5TH International Congress on Technology - Engineering & Science - Kuala Lumpur - Malaysia (2018-02-01)

Preliminary Study On The Lyso:ce/bcf-12 Phoswich Detector For Simultaneous Detection Of Beta And Gamma Radiations

The decommissioned site could be released from regulatory control only if the residual radioactivity is confirmed to be lower than the release criteria. For this, radioactive nuclides shall be identified and be considered in the evaluation of the radiation exposure pathways. At the decommissioned site, Co-60, Sr-90, I-131, Cs-137, Pu, Th, U are usually found [1]. Among those radionuclides most of the radionuclides are gamma emitter, but some nuclides are beta-emitter. [2] The samples collected from soil at the decommissioned site may include mixture of beta and gamma emitters, which requires the simultaneous detection of beta and gamma radiation to identify the radionuclides in them. At present, however, the samples are measured separately for beta and gamma radiation by using the different detectors. So, the separate measurements are time-consuming. This paper examined the feasibility of the phoswich detector for simultaneous detection of beta and gamma radiation, which is a kind of fiber-optic radiation sensor (FORS) [3]. The sensing probe of the FORS was fabricated using a bundle of the organic plastic scintillator(BCF-12) and inorganic scintillator, (Lu,Y)2SiO5:Ce (LYSO:Ce) and a mixture of epoxy resin. The measurement system consisted of a sensing probe with an aluminum foil reflector, transmitting plastic optical fiber, and a light measuring system comprising a Multi pixel photon counter(MPPC) and laptop computer. The sensing probe was connected with organic plastic scintillator bundle and inorganic crystal scintillator. Two sensing probes have the same diameter of 26 mm but the different heights of 20 and 30 mm, respectively. To characterize it, the FORS was applied to measure the disk type radiation source, Co-60, Sr-90, Cs-137. Figure 1 shows the experimental setup for the measurements. To prevent outside light from entering the FORS, it was enclosed in a black box. Before the measurement of disk sources, the background counts over 100 seconds were measured 5 times. Table 1 shows the average background counts. The average background count by the FORS was 12480.4 and the critical level (L_c) was 26 counts. After measuring background count, each disk source was measured 5 times by FORS. Table 2 summarizes the measurements. The total counts were larger than the critical level, which means its radioactivity were distinguishable from the backgrounds [4]. According to performance test, the FORS developed in this study could be used to measure beta and gamma radiations simultaneously. On the other hand, the FORS should be improved to produce more accurate measurements and radiospectroscopy through an iterative calibration process. It is expected that with some improvements, the FORS could be used as a phoswich radiation sensor for on-site scanning of the decommissioned site.
Hanyoung Joo, Jae Wook Kim, Joo Hyun Moon