Evaluation of activated area in the electrostatic accelerator facilities

K. Masumoto, H. Matsumura, T. Miura, G. Yoshida, A. Toyoda, H. Nakamura, K. Bessho,T. Nakabayashi,F. Nobuhara, K. Sasa, T. Moriguchi, H. Tsuchida,S. Matsuyama, M. Matsuda, A. Taniike

(1. High Energy Accelerator Research Organization, Tsukuba, Ibaraki, Japan;2. Japan Environmental Research Co., Ltd., Shinjuku, Tokyo, Japan;3. Tokyo Nuclear Service Co., Ltd., Country, Taito, Tokyo, Japan;4. Univ. of Tsukuba, Tsukuba, Ibaraki, Japan;5. Kyoto Univ., Uji, Kyoto, Japan;6. Tohoku Univ., Sendai, Miyagi, Japan;7. Japan Atomic Energy Agency, Tokai, Naka, Ibaraki, Japan;8. Kobe Univ., Kobe, Hyogo, Japan)

Abstract:In order to clear the activated area in electrostatic accelerator facilities, four accelerator facilities were selected and typical neutron emission experiments were performed. Neutron flux during operation and induced activity caused by charged particles on the accelerator and its surrounding area after irradiation were measured. Also the monitored neutron flux and calculated value by Monte Carlo calculation using PHITS code were compared. It was confirmed that the results between calculated data and measured data showed the good agreement with each other. Finally, it was concluded that we have to take care the activation of beam line and target. But, it is not necessary to treat accelerator tank, surrounding materials, and building concrete as radioactive materials in case of decommissioning.

Key words: Electrostatic accelerators; Neutron measurement; Activated area; Decommissioning

1 Introduction

In Japan, more than 100 electrostatic accelerators have been used in various scientific fields, such as nuclear, material, environmental, and archaeological sciences. As the beam energies and beam currents of almost all these accelerators are low, it is expected that activated area is limited and induced activity is very low. Although, it is necessary to sort the activated area or parts in case of decommissioning of each facility. Especially, the evaluation of neutron activation of surrounding materials is important. This duty is very troublesome for all facilities. Then, we studied the evaluation methodology of activated/non-activated area and try to clear the activated area for electrostatic accelerator facilities during operation in advance.

In case of these accelerators, the accelerated particles cannot penetrate outside of accelerator. Therefore, the activation of surrounding materials is only caused by neutrons produced secondary.

We already performed to clear the activation situation in the cyclotron room[1-2]. In this work, we studied followings, such as comparison of the several neutron monitoring methods for evaluation of neutron activation, monitoring of neutrons under several irradiation conditions, and the evaluation of activated area in the electrostatic accelerator facilities.

2 Experimental

2.1 Basic process for the zoning of activated

area

We select accelerator facilities after grouping of particle energy, current, beam loss, use and so on. Then, neutrons are measured during operation to estimate beam loss points and activation of surrounding materials. After operation, we measured surface dose rate, and induced radioisotopes in the accelerator room and on the accelerator components.

Additionally, Monte Carlo calculation of neutron transport and activation is performed for comparison to experimental results.

2.2 Facilities

Electrostatic accelerators in our study included types of Cock Croft Walton and Van de Graaff, which are single-end and tandem acceleration. But, neutron generators were excluded. After the preliminary study of neutron emission probability for 16 accelerators of terminal voltage from 1 to 20 MV by setting CR-39 neutron detectors in their facilities for about three months, four accelerators which were observed neutron emission were selected for our detailed study as shown in Table 1. In these four facilities, we obtained machine times for neutron measurement during operation using several irradiation conditions as shown in Table 1.

Tab.1 Selected electrostatic accelerators and irradiation condition

2.3 Neutron monitoring

Three kinds of neutron detectors such as nuclear track detector CR-39, activation detectors using Au foils, and thermoluminescence dosimeters (TLD) are set on accelerator components and inside the accelerator room during operation.

2.3.1CR-39

CR-39 was provided by Nagase Landauer, Ltd. CR-39 is a polycarbonate plastic. In order to detect neutrons, CR-39 is covered with a polyethylene plate as recoil proton radiator for fast neutron detection and a boron loaded plate as alpha radiator for thermal neutron detection[3].

2.3.2Activation detector using Au foils

Each Au foil is 6 mm in diameter and 20 μm in thickness. A pair of Au foils was prepared to paste on in each location to monitor thermal and epithermal neutrons. One foil is bare and the other is covered with Cd foils in 1 mm thickness to absorb thermal neutrons. Relative radioactivity (dose) of Au foils was simultaneously measured using an imaging plate and a Au foil was measured with a Ge-detector to obtain the absolute activity of198Au and the neutron flux was calculated[4].

