Development of a well-type phoswich detector for low concentration Krypton-85 measurement

The measurement of low-level Krypton-85 (85Kr) in low-volume influent samples from nuclear power station is required nowadays. We present the development of an original internal gas scintillator detector along with Low-level Automated 85Kr Analyzing System (LAKAS) for that purpose. The designed phoswich detector is consist of a thin BC404 cell in the inner side for the detection of 85Kr beta rays, and a well-type NaI(Tl) in the outer side for the reduction of background with anti-coincidence method, and a single photomultiplier tube is used for the fluorescence collecting make it very compact. Moreover, a lightweight shield and a high-speed digital pulses analyzer are constructed. The preliminary test with 85Kr gas shows that the typical Minimal Detection Activity of the detector is as low as 0.012 Bq, and the Minimal Detection Concentration of LAKAS system is about 5 Bq/m3, which indicate that the detection efficiency to 85Kr is comparable with the traditional liquid scintillation spectrometry and internal gas proportional counters.