CALCULATION OF SPECTRUM TO DOSE CONVERSION FACTORS FOR A NAI(TL) SCINTILLATION DETECTOR USING THE RESPONSE MATRIX

A method for converting a gamma-ray spectrum measured by a NaI(TI) sci ntillation detector to a dose, namely, conversion of a spectrum to an exposure, a spectrum to an air kerma, and a spectrum to a dose equival ent, is presented. A Monte Carlo simulation code was developed to calc ulate the response functions of the NaI(TI) scintillation detector for arbitrary gamma-ray energies. The effect of the detector cap, the non linear light output of the NaI(TI) crystal in the low-energy region, a nd all possible processes involved in photon and electron transport, s uch as the photoelectric effect, compton scattering, pair production, and bremsstrahlung, were taken into account in the course of the simul ation procedure. Response matrices of three different sizes (30 x 30, 40 x 40, and 50 x 50) were constructed by combining the response funct ions of a 3'' phi x 3'' NaI(TI) spectrometer for gamma-ray energies up to 3 MeV. Conversion factors for the measured spectrum to dose were e valuated by weighting the diagonal components of the inverse matrices so that the deviation deduced from the theoretical values might be the least. In order to confirm the applicability of the calculated conver sion factors, the factors were used in the conversion of a measured sp ectrum to a dose, and the result was compared with those obtained by t he G(E) operator and the energy-band methods.