A prototype low-cost secondary standard calorimeter for reference dosimetry with ultra-high pulse dose rates

Abstract

Objectives: Ultra-high pulse dose rate modalities present significant dosimetry challenges for ionisation chambers due to significant ion recombination. Conversely, calorimeters are ideally suited to measure high dose, short duration dose deliveries and this work describes a simple calorimeter as an alternative dosemeter for use in the clinic. Methods: Calorimeters were constructed featuring a disc-shaped core and single sensing thermistor encased in a 3D-printed body shaped like a Roos ionisation chamber. The thermistor forms one arm of a DC Wheatstone bridge, connected to a standard DMM. The bridge-out-of-balance voltage was calibrated in terms of temperature. A graphite-core calorimeter was calibrated in terms of absorbed dose to water (J/kg) in Co-60 and conventional 6, 10 and 15 MV X-rays. Similarly, an aluminium-core calorimeter was calibrated in a conventional 20 MeV electron beam and tested in a research high dose per pulse 6 MeV electron beam. Results: Calorimeters were successfully calibrated in terms of absorbed dose to water in conventional radiotherapy beams at approximately 5 Gy/min with an estimated uncertainty of ±2–2.5% (k = 2), and performed similarly in a 6 MeV electron beam delivering approximately 180 Gy/s. Conclusions: A simple, low-cost calorimeter traceably calibrated to existing primary standards of absorbed dose could be used as a secondary standard for dosimetry for ultra-high pulse dose rates in the clinic. Advances in knowledge: Secondary standard calorimeters for routine measurements are not available commercially; this work presents the basis of a simple, low-cost solution for reference dosimetry for ultra-high pulse dose rate beams.