrosion of Metals and Colloid Formation in Water under Intense Radiation Field
K. Bessho, H. Matsumura, Y. Oki1, N. Akimune2,
S. Sekimoto1, M. Hagiwara, K. Iijima, T. Sanami,
K. Masumoto, J. Hori1 and N. Abe1

tions, colloidal Cu concentration quickly reached to plateau within 2 h. More comprehensive analyses and discussion are in progress.

EXPERIMENTS: Cu vessels (I. D. 19 x 75 mm) filledwith pure water were irradiated by γ-rays generated at
the 60Co γ-ray irradiation facility, or bremsstrahlung andneutrons generated by 30 MeV electron beam hitting on aTa target (Φ50 x 62 mm) at the electron linear accelerator(LINAC). Samples at LINAC experiments were fixed at
the downstream position (80 mm from Ta target) andperpendicular position (80 mm from Ta target).
After the irradiations, water samples in Cu vessels
were poured out and treated with four kinds of ultrafiltration (UF) units for particle size separation. Estimatedpore sizes of the UF units were 200, 16, 7 and 3 nm.
Concentrations of Cu in the filtrates and unfiltered samples were determined by ICP-AES analyses.
RESULTS: Figure 1 (a) and (b) show soluble and colloidal concentrations of Cu in water as a function of reaction time. The data for LINAC experiments were obtained at the downstream sample position where flux ofneutrons can be neglected. Therefore, Fig. 1 shows theeffects of photon irradiation, and it demonstrates that
irradiation of high-energy photons affects the corrosionof Cu and colloid formation in water phase.
Concentration of soluble Cu species increased at photon irradiations. In γ-ray irradiation at the dose rate of
2.5-2.9 kGy/h, soluble Cu concentration increased withreaction time and reached to a plateau concentration of
0.6 mg/L. At the dose rate of 19-22 kGy/h, a plateau
concentration of soluble Cu was 1.7 mg/L. In the LINAC
experiments, soluble Cu concentration also increased
with irradiation times. These results imply that dissolution of Cu progresses under intense photon field and finally reaches to plateau concentrations dependent on thephoton intensity and/or energy.
Formation of colloidal Cu species was also noticeable
under intense photon field. In contrast with soluble frac-2(a) 0-3 nm
Cu (mg/L)

210(b) 3-200 nm

10020406080 100240120
Reaction Time (h)020406080 100240120
Reaction Time (h)Fig. 1 Cu concentrations in water in the copper vessels
after the irradiations as a function of reaction time.
(a) Soluble fraction:0-3 nm; (b) Colloidal fraction:3-200nm; Symbols △ :without irradiation; ◆ :γ-ray (2.5-2.9kGy/h); □:γ-ray (19-22 kGy/h);●: LINAC (current:10 µA)
RADIATION MONITOR FOR BURST NEUTRONS:A radiation monitor for burst neutron field has been developed to measure dose rate from beam loss of apulse-mode accelerator [1]. The monitor, which consistsof a 3He proportional counter, a 6.5 cm thick polyethylene moderator, equips a newly developed charge integrate circuit (current readout). The monitor was tested
under a burst neutron field that was generated using a Ta
target bombarded with electrons from LINAC, at 120degree, 10 m from the target. Another monitor with aconventional pulse readout circuit and the same moderator also tested for comparison. Photon dose of this field
was measured using an ionization chamber. The intensities of the burst field were varied with changing the beamenergy and pulse width. Fig. 2 shows experimental results on measured dose rate as a function of number ofneutrons generated at the target. The current readout typeneutron monitor showed linear response up to neutron
dose rate of 400 μSv/h. The photon dose rate that is fourtimes higher than neutron one, has less impact on theneutron monitor because of photon sensitivity.[1] The
pulse readout monitor failed at low dose rate owing toserious pulse pile-up.
1600
Cuurent readout

1400

Dose rate [μSv/h]

INTRODUCTION: At high-intensity accelerator facilities, intense radiations affect chemical states of elements inside the facilities. In the cooling-waters for various components, formation of metal related colloids inwater is one of the important subjects in the radiation
managements. In this work, metallic copper (Cu) contacted with water were irradiated by γ-rays, bremsstrahlung, or neutrons, as a model environment inside accelerator facilities. Radiation effects on corrosion of Cu inwater and formation of colloidal Cu species were studied.

Cu (mg/L)

3Radiation Science Center, KEK
1Research Reactor Institute, Kyoto University
2Graduate School of Engineering, Kyoto UniversityPulse readout

1200

Photon dose (ionization chamber)

1000
800
600
400
200
00

1E+122E+123E+124E+125E+126E+127E+12

Number of neutrons at target [n/s]Fig. 2 Comparison of the dose rate measured with thedifferent readout type neutron monitors. Photon dosewas measured using ionization chamber.
REFERENCES:[1] K. Iijima et al., Prog. Nucl. Sci. Tech., 1 (2011) 300

採択課題番号 24P11-3 高線量放射線場における水中金属材料の腐食およびコロイド生成挙動の解析（KEK・放射線）別所光太郎、松村宏、桝本和義、萩原雅之、飯島和彦、佐波俊哉（京大・原子炉）沖雄一、関本俊（京大院・工）秋宗尚弥

プロジェクト