A presumption calculating formula of the X-ray spectrum generated from a molybdenum target X-ray tube is presented. The calculation procedure is to add an amount of characteristic X-ray photons that corresponds to the ratio of characteristic photons and bremsstrahlung photons to the bremsstrahlung spectrum obtained using semiempirical calculation. The bremsstrahlung spectrum was calculated by using a corrected Tucker's formula. The corrected content was a formula for calculating the self-absorption length in the target that originated in the difference of the incident angle to the target of the electron and the mass stopping power data. The measured spectrum was separated into the bremsstrahlung component and the characteristic photon component, and the ratio of the characteristic photons and bremsstrahlung photons was obtained. The regression was derived from the function of the tube voltage. Based on this calculation procedure, computer software was constructed that can calculate an X-ray spectrum in arbitrary exposure conditions. The X-ray spectrum obtained from this presumption calculating formula and the measured X-ray spectrum corresponded well. This formula is very useful for analyzing various problems related to mammography by means of Monte Carlo simulations.
