McManus, Michael; (2022) Dosimetry of ultra-short high dose-per-pulse very high energy electrons. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Detailed characterisation of a standard plane-parallel ionisation chamber has been made when exposed to high dose-per-pulse Very High Energy Electrons (VHEEs). First-of-their kind absolute dosimetry measurements using a graphite calorimeter have been conducted in a novel VHEE beam and ionisation chamber correction factors such as ion recombination have been shown to be significant. Ion recombination has been shown to increase with increasing dose-per-pulse, with a collection efficiency as low as 4% for the highest dose-per-pulse investigated, 5.26 Gy/pulse. Current theoretical recombination models provide a reasonable description of the ion recombination behaviour. Moreover, the free-electron fraction component of ion recombination models was shown to vary with dose-per-pulse, contrary to what is typically expected, and an updated model proposed in this work has been shown to provide a better fit to data than currently available recombination models. Following the experimental campaigns, Monte Carlo (MC) simulations were conducted to determine stopping-power-ratios, perturbation factors and beam quality correction factors for a reference 12 MeV beam and 200 MeV user VHEE beams using the Geant4 general purpose MC code. A Fano test was conducted and several charged particle transport parameter configurations were found to pass the Fano cavity test. Modifications to current Geant4 default physics parameters were also determined in order to provide a passing Fano test at 200 MeV. Stopping-power-ratios were found to agree within uncertainties with that found previously using EGSnrc at 12 MeV, however, perturbation factors were found to vary more than previous studies. The stopping-power-ratio at 200 MeV was found to be approximately 6% lower than what is estimated in dosimetry protocols for a reference beam quality with similar measurement depth, with a total perturbation of approximately 5%. The beam quality correction factor, for the conversion of dose from the reference beam to that of the user beam, was found to lead to an approximately 10% reduction in measured chamber dose in comparison to what was originally determined. Correction factors for the graphite calorimeter have also been calculated for the 200 MeV VHEE beam with the vacuum gap correction factor shown to be within 1% of unity. Implementation of these new corrections to early experimental data largely remedies un-physical ion chamber measurements which showed greater than 100% ion collection efficiencies for a number of collecting voltages. It is now clear that improved dosimetry for VHEEs is vital to determine reasonable and accurate characterisations of secondary standard ionisation chambers.

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