Effects of crosslinking 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 transverse relaxation rate (R2) of bis-acrylamide-nitrogen-gelatin (BANG) aqueous polymer gel dosimeters have been investigated. The gel samples were irradiated in test tubes with 250 kV x-rays, and the water proton NMR transverse relaxation rates were measured at 0.47 T using a Carr-Purcell-Meiboom-Gill multiecho pulse sequence. Both the dose sensitivity (slope of the linear portion of an R2-dose response) and the maximum rate at which the R2-dose response saturated (R2max), were found to depend strongly on the crosslinker fraction and on the temperature of the R2 measurement. The dose sensitivity peaked at approximately 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 transverse 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 water proton transverse relaxation in the gel is controlled by an exchange of magnetization between the aqueous phase and the semi-solid protons associated with the polymer, and that the latter experience spectral broadening from immobilization which increases with crosslinking or cooling. Theoretical and practical implications of the above findings are discussed in the paper.