Characterization of a Disk-Shaped Graphite Calorimeter for Reference Dosimetry in Radiotherapy

We developed a disk-shaped graphite calorimeter, named Advanced KAERI Calorimeter (AKC), for reference dosimetry in both conventional radiotherapy (Conv-RT) and FLASH radiotherapy (FLASH-RT). The calorimeters were composed of a multilayered graphite core housed within a 3D-printed structure designed to match the dimensions of a widely used commercial ion chamber, thereby establishing compatibility with a designated solid water phantom. Their performance was evaluated using both low-dose-rate electron beams (∼4 Gy/m) and ultrahigh-dose-rate (UHDR) proton beams (∼55 Gy/s). Under conventional low-dose-rate irradiation, even subtle radiation-induced temperature changes were reliably detected with Type A uncertainties of approximately 1%. AKC #2 showed the best performance, exhibiting the lowest uncertainty with the electron beam and demonstrated a strong correlation (R2=0.86) with the incident proton fluence. It was also noted that the integration of the phantom significantly reduced the heat loss and improved the measurement stability under UHDR conditions. With the establishment of a well-defined traceability scheme under UHDR conditions, our AKC calorimeter offers a practical alternative to ion chambers for routine radiotherapy.