Masters of Science (Research)
Faculty of Engineering
Flynn, Alison, Investigation and characterization of a dual anti-Compton probe for anti-terrorism applications, Masters of Science (Research) thesis, Faculty of Engineering, University of Wollongong, 2011. https://ro.uow.edu.au/theses/3544
Trafficking of illicit nuclear materials such as highly enriched uranium and plutonium is a significant issue in today’s society. An important method to limit the distribution of these substances, is the application of gamma spectroscopy methods which can identify radioactive material even when attempts have been made to mask their presence. The Compton scattering of photons emitted from legitimate radioisotopes and/or medical isotopes can be used to mask identifying gamma lines of illicit material which occur in the same energy region.
The Centre for Medical and Radiation Physics (CMPR) has developed two Compton suppression detectors, the performance of which has been investigated in this work. The detectors are designed to be small in size and weight to form the basis for a hand-held detector that can be used by customs officials in the field. The detectors are based on two concentric CsI(Tl) scintillators: the inner is used for spectroscopy, and the outer vetoes any events which are scattered from the inner. One design uses silicon PIN photodiodes and the other is based on silicon photomultiplier (SiPM) array.
The scope of this project involved characterising the devices and comparing the performance of the two designs utilising detailed gamma ray spectroscopy and initial trials to examine the Compton suppression ability. The SiPM based scintillator readout system was found to have a resolution of approximately 15% for 511 keV gammas incident on the scintillator. Compton suppression was achieved using two different setups, one using a conventional PMT optically coupled to the outer scintillator, while a SiPM was optically coupled to the inner scintillator, the other using SiPM optically coupled to both the inner and the outer scintillator. The tests with the SiPM+PMT showed that the scintillator configuration could produce a Compton suppression of up to 80%. When both scintillators were coupled with the SiPM, the best Compton suppression achievable was 50% at room temperature.
The PIN photodiode system is a less elegant, but more robust system than that based on the SiPM. It was found that the PIN photodiodes based devices had an energy resolution of 6.9% for 80 keV gamma detected directly in the silicon. Unfortunately some instabilities in the leakage current developed over time in the photodiodes, possibly due to the thin oxide layer used in the large optical entrance window. This effect was further exacerbated by the optical coupling grease used to attach the scintillator. As a result any further photodiode characterisation was not attempted in this project. Future photodiode designs will incorporate a thicker oxide layer to avoid the observed instability issues.
From these results it was concluded that the Compton suppression systems showed promise for further development, with room for future optimisation of the results.