Effect of inorganic salts on a polymer gel dosimetry based on acrylamide

FACUNDO MATTEA; DAVID CHACÓN; MIRIAM C. STRUMIA; MAURO VALENTE

Budva

Conferencia; Fifth International Conference on Radiation and Applications in Various Fields of Research (RAD 2017); 2017

Polymer gel dosimetry is the only tissue equivalent dosimetry system able to record 3D dose distribution acting both as phantom and dosimeter at the same time ensuring post-irradiation stability. Therefore, a great effort has been made in the past two decades to obtain materials highly sensitive to ionizing radiation and with sufficient stability to preserve the dose information over time. A polymer gel dosimeter is constituted mainly of four components, an aqueous gelatin type material which limits the mobility of the reactive species and of the formed polymers after the irradiation, a monomer and a crosslinking chemical species that reacts with the radicals formed during the radiolysis of water molecules upon irradiation, and an antioxidant molecule, which consumes the oxygen present in the material and helps to avoid the inhibition effect of oxygen over the polymerization reactions. Among the different studies of the past decades aiming to improve the sensitivity of polymer dosimeters, the use of different monomers, crosslinking agents and antioxidants attained some level of success, giving place to commercial and registered materials, such as BANG, BANANA, and systems based on n-isopropyl acrylamide, methacrylic acid, acrylic acid, etc. Recently, several modifiers have been proposed to enhance the sensitivity of these materials like the use of co-solvents to enhance the solubility of the crosslinker or monomer, or the use of dopants like inorganic nanoparticles, organic molecules, or inorganic salts which could alter the disposition of water molecules around the chemical species improving the kinetics of the polymerization reactions. Despite the beneficial effects on the sensitivity of these modifications, most of these studies arrived to the conclusion that the thermal stability and the capacity of the gel dosimetry systems to preserve information over time decrease with the proposed modification. This effect is mainly due to the breakup of the gelatin structure in the dosimeter. Also, many of these studies suggest the use of a chemical modification of the supporting material to avoid this collateral drawback. In this study, the effect of different inorganic salts, namely MnCl2, MgCl2 and CaCl2, on the sensitivity of a dosimeter based on acrylamide (PAGAT) with a chemical modification of the gelatin support with glutaraldehyde has been investigated. A correlation between the elasticity of the supporting material and the dosimetric sensitivity has been found, suggesting that a great amount of the contribution of the inorganic salts on the sensitivity of the dosimetric material is related to the increase in the mobility of the chemical species. Therefore, a compromise exists between enhancing the sensitivity and preserving the spatial distribution of the dose. An optimal concentration of the gelatin modifier and inorganic salt was obtained for MgCl2, where the sensitivity of the standard PAGAT dosimeter has been enhanced by a factor of 1.8.