4.1 The cellulose triacetate (CTA) dosimetry system provides a means for measuring absorbed dose in materials (2-15). The dosimeter is a film containing cellulose triacetate and plasticizer. The dosimetry is based on chemical reactions in the film resulting in changes in the optical absorption properties in near ultraviolet region. The absorbance is measured at a specific wavelength using a spectrophotometer or equivalent photometric instruments.

4.2 Absorbed dose is evaluated by use of a calibration curve or response function traceable to nationally or internationally recognized standards.

4.3 Absorbed dose is usually specified in terms of absorbed dose to water. Absorbed dose to other materials may be evaluated by applying the conversion factors discussed in ISO/ASTM Guide 51261.

NOTE 2 - For a comprehensive discussion of various dosimetry methods applicable to the radiation types and energies discussed in this practice, see ICRU Reports 14, 17, 34, 35, and 37. 4.4 This dosimetry system may be used in industrial radiation processing of various products, for example radiation effect tests, polymer modifications, and sterilization of medical devices.

4.5 A one-dimensional absorbed-dose mapping on or within a product may be obtained by irradiating a dosimeter strip of appropriate length.

4.6 The absorbed-dose range indicated in 1.2.1 may be achieved by using triphenylphosphate (TPP) as the sole plasticizer in the dosimeter.

4.7 The effect on the dosimeter response due to changes in the irradiation conditions, such as absorbed-dose rate, temperature, humidity, and atmosphere should be considered when these are different from the calibration conditions.

4.7.1 The dosimeter has different responses to electron beams at relatively high dose rates than to gamma radiation at relatively low dose rates (see Fig. 1). Calibration should be carried out separately for each type of radiation.

4.7.2 The dosimeter response increases linearly with temperature from -10 to 60°C at 7 kGy/h, and from -10 to 40°C at 1.2 kGy/h (7), and with relative humidity (up to 80 %) during irradiation when irradiated at lower dose rates (<10 kGy/h) typical of gamma irradiators. The effects are not appreciable at higher absorbed-dose rates (>100 kGy/h). All these effects need to be considered before the dosimeter can be used routinely for processing (4,7,12,15). For high-dose-rate dosimetry (for example, electron beams) the influence of humidity on dosimeter response is not appreciable (4,7). For other conditions it is recommended to calibrate the dosimetry system by the final user under specific environmental conditions.