Doctor of Philosophy
School of Physics
Many elements possess short-lived isomeric states with half-lives ranging from seconds to minutes that are readily accessible via the interaction of high-energy photons with the nucleus, and the existence of these isomers can be exploited to perform rapid and non-destructive trace-element analysis. Using an 8 kW linear accelerator as a source of Bremsstrahlung X rays and Compton-suppressed high-purity germanium detectors to record the emission of characteristic γ rays, large-volume samples can be analysed with high precision at a rate far greater than existing assay methods. The formation cross- sections for several of these isomers cannot be found in nuclear reaction databases, and this presents an opportunity that is of both commercial and scientific interest.
Measurement of the γ rays emitted during the relaxation of these excited states can be impeded by the presence of other activation products that emit γ rays of a higher energy. The target photopeaks may be partially or completely obscured by the continuum of detected photon energies introduced by Compton scattering of these higher energy γ rays in the detector volume. It is possible to characterise and reject the scattered photons through the use of Compton suppression. The performance of our Compton suppression system is evaluated for different sources of background that are commonly encountered during analysis of mineral ores. The results indicate that up to 60% of the background due to Compton scattered γ rays can be successfully identified and flagged for rejection. The unfolding of reaction cross-sections from photonuclear yield data requires an accu- rate knowledge of the incident photon energy spectrum. A measurement programme was carried out to quantify the electron and X-ray spectra of the accelerator source across its 7–14 MeV operational range by measuring the induced activity of elements with known photonuclear reaction cross-sections.
Delaney, Justin, Investigating the physics of photon activation, Doctor of Philosophy thesis, School of Physics, University of Wollongong, 2021. https://ro.uow.edu.au/theses1/1089
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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.