Optimization of a large-area phoswich detector coupled with a 3-Inch photomultiplier tube for decommissioning

On June 19th, 2017, the nuclear power plant Kori No.1 was permanently shut down, and it will be decommissioned for about 15 years. During the initial five-year period, surveys of the site and building characterization are... [ view full abstract ]

On June 19th, 2017, the nuclear power plant Kori No.1 was permanently shut down, and it will be decommissioned for about 15 years. During the initial five-year period, surveys of the site and building characterization are essential to the decommissioning plan. According to the Multi-Agency Radiation Survey and Site Investigation Manual (MASSIM), surface contamination levels of alpha, beta, and gamma-rays for buildings and soil must be measured for characterization in the course of decommissioning. To do this, a large-area phoswich (phosphor sandwich) detector enables efficient measurement of surface contamination over a large area for alpha and beta particles. However, a light guide is required to increase light collection efficiency, because the size of a photomultiplier tube is limited. Therefore, the purpose of this study is to experimentally optimize the thickness of the light guide in order to improve detection efficiency (2π) and ultimately minimum detectable activity (MDA). The double-layer phoswich detector composed of a ZnS(Ag) crystal (0.08mm thickness) as the first layer and a plastic scintillator (0.5mm thickness) as the second layer was used. The detectors with an area of 100x100mm² and 150x150mm² were used to confirm the effect of the area size. A 3-inch Hamamatsu photomultiplier tube (PMT) operated at -1300V was connected with a CAEN DT5725 digitizer to process the anode pulse signal. The detection efficiencies for alpha and beta particles were measured by varying the 10, 30, 50 mm thickness of the light guide as shown in Figure 1. 100x100mm^{2 241}Am and ⁹⁰Sr were used as alpha and beta radioactive sources respectively. Five 1-minute measurements were made to evaluate the detection efficiencies for alpha and beta particles. Figure 2 shows the example of anode pulses induced by alpha and beta particles. The two pulses can be easily separated by using a charge comparison method because of the different decay constant of the scintillators. Table 1 shows the detection efficiency depending on the area size of the detector and the thickness of light guide. It indicates that the highest detection efficiency in both 100x100mm² and 150x150mm² phoswich detectors is at a 10mm thickness of light guide. In addition, the 100x100mm² phoswich detector shows higher detection efficiency. However, in terms of the MDA, the 150x150mm² phoswich detector is superior to the 100x100mm² phoswich detector.