Extending the dose range of the red 4034 PMMA dosimeter

Abstract

The working absorbed dose range of the radiation-processing dosimeter Harwell Red 4034 PMMA is normally quoted as 5–50 kilograys, because below a gamma radiation dose of 5 kGy spectrophotometric measurement precision deteriorates, and above 50 kGy the reduced slope of the optical absorbance versus dose curve, due to approaching saturation of the radiation induced colouration, would notionally cause significant increases in overall dose measurement uncertainty with increasing dose. However, dosimetric uncertainty studies are sparse, and there are no reported studies that would unequivocally support the choice of 5–50 kGy as the ideal range. Two batches of Red 4034 dosimeters have been studied over a wider gamma radiation dose range than normal, 1–100 kGy, and a mathematical model has been used to interpret the resulting specific optical absorbance and standard deviation data in the form of the spectrophotometric component of overall dosimetric uncertainty, expressed as a function of absorbed gamma radiation dose. The resulting spectrophotometric uncertainty versus dose curves support the choice of 5 kGy as a reasonable lower limit of measurable dose using normal measurement techniques. At doses above 50 kGy, however, the model suggests that the spectrophotometric components of dosimetric uncertainty are such that, provided other components of overall uncertainty are also small under the particular irradiation conditions used, 100 kGy could be acceptable as an upper limit.