COUMARIN CHEMICAL DOSIMETER FOR RADIATION-THERAPY

Aqueous coumarin was investigated as a possible dosimeter for radiatio n therapy applications. Coumarin-3-carboxylic acid in aqueous solution s converts upon irradiation to the highly fluorescent 7-hydroxy-coumar in-3-carboxylic acid. The intensity of the fluorescence signal is line arly proportional to the number of the formed 7-hydroxy-coumarin-3-car boxylic acid molecules, which in turn is proportional to the radiation -absorbed dose. Basic characterization of the dosimetric properties (L inearity with dose, energy and dose-rate dependence, postirradiation s tability, and reproducibility) was performed. The system exhibits near ly linear behavior with dose, in the range of 0.1 to 50 Gy, is stable for at least 166 days following the irradiation, is reproducible withi n the same solution (+/-2%) and is energy independent for 6- to 15-MV x-ray energies. A reduction of 18% of the fluorescence signal was obse rved by changing the dose rate from 0.8 to 4 Gy/min. The origin of the dose-rate effect was investigated and attributed to impurities, predo minantly transition metal ions. Removing the impurities by recrystalli zation of the coumarin and using ultrahigh-purity water reduced the do se-rate dependency to less than 4% in the examined dose-rate range (0. 8-4 Gy/min).