Problems encountered during the calibration of the new Cameco mobile lung counter: detector size or phantom limitation?

Abstract

This paper describes the calibration of the new Cameco mobile lung counter and, more importantly, the problems encountered with recommendations for their long-term solution. The new Cameco lung counting system, which is based on an array of four 80-mm-diameter Canberra BeGe detectors, has used the JAERI phantom for its primary calibration as it more closely resembles ICRP reference data for lung dimensions compared with the LLNL phantom. Although the JAERI phantom's lung dimensions offer advantages over the LLNL phantom's lungs, this phantom is still not ideal. The work described in this paper leads to the conclusion that the LLNL be modified to more closely resemble the ICRP reference data if large area germanium detectors comprise the lung counter. Overlay plate stacking was necessary to achieve the range of chest wall thicknesses found within the Cameco work force (2-8 cm) when using the JAERI phantom. This technique has proved to be robust and is useful for extending the calibration range. Cameco is using group monitoring, which adds spectra to simulate very long counting times (10-20 h), and it is essential that all materials be low background. This was not initially the case here as found from overnight background counts.