In radiation calorimetry by using laser beams and interferometry setups, variations induced by dose absorption in phantom can be precisely measured. Dose absorption and the induced temperature change result in refractive index variation of the material. In order to be able to measure the low amount of absorbed dose in the phantom, temperature dependence of refractive index of the material must be precisely known. Water and Plexiglas phantoms can be used as tissue-equivalent materials and a multitude of reports have been published in recent years regarding their use for radiation calorimetry. In this study, temperature dependence of refractive index of water and Plexiglas (poly- methyl methacrylate) is measured using a laser interferometry setup. The results show that the refractive index of Plexiglas has more variations with temperature compared to water. This shows that use of Plexiglas as the absorbing material for optical calorimetry, due to its nearly zero heat defect, increases the accuracy of absorbed dose measurements.

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