Using a Monte-Carlo simulation code, a practical method to calculate the X-ray spectrum in the radiography was proposed. The geometry used in the simulation was composed of the target and total filtration. This geometry could calculate both the generation of the X-rays at the target and its attenuation in the total filtration. The features of this method are to simulate the heal effect and to obtain the X-ray spectrum corresponding to the all irradiated field. Because it was difficult to reproduce the electric field in the simulation, almost all incident electrons were scattered at the target and some of them were converted to the bremsstrahlung X-rays in the total filtration. This phenomenon did not occur in the real X-ray equipment. Therefore, we considered that our method was appropriate to use a case in which the bremsstrahlung X-rays generated in the total filtration was negligibly-small compared with the X-ray spectrum generated at the target. We investigated the possibility to apply the method for the tube voltage of 40-120 kV. As a result, we found that a fraction of inessential bremsstrahlung X-rays was at most 0.4%. Namely, using the proposed method, the X-ray spectrum can be reproduced with accuracy of 0.4%.
