What Spectral Methods Can Tell Us about Heat Transfer Effects in Water Calorimeters

In water calorimetry, numerous ancillary effects – such as radiation-induced chemical reactions, scattering and excess heat from nonwater materials, and dose nonuniformities within the phantom – complicate the determination of absorbed dose from measurements of radiation-induced heating. Corrections for thermal transport due to excess heat and dose nonuniformities can be difficult to assess because the effects are delayed by variable amounts of time – from seconds to hours – that depend upon the geometry of the probes, the calorimeter vessel and the radiation beam. Typically, such corrections involve finiteelement modeling of these components that is analyzed in the time domain and, accordingly, is sensitive to timing details of the source. We have developed a technique that circumvents this difficulty by using periodic modulation of the radiation source and measuring an effective frequency response, or system transfer function, of the calorimeter. By tracing the frequency dependence of systematic deviations from nominal or applied dose rates due to heat conduction, our approach provides a basis for assessing systematic errors for all radiation exposure times, including those that can not be handled by data analysis techniques like midpoint extrapolation.