Car-borne gamma spectrometry: a virtual exercise in emergency response

In recent years car-borne gamma spectrometry has expanded from its role as a geological survey platform to being a useful asset in searching for orphan sources and for surveying in the aftermath of an incident involving the release of radioactive materials. The opportunities for gaining practical experience in the field however are limited by cost considerations and practicability. These limitations are exacerbated by the fact that field data can differ significantly from data generated in the laboratory. As a means of exercising existing emergency measuring/surveying capability and introducing car-borne measurements to a larger group, a virtual exercise was devised. The exercise ORPEX (Orphan Sources and Fresh Fallout Virtual Exercise in Mobile Measurement) featured two typical emergency scenarios: a search for orphan sources and surveying to delineate fallout from a local release point. Synthetic spectral data were generated for point sources and inserted into genuine car-borne measurement data. Participants were presented with a typical software tool and data and were asked to report source locations and isotopes within a time limit. In the second scenario, synthetic data representing fallout from a local fire involving radioactive material were added to real car-borne data, participants being asked to produce maps identifying and characterising the regions of contamination. Fourteen individual organisations from seven different countries supplied results which indicated that for strong sources of isotopes with simple spectra featuring high energy peaks, location and identification was not a problem. Problems arose for isotopes with low energy signals or that presented a weak signal even when visible for extended periods. Experienced analysts tended to perform better in identification of sources irrespective of experience with mobile measurements whereas those with experience in such measurements were more confident in providing more precise estimates of location. The results indicated the need for the inclusion of less frequently encountered sources in field exercise related to mobile measurements.