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 silicon PIN-photodiodes 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 8012 A/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 3.9%. The initial detection efficiency for ²³³U was 8.8-19.5% and declined with time due to aqueous solution diffusing through the microencapsulant. The initial detection efficiency decreased with increased particle size, increased porosity, and increased contact time. The slow component of the detection efficiency "half-life" was measured to be 36 h when 1000 Bq mL^{-1 233}U in 0.1 HNO₃ solution circulated in a closed loop through the flow-cell at a flow rate of 1.0 ml min^-1. The alpha minimum detectable concentration of a CsI:Tl flow-cell with approximate active volume of 0.075 ml was measured to be 12.4 Bq mL^-1 for a 120-s count time.