Calibration of a Sodium Iodide Gamma-Ray Spectrometer for Measuring the Air Kerma Rate Due to 41Ar


R. L. Grasty, B. R. B. Walters, L. R. Lamarre


A portable gamma-ray spectrometer with a 7.5 x 7.5-cm (3 x 3-inch) sodium iodide detector was calibrated for air kerma using 10 radioactive sources of known activity covering an energy range from 60 keV to 1,836 keV. The gamma-ray spectrum was measured with each source at a fixed distance from the center of the detector and at different angles to the detector axis. Ten weighting coefficients were used to convert the energy deposited in 10 selected regions of the spectrum to air kerma rate. The energy deposited in the 10 energy regions of the spectrum was confirmed by Monte Carlo calculations. Additional confirmation of the calibration procedure was provided by comparing measurements with a similar sodium iodide spectrometer in Finland with those using a calibrated pressurized ionization chamber. Measurements of air kerma rate due to Ar-41 were made at the boundary of the Pickering nuclear generating station near Toronto, Ontario. Natural background variations in the Ar-41 window around 1,294 keV were removed by monitoring the K-40 photopeak at 1,460 keV and the photopeaks around 1,760 keV from Bi-214. Using a 10-minute sampling period, the sensitivity of the system for Ar-41 was found to be about 0.2 nGy/hr.


This abstract was presented at the 32nd Annual Midyear Meeting, "Creation and Future Legacy of Stockpile Stewardship Isotope Production, Applications, and Consumption", Instrumentation, Monitors, and Devices Session, 1/24/1999 - 1/27/1999, held in Albuquerque, NM.

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