Development of a flow-cell alpha detector utilizing microencapsulated CsI:Tl granules and silicon PIN-photodiodes

A flow-cell detector was developed utilizing microencapsulated granular CsI:Tl and Si PIN-photodiode for gross alpha radiation quantification. The CsI:Tl scintillator was crushed and sieved to 63-90 and 90-125 μm particle size and encapsulated with 10% (ratio of coating layer mass to particle mass) of Parylene C polymer. The radiation detector consisted of a CsI:Tl flow-cell and two silicon PIN-photodiodes (PDs). Signals from the PDs were processed by charge sensitive preamplifiers and shaping amplifiers then digitized by a GaGe CompuScope 8012A/PCI 12-bit dual channel 50-MHz digital oscilloscope card residing in a Pentium III 450 MHz PC. The digitized signal was processed by a LabVIEW-based data acquisition program for pulse height and pulse shape analyses in conjunction with a software-realized coincidence mode with a resolving time of 800 ns. By setting an appropriate region of interest (ROI) for alpha events in a dual parameter histogram, the spillover of beta events into alpha ROI was less than 0.03%. The detection efficiency for ²³³U was about 10% in the first several hours and declined with time due to aqueous solution diffusing through the microencapsulant. The long component of detection efficiency "half-life" for 63-90 and 90-125 μm CsI:Tl granules was measured to be 36 hours when 1000 Bq mL^{-1 233}U in 0.1 HNO₃ solution circulated through the flow-cell continuously at a flow rate of 1.0 ml min^-1. The alpha minimum detectable concentration of a CsI:Tl flow-cell with an approximate active volume of 0.075 ml was measured to be 12.4 Bq mL^-1 for a 120-s count time.