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Standardisation of Kr-85.

Phillips, H C; Johansson, L C; Sephton, J P (2010) Standardisation of Kr-85. Appl. Radiat. Isot., 68 (7-8). pp. 1335-1339.

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For the purpose of a BIPM key-comparison of 85Kr, a primary standardisation using internal gas proportional counting was performed. The supplied activity for the comparison was approximately 40 MBq and a two-stage dilution was required to reduce the activity concentration to a level suitable for gas counting. An new rig was constructed to perform the dilutions. Nevertheless, the dilutions introduced significant uncertainties in the final result and hence, methods suitable for measurement at higher activity levels were also used. Calibration factors for 85Kr in ampoules were determined for a HPGe detector and a NaI(Tl) detector derived from internal gas proportional counting.
A series of dilution ampoule at 6 MBq and 1 MBq were prepared in the new NPL gas handling rig using inactive krypton as a diluent. Internal gas proportional counting was performed on each of the 1 MBq dilution ampoules. The activity transferred to the counting system was estimated using pressure and volume data and the total ampoule activity calculated. Counting losses below the threshold were assumed to be 0.2%. The effect of changing the composition of the counting gas by inclusion of krypton was evaluated and found to not significantly change the gas gain, i.e. losses below the noise threshold (approximately 120 eV) remained essentially constant. The proportional counters were assumed to be 100% efficient with an uncertainty of -0.5% (k=1).
Both the 1 MBq and the 6 MBq ampoules were assayed by gamma spectrometry using HPGe and NaI(Tl) detectors, gated on the 514 keV 85Kr peak. This method resulted in an activity value with a smaller uncertainty than the primary method. Activity values for the three methods employed were consistent within the uncertainty of measurement.

Item Type: Article
Subjects: Ionising Radiation
Ionising Radiation > Radioactivity
Last Modified: 02 Feb 2018 13:15
URI: http://eprintspublications.npl.co.uk/id/eprint/4704

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