Combined use of an activation detector and an imaging analyzer for measuring the distribution spatial fluence in an accelerator building

At the accelerator facilities, neutron produced by the nuclear reactions of high energy particles often causes activation of accelerator components and building and radiation damage of electronics circuit. In case of usual activation detector method, each detector is measured with Ge-detector and multichannel analyzer. This method needs time-consuming measurement and tedious calculation. Therefore, we developed the rapid and simple method of neutron flux evaluation. Neutron fluence was measured as SF-Cyclotron, Center for Nuclear Study, the University of Tokyo. The neutron was monitored under the different operation conditions, such as 30 MeV proton, 30 MeV deuteron and 40 MeV alpha. In order to obtain two-dimensional map of neutron flux, small tips of gold and aluminum foils were set in the accelerator room and experimental rooms. The 197Au(n,γ)198Au, 197Au(n,2n)196Au and 27Al(n,α)24Na reactions were used as the monitor reaction. The maximum number of monitoring points in the accelerator room was about 60. After the end of operation, activation detectors were collected and paste on the plan and the elevation for the accelerator building to show the monitoring points of each detector. The imaging plate (BAS III, Fuji Photo Film Co.) and the plan (or the elevation) were stuck together to develop the activities of detectors simultaneously. The imaging plate was measured with the bioimaging analyser (BAS-1000, Fuji Photo Film Co.). The intensity of neutron each point on the map was calculated and the color image of neutron intensity was obtained as a two-dimensional map by the software called MacBAS. The conversion factor between PSI and Bq was calibrated by the gamma-ray spectrometry using Ge-detector in advance. The neutron fluence of 104 n/cm2/sec was easily detected by the exposure of 10 min. The linearity of PSI vs. radioactivity and PSI vs. exposure time was very good. The neutron fluence at the floor and wall near the beam extraction side of cyclotron was high. The fluence of the opposite side of beam extraction was low. Therefore, neutron was mainly produced at the surroundings of beam extraction of cyclotron. The neutron flux was decreased with the distance from the cyclotron along with the beam direction. On the other hand, the gradation of vertical direction was gentle. This method can directly provide the map of neutron fluence by the scanning of an imaging plate. These image data are very useful to grasp the spatial distribution of neutron at a glance. The quantitative data are also useful for radiation protection. The radioactivity induced in the concrete wall was measured and compared with the result calculated by the neutron flux and the operation condition of cyclotron. If the area of high neutron fluence was covered with a boron sheet, the activation of floor and wall can be effectively reduced. (author)