A thin dyed-plastic dosimeter for large radiation doses

A commercially available, thin, dyed-plastic film (mean thickness 30 μm) having a transparent green color, changes color progressively to brown shades and then to a deep red shade upon irradiation at successively higher values of absorbed dose (gamma radiation) in the range 8–60 kGy. The analysis for dosimetry is made with spectrophotometric measurements (at visible light wavelengths) of pre-irradiation absorbance, A₀, and radiation-induced absorbance, A_i. Either the wavelength of the maximum of the absorption band that bleaches upon irradiation (γ = 425 nm) or the wavelengths of the maxima of the two overlapping radiation-induced absorption bands (γ = 525 or 556 nm) are used to make these measurements. The most reproducible calibration consists of the ratio, $\text{A}{}_{\mathrm{<mtext>i</mtext>}}\text{A}{}_0$, at a given wavelength, as a function of the absorbed dose. The new film dosimeter is flexible, rugged, and relatively stable at temperatures between −80°C and +50°C, and in daylight. The radiation-induced color, in terms of the absorbance, A_i, at selected wavelengths, is unstable for about two days subsequent to irradiation, after which the values of $\text{A}{}_{\mathrm{<mtext>i</mtext>}}\text{A}{}_0$ remain stable for at least several months, even under illumination with white fluorescent or indirect daylight. The film color is unstable at high temperatures (60°C and above). The film sensitivity to gamma radiation increases gradually with rise in irradiation temperature up to 50°C and then decreases sharply at temperatures above 60°C. There is only slight variation of response with change in relative humidity during irradiation.