Optimizing light guide shape of beta-ray detector for surface contamination monitor

Object surface must be inspected for possible radioactive contamination when objects are moved from nuclear facilities and decommissioning sites. Currently, large vehicles are manually inspected for contamination. To avoid exposing the inspection workers to radiation, an automatic contamination inspection instrument for large vehicles should be developed. Moreover, such an instrument will require a compact and light detect. The light guide of the detector should be designed that can make the detector compact and sensitive enough. We used a 150×150×0.5 mm 3 plastic scintillator for our developing monitor. In particular, the size of the light guide should be optimized before producing the detector, and here, Geant4, a radiation simulation including optical processes, can be used for determining a suitable light guide size. In this study, the light collection efficiency, the instrument efficiency, and uniformity were estimated with Geant4. Based on the estimations, we found the optimal light-guide height and photodetector window size to be 40 mm and 20 mm, respectively, for a scintillator of 150×150×0.5 mm 3 . We fabricated a detector incorporating the optimized light guide and examined its performance in an experiment with radioactive sources 137 Cs and 60 Co. The measures of instrument efficiency and uniformity were consistent with those predicted by the simulation. If this detector were installed in the instrument, it would be able to inspect a large vehicle in 250 seconds under a gamma-ray background of 1.0 μSv/h .