Room: Track 5
Ultra-short pulsed Very High Energy Electrons (VHEEs) are found to be of interest for radiotherapy and, in addition, to operate within parameters that are known to produce the FLASH effect. This study aims to characterize a commonly used plane-parallel ionization chamber (PPIC) for reference dosimetry of ultra-short pulsed VHEEs by comparing its response with that of a graphite calorimeter (GC).
A PTW-34001 Roos-type PPIC was compared with a portable GC in an electron beam that delivered doses greater than 5 Gy/pulse, with an instantaneous dose-rate per-pulse of up-to 108 Gy/s. From this comparison the ion recombination in the PPIC is derived. To refine this evaluation, Monte Carlo calculations of the IC’s fluence correction factor is required and a Fano test has been conducted for the GEANT4 Monte Carlo code to determine its ability to simulate dose to the PPIC cavity by 200MeV electrons.
This work shows that this beam type produces significant ion recombination effects in the PPIC, with collection efficiencies as low as 4% for a 75V collecting voltage. The standard two-voltage method shows large deviations (up-to 50%) from the measured value of ion recombination above approximately 0.5 Gy/pulse. Previously investigated analytical high dose-per-pulse recombination models show qualitative agreement with the data, however, can still show large deviations over the dose-per-pulse range investigated. Measurement uncertainties have been evaluated and range between 0.78% and 5.8% depending on the beam configuration. The Fano test has shown deviations from dose uniformity within 0.1 %.
The experimental study demonstrates that the high recombination correction factors of the PPIC can be measured and satisfactorily modelled. Confidence in particle transport, based on the Fano tests, in turn allows confidence in determination of fluence perturbation correction factors. These are the first steps towards a standard dosimetry protocol for VHEE high dose-rate radiotherapy.