Endothermic Processes in Tissue-Equivalent Plastic

The use of tissue-equivalent plastic as the absorbing element for absolute calorimetric measurement of dose is limited by uncertainties existing in the heat lost by endothermic processes induced in the plastic by the absorbed radiation. Measurements have been conducted to determine experimentally the difference between the absorbed energy and the resulting thermal energy, a thermal defect, for Shonka-type tissue-equivalent plastic when irradiated by monoenergetic protons. Results of this work indicate that the thermal defect varies with total absorbed dose and is nearly twice as great as has been predicted for a similar plastic on the basis of independently determined G values. An extrapolated zero-dose value for the thermal defect was found to be 4.17 ± 0.2%. The thermal defect decreases with accumulated absorbed dose and appears to reach a saturation value of 3.67% for doses above 10⁸ rads.