International intercomparison exercise of airborne gamma-spectrometric systems of the Czech Republic, France, Germany and Switzerland in the framework of the Swiss exercise ARM17

Abstract

Participants in this international intercomparison exercise performed from June 26th to June 30 th were seven measuring teams of the Czech Republic, Germany, France and Switzerland. In advance to the intercomparison exercise, two measuring areas around the nuclear power plants Gösgen and Mühleberg were inspected by Swiss teams CH01 and CH02 on June 22nd and June 23rd following a bi-annual schedule on behalf of the Swiss nuclear safety inspectorate (ENSI). No artificial radionuclides were detected outside the premises of the power plants. The main measurement task for the international exercise tried to simulate a scenario in which the deployment of airborne gamma-spectrometry is essential and international support would be requested. The scenario assumes a theft of a strong radioactive 137Cssource. The get-away car including the empty source shielding was found abandoned by the police, suggesting that the thieves took flight on foot taking the source with them. Police investigations determined a perimeter inside which the thieves and the source should be located. The associated area adds up to seven percent of Switzerland (2900 km2). Two 137Cs-sources were placed in the search area. Both sources were identified and located during the exercise. Additional tasks consisted in combining the measured data in the source search area into composite maps of the natural radioactivity and in several reference areas for a direct comparison of system performance. The results of the measurements showed some minor differences between the results of the measuring teams. Several supplementary tasks were included into the exercise to provide informations for potential optimisation of data evaluation to the participants. An altitude profile over a Swiss lake yielded information on system background and the influence of cosmic radiation. A reference area with mainly flat topography measured prior to the exercise with in-situ gammaspectrometry on the ground by staff of the Nuclear Biological Chemical - Explosive Ordnance Disposal (NBC-EOD) Centre of Competence was introduced to check on individual calibration factors and the influence of different ground clearances on the measurement results. A second reference area with alpine topography and a transversal going from flat to alpine terrain was introduced to provide information on the influence of topography on the measurement results. The detailed analysis of the results of the supplementary tasks in combination with post-processing of raw data after the exercise helped to reduce some of the differences in the composite mapping observed during the exercise.