Second International Intercomparison of Environmental Dosimeters

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The second international intercomparison of environmental dosimeters under both field and laboratory conditions was held in New York in 1976. Eighty-five participants from 26 countries submitted 133 sets of 6 dosimeters, primarily TLDs. Upon receipt, the dosimeters were placed in a shielded location (V 1.5 microR/hr). At the beginning of the intercomparison period, the field dosimeters were placed in close proximity, about 1 m above the ground, in a field location for 3 months while the laboratory and control dosimeters remained in the shield. Six weeks after the start of the study, the laboratory dosimeters were briefly removed from the shield and given a known but unrevealed gamma radiation exposure. At the end of the 3 months, the dosimeters were returned to the participants, who evaluated them and reported their results. The significant findings included: (1) Thirty-six and 81% of the laboratory results were within 10 and 30% of the estimated delivered exposure of 21.3 mR, and 32 and 80% of the field results were within 10 and 30% of the average field exposure, 16.4 mR. (2) No significant differences were found among the results for three of the most commonly used phosphors, LiF, CaF2: Dy, and CaS04: Cy. CaF2: Mn, while exhibiting good agreement for the laboratory exposure, yielded an average field exposure 10% lower than the average for all dosimeters. (3) The laboratory results correlated with the calibration sources used by the participants, with the exposure estimates ranging from high to law for calibrations performed with 60Co, 131Cs and 226Ra, respectively. (4) Analysis of the field and laboratory results revealed that the standard deviation of the results obtained by participants calculating their calibration exposures, based on source strength and distance, exceeded that for those actually measuring their calibration exposures by a factor of two. (5) The field results for LiF revealed a significant correlation with packaging thickness, suggesting that phosphors with surrounding material of a thickness less than about 300 mg/cm2 responded to environmental beta radiation as well as gamma radiation. (6) Control dosimeters indicated transit and storage exposures of a few mR up to slightly over 100 mR, with the majority ranging from 1 to 15 mR. The typical transit time was 2 weeks. Future intercomparisons of this type are planned by the authors.

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