Are There Large Variations in Low-Energy Response Between Your Bicron Micro-Rem Survey Meters?

M.K. Murphy; G.L. Carter; A.A. Kriss; P.J. Smith; and R.K. Berg (Battelle-PNNL)

At Pacific Northwest National Laboratory (PNNL), evaluations of a group of Bicron Micro-rem survey meters calibrated to Cs-137 indicated as much as a ±40% (one standard deviation) variation in the response to a calibrated Am-241 field on the X1 scale, compared to about ±12% for the upper three scales. Further evaluations also indicated that, for any given Micro-rem meter unit, the Am-241 response (relative to Cs-137) between the lower and upper scales could be different by as much as a factor of two. This low-energy response can be significantly different between the lower and upper scales because the X0.1 and X1 scales analyze the detector signal completely different than the upper three scales. The X1 scale calibration pot and the "HV Set" pot can be adjusted in a way that results in a relative Am-241 response well within ±10% across all scales for a given Micro-rem meter unit. Performing these adjustments on a population of Micro-rem meter units improves the Am-241 response variation between units from the ±40% to about ±12%. This potential large variation in low-energy response can easily go unnoticed by the instrument calibrator and user, unless they are aware of this issue. To resolve the problem, the PNNL calibration procedure for the Micro-rem survey meters was revised to include an Am-241 calibration in addition to the Cs-137 calibration, along with the appropriate pot adjustments. Using NIST-traceable Am-241 fields, during the routine Cs-137 calibration of the Micro-rem meter, the technician would simply adjust the HV Set pot and 1X Cal pot together until the magnitude of the over-response to Am-241 on the 1X scale matched the over-response on the 10X scale. This results in a consistent Am-241 response across all scales of a given Micro-rem meter unit, as well as consistency across a group of Micro-rem meters. Before and after calibration data and calibration procedure recommendations will be presented, as well as energy response curves obtained for the energy range of 17-662 keV.

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