Development of a 4πβ (Plastic Scintillator) - 4πγ Detection System for Radionuclide Standardisation

This thesis describes the development of a 4πβ - 4πγ detection system to perform primary standardisations of activity at the Radionuclide Metrology Laboratory at Australia’s Nuclear Science and Technology Organisation (ANSTO). Characterisation of the detectors, plastic scintillator for β-detector and well type NaI(Tl) detector for γ-detector, has been investigated by using Monte Carlo GEANT4 simulation and experimental measurement. Modelling of energy deposition in the scintillator and optical photon transport were employed the GEANT4 simulations. Several issues of the well type NaI(Tl) detector related to potential dead layer/inactive materials, full energy peak efficiency, coincidence-summing correction, and energy resolution broadening have been addressed. An optimum configuration of plastic scintillator detector for 60Co measurement was obtained to maximize electron detection probability and minimize photon interference probability at the same time. A hardware configuration of the 4πβ−4πγ instrument which utilizes a CAEN N6751 digitizer as the digital acquisition device has been implemented. The digitizer applied Pulse Shape Discriminator technique to process detector signals and generate binary list-mode files that represent measurement data of detectors. An offline-analysis method (OAM) based on Python program for the coincidence counting was written to analyse the list-mode data. This method effectively applied time settings (such as dead time, delay and resolving time), algorithm of coincidence counting and its correction, correction of background and decay, activity concentration calculation, and efficiency extrapolation with weighted linear fit as well. Using this method, accurate calculations of absolute radioactivity measurement can be performed repeatedly offline since raw detector signals were first experimentally measured, reducing time and unwanted fluctuations of the measurement.