Measurement of radioactivity in aqueous solutions by heterogeneous scintillation detection with comparison to stochastic models

Digital instrumentation for heterogeneous scintillation flow-cell radiation detection has been developed and optimized using Monte Carlo computer codes for the quantification of gross alpha and beta radionuclides in aqueous streams. A digital alpha/beta pulse shape discriminating data acquisition system utilizing CsI:Tl granules microencapsulated with 3% (ratio of coating layer mass to scintillator granule mass) Parylene C polymer and photomultiplier tubes was developed for on-line measurement of alpha and beta emitters. Also, a flow-cell detection system utilizing 10% microencapsulated CsI:Tl granules and silicon PIN-photodiodes was used for gross alpha quantification. The comparison between the performances of these two systems demonstrated that the high intrinsic noise of photodiode operated at room temperature limits its alpha/beta pulse shape discriminating capability, and the thicker polymer coating on CsI:Tl significantly improved its “half-life” but at the expense of reduced detection efficiency. Monte Carlo modeling was performed on heterogeneous scintillation flow-cell detectors to investigate the effects of geometrical and optical properties of the flow-cell detector on the percentage of charged particle energy deposited in the scintillator and the scintillation photon collection efficiency. The simulation results showed that geometrical factors including packing geometry, scintillator granule size and initial location of radioactivity have strong effects on charged particle energy deposition in the scintillator, while scintillation photon detection was mainly affected by optical factors such as index of refraction of the flow-cell detector components, reflectivity of the flow-cell holder internal surfaces, etc. The simulated geometrical detection efficiencies were shown to be comparable to experimental absolute detection efficiencies. Under typical experimental conditions, the models indicate that the charged particle energy deposition has a greater effect on the absolute detection efficiency than the scintillation photon detection.; The development of digital instrumentation demonstrates the feasibility and versatility of digital signal processing techniques on radiation pulse processing. The Monte Carlo modeling provides a tool to understand fundamentally the operation of heterogeneous scintillation flow-cell detectors. The results from both the experimental testing and computer modeling of heterogeneous scintillation flow-cell detectors will aid the future instrumentation development of such detectors which have application in environmental monitoring and remediation activities.