Evaluation of a simple graphite calorimeter for ultra‑high dose‑rate electron beam dosimetry

OBJECTIVE: This study evaluates the performance of a secondary‑standard graphite calorimeter (SSCal) for absorbed‑dose measurements in ultra‑high dose‑rate (UHDR) electron beams, motivated by the need for reliable, traceable dosimetry to support the development of FLASH radiotherapy.
Approach. The SSCal was calibrated against the primary absorbed‑dose standard at the National Physical Laboratory (NPL) and characterised in conventional 6-10 MeV electron beams. Monte Carlo (MC) simulations were used to derive detector‑specific perturbation and field‑size correction factors, including conversion from absorbed dose to graphite to absorbed dose to water. The SSCal was then tested in a 9 MeV UHDR beam (0.06-5.78 Gy pulse⁻¹) at the Centro Pisano FLASH Radiotherapy (CPFR) facility. Absorbed dose was determined using two independent approaches: (1) application of the calibration coefficient derived at NPL and (2) first‑principles calorimetry. Alanine dosimetry provided an independent traceability route for cross‑validation.
Main Results. Calibration coefficients obtained at NPL showed minimal dependence on field size or beam quality. MC simulations indicated that perturbations were dominated by internal air gaps, with increasing magnitude at smaller field sizes. Corresponding beam quality correction factors enabled accurate application of the NPL calibration to the UHDR beam. Across all dose‑per‑pulse (DPP) values, the two dose determination methods agreed within 1%. The SSCal exhibited excellent linearity with pulse number and stable response for three pulses or more. Deviations at low pulse numbers were attributed to transient beam current transformer (BCT) effects and potential heat‑transfer non‑linearities. Agreement between SSCal and alanine was within ±1%, except at the highest DPP (5.71 Gy pulse⁻¹), where a 2.6% deviation was observed, consistent with these low‑pulse effects.
Significance. The results demonstrate that the SSCal provides stable, linear, and largely dose‑rate‑independent absorbed‑dose measurements in UHDR electron beams when appropriate MC‑derived corrections are applied, supporting its suitability as a reference‑quality dosimeter for FLASH research and development.

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