The Effects of Crosslinker and Monomer to the Polymerization of Polymethacrylamide Gel Dosimeters by Direct and Indirect Methods

Radiation-induced polymerization in Polymethacrylamide Gels (PMAAGs) potentially used for 3D dose verification in radiotherapy has been studied using both Raman spectroscopy and Nuclear Magnetic Resonance (NMR) method. The dosimeters are composed of aqueous Methacrylamide (MAA) monomer and N, N’ methyelene-bis-acrylamide (BIS) crosslinker at various concentrations from 2 to 6% (w/w) and gelatin at 6% (w/w). The dosimeters were irradiated to doses up to 30 Gy using ⁶⁰Co teletherapy γ-ray source at a constant dose rate. The formation of the polymer increases with increasing dose and was followed directly by Raman spectroscopic measurement for CH₃ stretching mode assigned to polymethacrylamide and indirectly by the transverse spin-echo pulse NMR R₂ relaxation rate of water protons within the polymer gel network. The half dose D_1/2 values of both direct and indirect methods were used to evaluate the effects of initial concentrations of monomer and crosslinker to the dose required to produce 50% of the polymer in PMAAGs. The PMAAGs containing more crosslinker than monomer show larger D_1/2 values, indicating that the crosslinker has a larger effect on the increase in dose required to produce 50% of the polymer. The D_1/2 value of the direct method is consistently higher than that of the indirect method, indicating that the indirect method is more sensitive to the dose response, but fundamentally does not measure the amount of polymethacrylamide formed. There is a correlation between D_1/2 value and concentrations of monomer and crosslinker. The correlation factor, k_C of the crosslinker is always greater than k_M of monomer, for both the direct and indirect methods, suggesting the crosslinker reacts more efficiently than monomer to produce 50% of the polymer of polymethacrylamide.