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Corrections to air kerma rate measurements of 137Cs environmental sources for free space conditions.

Shipley, D R (1996) Corrections to air kerma rate measurements of 137Cs environmental sources for free space conditions. NPL Report. CIRA(EXT)013

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Abstract

Air kerma rate measurements between 2 m and 4 m from a set of 137CS reference environmental sources (with both stainless steel and brass encapsulation) have been made at NPL using a calibrated Reuter-Stokes high pressure ionisation chamber. Monte Carlo techniques are used to calculate the air kerma rate components over a similar range of distances from these sources. The calculated air kerma values are compared with measurements and the deviation of the data from the inverse square law is investigated. Corrections to the measurement data are then derived to obtain the air kerma rate in free space at 2.5 m from each source.
Simulations of the experimental setup using a collimated 137CS environmental source with stainless steel encapsulation reproduce the air kerma rate measurements reasonably well. The calculations indicate that there is a significant contribution to the measured air kerma rate (typically 6%) due to scattered photons and that this contribution does not vary greatly with distance from the source. This scatter contribution is approximately 0.6% greater for sources with brass encapsulation. m both cases, there is approximately a fourfold increase in the scatter component if the beam collimator is removed.
The overall correction (defined as the product of individual corrections for chamber size effect, air attenuation and radiation scatter) to the inverse square law to obtain the air kerma rate in free space at 2.5 m from reference 137CS sources with stainless steel and brass encapsulation is 0.961 and 0.955, respectively. The total uncertainty in these corrections is estimated to be 0.27% at the 20 level which is much smaller than the estimated random uncertainties in the experimental measurements (which range from 2.6% for the highest activity source to 8.6% for the source with lowest activity).

Item Type: Report/Guide (NPL Report)
NPL Report No.: CIRA(EXT)013
Subjects: Ionising Radiation
Ionising Radiation > Dosimetry
Last Modified: 02 Feb 2018 13:18
URI: http://eprintspublications.npl.co.uk/id/eprint/576

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