engleski

Quantification of radioactivity in air - influence of sampling on the time correction of results

Quantification of radioactivity in air is carried out in a few steps: air is pumped through a filter, small particles of dust which contain radionuclides are caught on the filter, and the filter is subsequently measured by gamma- ray spectrometry. Radioactivity of the filter represents radioactivity in the volume of air pumped through the filter. For short-lived radionuclides it is necessary to perform a time correction, which is done in three steps. The first step refers to sampling time, the second to the time between a sampling and a measurement, and the third to the duration of a measurement. In this paper, we consider the first part of the time correction. This part of the correction depends on sampling time and the flow rate through a filter. Usually, it is assumed that a flow rate is constant in time, and with this assumption one calculates the first part of the correction. We have observed that the flow rate through our filters is not constant but instead decreases in time linearly. For that reason, we derive a general formula for the first part of the correction, which is applicable to any time dependence of the flow rate. We apply this formula to the cases of constant and linearly decreasing flow rates and compare these two corrections. The comparison shows that the difference starts to be significant when the sampling time and radionuclide half-life become comparable. The difference grows as the sampling time increases and radionuclide half-life decreases.

radioactivity; air; sampling

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