2.3.3TLD

TLDs (Panasonic UD813PQ4) were used for neutron detection. In this case, pairs of TLDs were used; one bare TLD and another TLD covered with a cadmium sheet (0.5 mm in thickness) to monitor the effect of thermal neutrons. UD813PQ4 has four elements, two elements were made of6Li210B4O7(Cu) for the detection of photons and thermal neutrons, and the other two elements made of7Li211B4O7(Cu) for the detection of photon only. Thermal neutron dose was calculated by the difference of thermoluminescence signal between 2 types of element[5]. Additionally, we confirmed the effect of epithermal neutrons by a signal obtained by TLD covered with cadmium. The dose was converted to neutron fluence by using the conversion factor obtained in our calibration field using a graphite pile.

2.4 Residual dose measurement

Before and after experiments, a NaI(Tl) scintillation survey meter (Hitachi, TCS-171) and a LaBr3scintillation spectrometer (Mirion, InSpector 1000) are mainly used to detect surface dose rate, activated area, and radioactive nuclides. Floor concrete under the target was also measured by a Ge-detector to detect the nuclides induced in concrete after the experiment.

2.5 Monte Carlo calculation of neutron distribution and neutron activation

In order to estimate the behavior of neutrons, we used the Monte Carlo calculation code, PHITS (v. 2.88)[6]for neutron transport calculation and DCHAIN-SP (v. 2004) for activation calculation in case of proton and deuteron irradiation for seven target materials such as Li, Be, Cu, Ta and so on.

3 Results and discussion

3.1 Neutron monitoring for preliminary study

3.1.1Accelerators of 1 to 3 MV terminal voltage

Neutrons were not detected in seven accelerators, except Kobe University 1.7 MV accelerator.

3.1.2Accelerators of higher than 3 MV terminal voltage

In the facility of three 5 MV accelerators for AMS, very low flux of neutrons were detected on the Faraday cup. The order was from 10-1to 100cm-2·s-1. In the facility of 3 MV single-end type accelerator, low flux of neutrons were also detected around the beam exit window. And slightly higher flux of neutrons was observed in a 2.5 MV single-end type accelerator because of the (d,d) reaction experiment.

In four accelerator facilities as shown in Table 1, neutrons were clearly observed. Then, we selected these facilities as the targets of full scale investigation.

3.2 The 1.7 MV tandem accelerator of Kobe University

In this facility, 1.7 MV tandem accelerator pelletron 5SDH-2 (National Electrostatic Corp.) has been used for material science by using proton, deuteron, and heavy ion beams. We selected the deuteron experiment using Be target, because the9Be(d,n)10B reaction has been used for neutron production several times per year and this is a major cause of neutron activation. Experimental condition is shown in Table 1.

Thermal neutron flux measured by Au foils was 6×103, 5×102, 1 × 102cm-2·s-1near the Be target, on the floor just under the target and the floor of down stream side, respectively. The results of TLD show almost the same value. The obtained results of CR-39 were one order higher than those of Au and TLD.

We measure the activity of beam line by LaBr3spectrometer and floor concrete by Ge detector before experiment. No activity of152Eu and60Co induced by neutron capturer reactions, could be detected on the surface of concrete. The gamma-rays from58Co and54Mn were observed on the target chamber. After experiment, we also measured the induced activity in the same way. On the beam line,13N (t1/2=9.97 m) from12C and56Mn (t1/2=2.58 h) from56Fe were mainly observed. And on the chamber,27Mg and24Na from27Al were also detected. Gamma rays of24Na and56Mn were detected on the surface of concrete.

It was found that activated area of this facility is limited to the beam pipe and the target chamber and the residual activities of accelerator tank, surrounding materials, and building constructions are negligible.

3.3 The 4.5 MV Dynamitron of Tohoku Unive-rsity

In this facility, a 4.5 MV Dynamitron (RPEA-4.5, Radiation Dynamics Corp.), which is a Cockcroft-Walton type single-end accelerator, has been used not only for nuclear science but also material, biological and environmental sciences by using proton beams. In case of preliminary study, neutron emission was occurred from the Li-target and the copper slit for micro beam production. Then, protons of 2.5 MeV were transported to the Li-target for 8 h and to the copper slit for 7 h. Experimental condition is shown in Table 1.

In case of the experiment using the Li target, maximum thermal neutron flux (3.3 × 102cm-2·s-1) and maximum epithermal neutron flux (3 × 101cm-2·s-1) was detected on the floor just under the Li target and near the Li target, respectively. The results of TLD show almost the same value. The obtained results of CR-39 were one or more order higher than those of Au and TLD. In case of the slit, activity of198Au could not be detected. By the TLD results, maximum thermal neutron flux (2 × 101cm-2·s-1) was observed on the upper part of the slit.

