High dose-per-pulse electron beam dosimetry: Usability and dose-rate independence of EBT3 Gafchromic films

The aim of this study was to assess the suitability of Gafchromic EBT3 films for reference dose measurements in the beam of a prototype high dose-per-pulse linear accelerator (linac), capable of delivering electron beams with a mean dose-rate (Ḋ_m ) ranging from 0.07 to 3000 Gy/s and a dose-rate in pulse (Ḋ_p ) of up to 8 × 10⁶ Gy/s. To do this, we evaluated the overall uncertainties in EBT3 film dosimetry as well as the energy and dose-rate dependence of their response.

Our dosimetric system was composed of EBT3 Gafchromic films in combination with a flatbed scanner and was calibrated against an ionization chamber traceable to primary standard. All sources of uncertainties in EBT3 dosimetry were carefully analyzed using irradiations at a clinical radiotherapy linac. Energy dependence was investigated with the same machine by acquiring and comparing calibration curves for three different beam energies (4, 8 and 12 MeV), for doses between 0.25 and 30 Gy. Ḋ_m dependence was studied at the clinical linac by changing the pulse repetition frequency (f) of the beam in order to vary Ḋ_m between 0.55 and 4.40 Gy/min, while Ḋ_p dependence was probed at the prototype machine for Ḋ_p ranging from 7 × 10³ to 8 × 10⁶ Gy/s. Ḋ_p dependence was first determined by studying the correlation between the dose measured by films and the charge of electrons measured at the exit of the machine by an induction torus. Furthermore, we compared doses from the films to independently calibrated thermo-luminescent dosimeters (TLD) that have been reported as being dose-rate independent up to such high dose-rates.

We report that uncertainty below 4% (k = 2) can be achieved in the dose range between 3 and 17 Gy. Results also demonstrated that EBT3 films did not display any detectable energy dependence for electron beam energies between 4 and 12 MeV. No Ḋ_m dependence was found either. In addition, we obtained excellent consistency between films and TLDs over the entire Ḋ_p range attainable at the prototype linac confirming the absence of any dose-rate dependence within the investigated range (7 × 10³ to 8 × 10⁶ Gy/s). This aspect was further corroborated by the linear relationship between the dose-per-pulse (D_p ) measured by films and the charge per pulse (C_p ) measured at the prototype linac exit.

Our study shows that the use of EBT3 Gafchromic films can be extended to reference dosimetry in pulsed electron beams with a very high dose rate. The measurement results are associated with an overall uncertainty below 4% (k = 2) and are dose-rate and energy independent.