RESPONSE OF PLASTIC DOSIMETERS TO GAMMA-RAYS AND ELECTRONS AT HIGH ABSORBED DOSE-RATES

Several clear plastics and dyed plastics are commercially available for dosimetry in intense radiation fields, particularly for radiation processing applications using gamma rays from intense cobalt-60 sources and electron beams from accelerators running at potentials from 0.1 to 10 MeV. Some of these widely used dosimeters are: colorless cellulose triacetate, polymethylmethacrylate, or polyvinylchloride, showing induced absorption bands in the ultraviolet spectrum; radiochromic dyed plastics, such as nylon, polychlorostyrene, polyvinylbutyral, polyvinylpyrrolidone, showing induced absorption bands in the visible spectrum; blue cellophane, showing a bleaching in the visible absorption spectrum. A major source of error in making dose interpretations by spectrophotometric analysis of these dosimeters comes from dose-rate dependence of response. The present work shows measurements at high dose rates (e.g. 100 and 1050 rad · s-1 with gamma radiation, 106 rad · s-1 with a scanned beam of 2-MeV electrons, 1010 rad · s1 with a pulsed beam of 10-MeV electrons and 1011 to 1014 rad · s-1 with single pulses of 2-MeV electrons). For calibrations with gamma rays, only those plastics containing the radiochromic triphenylmethane cyanides or methoxides, i.e. nylon, polychlorostyrene, polyvinylbutyral, or polyvinylpyrrolidone, can correctly interpret dose determinations from electron beam irradiation. The other systems show a marked rate dependence of response. Of radiochromic dye systems, only polyvinylpyrrolidone as host matrix can be read with precision immediately after electron irradiation, since a slow build-up of the absorption band during the first hours after irradiation occurs in the other radiochromic plastic systems. © 1979.