Delayed Darkening of Radiation-Exposed Radiochromic Dye Dosimeters

Thin dosimetry films, type FWT-60, consisting of hexahydroxyethyl pararosaniline cyanide dye in a Dupont Nylon resin, have been found to exhibit a considerable delayed darkening that may, depending on relative humidity, last for tens of hours at room temperature, until full color saturation is reached. Erroneous dosimetry measurements may therefore result if optical density measurements are made before this delayed darkening has proceeded to saturation. This effect has been investigated using both 1 MeV electrons and Co-60 gamma rays, and has been found to be independent of dose rate in the range 0.7 to 70 Gy, radiation species (electrons or gamma rays), and the percentage of total radiation dose which it represents. The amount of delayed darkening, i.e., the percentage of optical density at saturation, was found to depend very strongly on the relative humidity at which the films were equilibrated a few hours before irradiation. The delayed darkening which remained 10 minutes after irradiation was found to be 45% for nominal 0% RH (1 hour in vacuum), 23% for 25-35% RH, and less than 3% for 65-75% RH. Heating the films for 1 minute at 45°C effects a decrease in delayed darkening from 45% to 20%, and for 5 minutes, the decrease is to less than 5%. A number of recommendations are made to account for delayed darkening in FWT-60 films in order to maximize accuracy and repeatability of dosimetry measurements.