IMPROVING CALIBRATION ACCURACY IN GEL DOSIMETRY

A new method of calibrating gel dosimeters (applicable to both Fricke and polyacrylamide gels) is presented which has intrinsically higher a ccuracy than current methods, and requires less gel. Two test-tubes of gel (inner diameter 2.5 cm, length 20 cm) are irradiated separately w ith a 10 x 10 cm(2) field end-on in a water bath, such that the charac teristic depth-dose curve is recorded in the gel. The calibration is t hen determined by fitting the depth-dose measured in water, against th e measured change in relaxivity with depth in the gel. Increased accur acy is achieved in this simple depth-dose geometry by averaging the re laxivity at each depth. A large number of calibration data points, eac h with relatively high accuracy, are obtained. Calibration data over t he full range of dose (1.6-10 Gy) is obtained by irradiating one test- tube to 10 Gy at dose maximum (D-max), and the other to 4.5 Gy at D-ma x. The new calibration method is compared with a 'standard method' whe re five identical test-tubes of gel were irradiated to different known doses between 2 and 10 Gy, The percentage uncertainties in the slope and intercept of the calibration fit are found to be lower with the ne w method by a factor of about 4 and 10 respectively, when compared wit h the standard method and with published values. The gel was found to respond linearly within the error bars up to doses of 7 Gy, with a slo pe of 0.233 +/- 0.001 s(-1) Gy(-1) and an intercept of 1.106 +/- 0.005 Gy. For higher doses, nonlinear behaviour was observed.