STUDY OF THE EFFECTS OF NEGLECTING OR CONSIDERING THE RAYLEIGH SCATTERING IN MONTE CARLO SIMULATION CODES APPLIED FOR THE MEASUREMENT OF LARGE VOLUME SAMPLES

摘要

The Monte Carlo method is often used for the simulations of gamma spectrometric measurements that are typical for germanium detectors. In contrast, there are few simulations for the measurements of radioactive waste drums. When such simulations are applied they often rely on simplified models and approximations. For the measurement of a standard radioactive waste drum the detection efficiency is in the range 10~(-6) - 10~(-8). This means that a very large number of photons should be simulated in order to obtain a result with satisfactory uncertainty. The question arises if it is possible to limit the simulation only to a much reduced group of photons that contribute predominantly to the detector signal. For the case of radioactive sources of small sizes, up to several dm3, it has been shown that the Rayleigh scattering does not have a significant contribution to the detector signal, but the case of radioactive waste drums with typical sizes in the 200 dm3 range has not been investigated yet. We investigate whether the Rayleigh scattering could be neglected in the calculation of detection efficiency for the measurement of waste drums. If the answer to this question is a positive one, then the simulation of emitted photons in every point inside the source can be reduced to the simulation of only those photons that are emitted inside the solid angle in which the detector is seen from the emission point. We investigate also the possibility to reduce the simulation time by avoiding the simulation of photons that are emitted towards the detector but suffer interactions before exiting the drum. In this hypothesis the transport of photons through the drum is possible to be described by using an attenuation factor that is calculated analytically based on the total attenuation coefficient of the photons.