Development of an alpha- and beta-imaging detector using a thin-stilbene plate for radon-222 progeny measurements

摘要

Radon-222 is a naturally-occurring radioactive gas, and the measurement of this isotope and its progeny is of interest from the viewpoint of protection against internal exposure. The author has developed a new alpha/betaimaging detector combined with a waveform digitizer capable of imaging alpha- and beta-particle locations simultaneously. A 0.9-mm-thick stilbene plate was optically coupled to a position-sensitive photomultiplier tube (PSPMT), and the analog signals from the PSPMT were transferred in parallel to the waveform digitizer. For 5.5-MeV alpha particles, the detection efficiency was 97.2% for 2pi steradians, and the energy resolution of the 5.5 MeV alpha peak was 21.6 +/- 0.18% full width at half maximum (FWHM). The imaging detector was able to discriminate between alpha and beta particles via the pulse-shape-discrimination (PSD) technique, as well as being capable of alphaand beta-particle imaging. The imaging detector was used for Rn-222-progeny measurements. When measuring the Rn-222 progeny, both Bi-214 and Po-214 are imaged simultaneously, and a correlation is found between the positions of Bi-214 and Po-214. The developed detector will be useful as a Rn-222 detector and in alpha-continuous-air monitoring for nuclear facilities.