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A Study of the Irradiation-Temperature Coefficient for Alanine Film and Pellet Dosimeters at Elevated Temperature

Summary:

Correcting the response of a dosimeter for the average temperature during irradiation processing improves the accuracy of the dose measurement. 

Description:

The relationship between the dosimeter's radiation response to the absorbed dose and its temperature during irradiation is termed the irradiation-temperature coefficient, typically expressed in percent change per degree. The temperature rise in dosimeters irradiated with high-intensity ionizing radiation sources can be appreciable. This is especially true for electron-beam processing in which dosimeter temperatures can approach 80 °C. However, the temperature coefficients determined for commercial dosimeters have been characterized only up to ≈ 50 °C. The first NIST study revealed modest (0.5 % to 1.0 %) deviations from the predicted value at temperatures above 70 °C for absorbed doses of 1 kGy and 20 kGy.  However, these data were inconsistent with a National Physical Laboratory (NPL) manuscript published coincidently that attempted to address the same topic. Though similar in nature, the NPL study used dosimeters from a different manufacturer and of a different experimental design. The NPL study found significant deviations from linearity in the alanine temperature responses that were dose dependent.  A follow-up study by NIST was undertaken to resolve this discrepancy.  This study co-irradiated alanine dosimeters from each manufacturer used by NIST and NPL over a wide range of absorbed doses and irradiation temperatures.  The NIST study found that though there was a slight variation in the temperature coefficient between the two alanine dosimeter sources (a finding that was expected), both systems were linear with irradiation temperature up to 70 °C and the NPL observations of non-linearity were not reproduced.  It was presumed that the NPL findings of non-linearity resulted from their experimental design. These data confirmed that there is no fundamental difference in the two commercial alanine dosimeter sources and that temperature corrections could be made on industrial irradiations at the extremes of irradiation temperature and absorbed dose.