Monte Carlo simulation of GM probe and NaI detector efficiency for surface activity measurements

This paper deals with the direct measurement of total (fixed plus removable) surface activity in the presence of interfering radiation fields. Two methods based on Monte Carlo simulations are used: one for a Geiger–Muller (GM) ionisation probe and the other for sodium iodide (NaI) detector with lead collimators; equations for the most general case and the geometry models for Monte Carlo simulation of both (GM and NaI) detectors are employed. Finally, an example of application is discussed. •Two methods for direct measurements of beta/gamma surface activity are proposed.•Monte Carlo simulated efficiency of detectors was validated and tested.•The calculated and measured efficiencies of detection systems were very similar.•The comparison between two different methods shows good agreement.•Methods can be used for rapid and accurate direct measurements of surface activity.