Room: Track 3
Purpose: One of the challenges in investigating FLASH radiation therapy is the accurate measurement of the absorbed dose at the ultra-high dose per pulse values seen in these beams (1 Gy/pulse). Ion chambers are not suitable, due to the very large (> 30% @ 1Gy/pulse) ion recombination correction, and solid state detectors (diode, diamond) are similarly affected. Passive dosimeters, such as radiochromic film, can be used but do not provide real-time data. In this work, an absorbed dose calorimeter is investigated as a suitably accurate active dosimetry system for electron beams delivered at FLASH dose rates.
Method: The calorimeter is an aluminium open-to-atmosphere calorimeter, operated in quasi-adiabatic mode. The dose deposited in the aluminium core was determined from the product of the specific heat capacity and the radiation-induced temperature rise.
The measurement was carried out at the Metrological Electron Accelerator Facility of the German National Metrology Institute, PTB. Ten pulses were delivered at a pulse repetition frequency of 5 Hz to the calorimeter and the measurement was repeated 5 times. The pulse charge, and thus the dose per pulse, was varied between 0.5 to 1.5 Gy per pulses. The pulse charge was measured non-destructively with a calibrated beam current transformer (ICT).
Results: The response of the calorimeter was proportional to the dose per pulse in the investigated range, 0.5 to 1.5 Gy deposited per pulse. On average, the standard deviation of the calorimeter response was 0.1 %. The absolute deviation of the calorimeter dose measurement was 0.24 % from the linear fit compared to the ICT (uncertainties 0.05 %) and maximum deviation was 0.5 %.
Conclusion: This preliminary investigation confirms the suitability of the simplified calorimeter design for FLASH dosimetry. The design could be applied to other beam modalities (photons, protons) and operated in clinical facilities.