Methodology for monitoring the residual activity in silicon rods irradiated with thermal neutrons

A calibrated detection system developed for monitoring the residual activity in silicon rods irradiated for neutron transmutation doping is described. This residual activity is mainly due to 31Si and 32P, produced by thermal neutron capture reactions. The set-up consists of a scanning device, developed for positioning the rods, coupled to a GM detector for measuring β-rays coming from the rod. The set-up has been calibrated using a 32P aqueous solution standardised in a 4πβ gas-flow proportional counter system. The validity of the experimental procedure has been checked by Monte Carlo calculations. An additional check has been made by measuring the residual activity of a silicon rod irradiated in a well-known neutron fluence and the formalism used for calculating the activity on the basis of irradiation parameters is given. The results are compared to the radiological limit imposed by IAEA regulations in order to consider the material as exempt before it could be released for industrial applications.