After irradiation,7Be and65Zn were observed near the Li target and65Zn and67Ga were observed near the Cu slit. On the floor under beam transport line, natural radionuclides were only detected by the gamma-ray spectrometry using a Ge-detector.

3.4 The 6 MV tandem accelerator of the University of Tsukuba

In this facility, a 6 MV Pelletron tandem (18SDH-2, National Electrostatics Corp.) has been used for various fields, such as nuclear, material, biological and environmental applications. Especially, AMS using from10Be to129I has been actively used. This facility has accelerator room including AMS line, beam transport room and experimental room.

We intended to do the experiment to produce neutrons by proton bombardment on Ta and SUS304 to simulate the activation of accelerator components. Experimental condition is shown in Table 1. A plate of SUS304 of 1 mm thickness set in the target chamber was bombarded by 6 and 12 MeV with beam current of 1 μA for 2 h. A plate of Ta of 1 mm thickness was also bombarded by 12 MeV protons.

By the TLD results, thermal neutron flux was very low and the maximum value (2.7 × 101cm-2·s-1) was observed at 1.1 m above the target chamber in the case of 6 MeV proton irradiation.

In case of 12 MeV proton irradiation, thermal neutron flux was increased to 7.1 × 102cm-2·s-1for SUS and 2.5 × 102cm-2·s-1for Ta at the same position of 6 MeV experiment. Thermal neutron flux was 5.9 × 101cm-2·s-1on the surface of the middle of accelerator tank.

Thermal neutron flux data contain large error in case of Au-foils, because the cadmium ratio became near one. Data of CR-39 showed very high value comparing with TLD, because the interference of fast neutron to the region of thermal neutron tracks might be very large.

In advance our experiment to check the effect of a preceding experiment, activity was measured by LaBr3spectrometer. Gamma rays of52Mn and56Co were observed on the beam duct in the accelerator room. Gamma rays from95mTc,96mTc, and99mTc were also observed on the beam duct in the beam transport room. In the experimental room, such nuclides could not be detected.

After proton irradiation of SUS304 and Ta,60Co and56Mn were detected by LaBr3spectrometer. Gamma rays of24Na and56Mn were detected on the surface of concrete just under the target chamber by using the Ge-detector shielded with the lead collimator.

3.5 The 20 MV tandem accelerator of JAEA

JAEA-Tokai tandem accelerator (20 MV Pelletron tandem 20UR, National Electrostatics Corp.) is the largest electrostatic accelerator in Japan. It was mainly used for nuclear and material science. This facility is composed of accelerator room, beam transport room and experimental room, which are separated by the concrete wall. This accelerator is set vertically in the accelerator room, which has 6 floors. Terminal voltage is 18 MV. Charged particles from H to Bi have been accelerated. In the beam transport room, beam is switched to 12 beam lines. In order to evaluate the activation of accelerator room and beam transport room during operation, experiment was performed under the condition shown in Table 1.

Residual radioactivity was measured by LaBr3spectrometer before and after the experiment by LaBr3spectrometer. In the accelerator room,60Co could not be detected. After our experiment,56Mn was observed on the surface of the accelerator tank and24Na and56Mn were observed on the surface of floor concrete. In the beam transport room, maximum surface dose rate before our experiment was 2.1 μSv/h on the surface of Faraday cup. After our experiment, gamma-ray from56Mn and 511 keV annihilation peak were observed.

3.6 Monte Carlo calculation of neutron distribution and neutron activation

As the preliminary study, we compared with the energy dependence of neutron production rate between literature value and calculation in case of Li to Ta target. Then we decided to use TENDL-2014 for cross section data. Monte Carlo calculation using PHITS was performed in the case of the 6 MV tandem accelerator of the university of Tsukuba. It was clearly drawn that neutrons, which produced by the bombardment of proton to the target, were widely distributed in the experimental room and gradually thermalized by scattering. The calculated data were in good agreement within the value of 0.68 to 5.2 as C/M with TLD results.

4 Conclusion

4.1 Thermal neutron monitoring

We used three types of monitors. In case of accelerator facilities, we have to consider the effect of gamma rays and fast and epithermal neutrons. It is important to compare the data each other to confirm the interference effect. See ref.[7]for details. Residual activity measurement will expected to become the effective estimation method for thermal neutron flux.

4.2 Thermal neutron flux

We performed the typical experiment for neutron emission. At the beam sorting section of JAEA tandem accelerator, thermal neutron flux was the order of 103cm-2·s-1. At the other three facilities, thermal neutron fluxes on the floor and wall of the accelerator room were the order of 102cm-2·s-1during experiment.

4.3 Activation area of electrostatic accelerator

It was found that we have to take care the activation of beam line and target. But, it is not necessary to treat accelerator tank, surrounding materials, and building concrete as radioactive materials in case of decommissioning.