EFFECTS OF CROSS-LINKING AND TEMPERATURE ON THE DOSE-RESPONSE OF A BANG POLYMER GEL DOSIMETER

The effects of varying the weight fraction (%C) of the crosslinker N,N '-methylene-bisacrylamide (bis) per total amount of monomer (6% w/w), and the NMR measurement temperature, on the dose response of the trans verse relaxation rate (R(2)) of bis-acrylamide-nitrogen-gelatin (BANG) aqueous polymer gel dosimeters have been investigated. The gel sample s were irradiated in test tubes with 250 kV x-rays, and the water prot on NMR transverse relaxation rates were measured at 0.47 T using a Car r-Purcell-Meiboom-Gill multiecho pulse sequence. Both the dose sensiti vity (slope of the linear portion of an R(2)-dose response) and the ma ximum rate at which the R(2)-dose response saturated (R(2)(max)), were found to depend strongly on the crosslinker fraction and on the tempe rature of the R(2) measurement. The dose sensitivity peaked at approxi mately 50% C, and, for this composition, varied from 0.14 s(-1)Gy(-1) at 40 degrees C to 0.48 s(-1) Gy(-1) at 10 degrees C. The maximum tran sverse relaxation rates ranged from 0.8 s(-1) at 33% C and 40 degrees C to 11.8 s(-1) at 83% C and 5 degrees C. These results suggest that w ater proton transverse relaxation in the gel is controlled by an excha nge of magnetization between the aqueous phase and the semi-solid prot ons associated with the polymer, and that the latter experience spectr al broadening from immobilization which increases with crosslinking or cooling. Theoretical and practical implications of the above findings are discussed in the paper.