Evaluation of an ultra-thin plastic scintillator to detect alpha and beta particle contamination

Abstract

To date, alpha and high beta particle emitters have been detected in the reactor buildings of the Fukushima Daiichi Nuclear Power Station (FDNPS). Since the beta radiation levels in the FDNPS buildings are extremely high, a commercial beta survey meter, such as a Geiger–Muller (GM) counter, would no longer have the ability to measure the beta contamination levels. In order to solve this issue, we utilized ultra -thin plastic scintillators to increase the detection rate of alpha and beta contamination. In this study, ultra-thin plastic scintillators with varied thicknesses of 7, 22, 24, 31, 39, and 55μm were prepared. To test their sensitivity, each scintillator was optically coupled to a glass plate and a 2-in square position-sensitive photomultiplier tube, and exposed to either an alpha, beta, or gamma source. Results from alpha spectrometry show that only the 55μm-thick plastic scintillator fully absorbed the alpha particles (5.5 MeV) with a 16.7% Full Width at Half Maximum. For alpha imaging under a high beta background, the 7μm-thick plastic scintillator was found to be the best choice, with an alpha to beta ratio of 652. This plastic scintillator also had the lowest sensitivity to beta particles, measuring only 41.74 ± 0.93 cps when exposed to a 1MBq 90Sr/90Y source in contact. It would therefore be possible to use the 7μm-thick plastic scintillator, to directly measure the surface contamination levels at the FDNPS site in real-time, where previous methods have fallen short Thus, the developed detector would be a useful tool for the detection of alpha and beta contamination in the FDNPS.