Comparison of calibration results for α, β surface contamination monitors

Surface contamination monitors are used intensively in many facilities, like in the nuclear medicine departments for clearance measurements and decontamination and in decommissioning of nuclear installations. For a reliable use, all surface contamination monitors should have a valid calibration with traceability to the international standards. A comparison exercise for calibrations in terms of efficiency in 2π steradian for surface contamination monitors was organized between five dosimetry calibration laboratories, members of EURAMET (The European Association of National Metrology Institutes). Four β-beta radionuclide sources, Sr-90, Cl-36, Cs-137, Co-60, and one α-alpha source, Am-241, were used for calibrations during this exercise. Two transfer surface contamination monitors, one Canberra SABG 100 and one Berthold LB 124 with 100 cm2 and 343 cm2 effective area, respectively, were used. The monitors were circulated between the partners and were periodically returned to the pilot laboratory for stability checks. The efficiencies in 2π steradian measured by the participants do not fully agree for all sources and both detectors despite the fact that all laboratories used similar types of surface contamination sources and had proper traceability to primary standards for surface emission rate measurements. As no evident difference in measurements of surface emission rates is expected between the primary standard laboratories that calibrated the sources, it is very probable that the observed differences are mainly due to the calibration procedures used by each laboratory and that the measurement uncertainties of efficiency in 2π steradian are underestimated. This report presents the results of this pilot comparison for calibration of surface contamination monitors and stresses the need to organize similar generalized exercises. This report also identifies some gaps in procedures for calibration of surface contamination monitors.