Heat Transfer Effects in a Water Calorimeter for Measuring the Absorbed Dose of Therapy-Level Radiation Beams

R. E. Tosh, and H. H. Chen-Mayer
National Institute of Standards and Technology, Gaithersburg, MD, USA

Water calorimetry that directly measures the temperature rise (at the mK level) due to radiation heating is used as a primary standard for therapy-level gamma-radiation beams. The temperature rise is measured at a given point in space where the spatial distribution of the absorbed dose is non-uniform, and therefore is subject to heat conduction and convection distortions that must be corrected to recover the accurate dosimetric value.

Corrections are typically estimated using finite-element modeling of these processes analyzed in the time domain which is sensitive to timing details of the source.

By using periodic modulation of the radiation source and measuring an effective frequency response, a system transfer function of the calorimeter can be derived.