Doctor of Philosophy
School of Physics
A rarely studied feature of many naturally abundant isotopes is the possibility of exciting the nucleus using high energy photons into an isomeric state with a half-life of microseconds up to minutes and hours. These photonuclear interactions have been explored as a method of performing non-destructive trace elemental analysis of mineral ores using a technique known as Gamma Activation Analysis (GAA). In GAA, samples are irradiated with a linear accelerator to induce short-lived radioactivity in a sample, transferred to a shielded pair of high purity germanium detectors where their characteristic decay is measured. The time taken to mechanically transfer samples precludes the measurement of isomers with half-lives shorter than a second, which requires any measurement apparatus for these isomers to be near the intense exciting radiation and measure between rapid accelerator pulses. The production cross sections for these isomers do not exist in experimental reaction databases, and several isomers with a half-life less than a second have never been measured experimentally, presenting an opportunity of scientific and commercial interest. The element of primary commercial interest is the measurement of arsenic due to its presence in some mineral gold ores, which is possible with this technique due to the isomer 75mAs with a half-life of 17.6 ms.
To operate in this intense, high radiation dose environment in close proximity to the accelerator, a radiation hard detector system was designed, modelled, constructed and tested using several interchangeable cerium-based scintillation detectors chosen for their radiation resistance, low afterglow and suitable resolution. These scintillators are CeBr3, GYGAG:Ce and GAGG:Ce:Mg, with each having overlapping complementary strengths and weaknesses for operation in this environment. The most severe form of radiation induced damage observed in these scintillators is that of long-lived afterglow, with all three scintillators exhibiting both intermediate (following an accelerator pulse) and long lived (over many minutes) luminescence proportional to the radiation dose absorbed in the scintillation crystal. The previously unknown relative intensity and decay time of these components was characterised in each of the three scintillators by measuring the baseline signal intensity following both a single pulse and an entire extended irradiation of many minutes in two separate measurements. These separate results were normalised and compared to a single pulse with the assistance of Monte Carlo simulations to equate differences in dose rates. With the CeBr3 and GAGG:Ce:Mg scintillators, the short-lived afterglow components rapidly decayed, leaving the residual light intensity of a hundredth of a percent of the fluorescent intensity of the exciting pulse. The GYGAG:Ce took longer, taking some 1 ms to return to the same intensity. Using these measurements, the complete relative luminescence decay of each scintillator was fit using nonlinear least squares, with up to 6 different decay time components depending on the scintillator material.
Delpiano, Shaun Carlos, Measuring sub-second isomers using photon activation, Doctor of Philosophy thesis, School of Physics, University of Wollongong, 2023. https://ro.uow.edu.au/theses1/1548
FoR codes (2008)
020202 Nuclear Physics, 020504 Photonics, Optoelectronics and Optical Communications
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.