Dosimetric features and kinetic analysis of thermoluminescence from ultra-high molecular weight polyethylene

Abstract

Thermoluminescence (TL) from beta irradiated ultra-high molecular weight polyethylene has been studied for measurements between 30 and 200 °C. An aliquot studied in this work produced TL glow curves consisting of two peaks, the main peak at 88 °C and a weaker intensity peak at 148 °C for heating at 1 °C s−1 following an excitation dose of 215 Gy. The position of the main peak is poorly reproducible for heating rates of 0.2 and 0.6 °C s−1 investigated with the peak position decreasing when the sample is freshly irradiated and the TL re-measured. The said change in peak position is however less of an effect for measurements made at 1 °C s−1 with the peak position being fairly reproducible in this case. Further measurements of the dosimetric properties of ultra-high molecular weight polyethylene showed that its dose response is linear from 26 Gy to about 161 Gy but exhibits slower growth in intensity with dose from about 860 Gy after regions of sub- and supra-linearity in between. If the TL is not measured immediately after irradiation, the signal fades with the delay approximately exponentially. In addition, a number of tests including phosphorescence analysis showed the possibility that the order of kinetics might not be unique but sensitive to several factors including measurement temperature. Thus for instance, the dependence of the peak position on the stop temperature in the partial heating procedure Tm − Tstop implied first-order kinetics but analysis of the geometrical factor μg for the same set of data gave μg = 0.46 ± 0.03 a value corresponding to characteristics somewhat intermediate between first and second order. In comparison, the results of analysis of the phosphorescence recorded at several temperatures on the rising edge of the main peak were only in agreement for measurements at 40 °C with general-order analysis suggesting second-order kinetics apply as did TL-like transformation of the monotonic phosphorescence decay. Both results were also consistent with an additional finding that the time dependence of the isothermal decay at 40 °C was consistent with second-order kinetics. Analysis of the TL for the activation energy using the initial rise method, variable heating rate procedure, TL-like transformation of phosphorescence and the peak shape estimates, produced a self-consistent set of values equal to 0.76 ±0.05 eV, 0.66±0.03 eV, 0.77±0.06 eV and values of the order of 0.8 eV, respectively. The kinetic parameters found in this study compare favourably with literature values.