Absorbed dose determination in photon fields using the tandem method

An alternative method for the determination of the absorbed dose and the effective energy of photons with unknown spectral distributions is developed. It is a 'tandem' system formed by two thermoluminiscence dosimeters of different energy dependence. Thermoluminiscence dosimeters of LiF: Mg, Ti; CaF2: Dy and a reading system Harshaw 35000 are used. The dosimeters are characterized with 90Sr - 90Y, calibrated to the energy of 60Co and irradiated with seven qualities of X-radiation beams, as suggested by the ANSI No. 13 and the ISO No. 4037. The responses determined for each type of dosimeter are fitted to a function that depends on the effective energy of the photons. The fitting is carried out by means of the minimization algorithm of Rosenbrock. The mathematical model used for this function has five fitting parameters and it is composed of a gaussian and a linear function. The results show that the obtained analytic functions reproduced the experimental data of the responses, with errors smaller than 8%. The ratio of the responses of CaF2: Dy and of LiF: Mg, Ti as a function of the energy of the radiation, allows to determine the effective energy of the photons and the absorbed doses with errors smaller than 15% and 20%, respectively