Protocol for Establishing and Maintaining the Calibration - NPL ...

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24 Good Practice Guide 93 Chapter 2 low energy emissions. The copper filter [22] essentially eliminates the low energy emissions but only reduces the high-energy gamma emissions by a small factor (
Chapter 2 Good Practice Guide 93 25 The coefficients a 1 and a 2 may be calculated and also applied in practice using an Excel spreadsheet. In many cases, it may be possible to set limits on the volume that can be measured without the need to apply a correction factor, provided the magnitude of the correction is much less than the overall uncertainty of the measurement. For example, in the case of 99m Tc, the correction is no greater than 1.0 % for all volumes between 1 and 10 ml in this vial. If the combined uncertainty from all other sources is say 3 %, it might be reasonable to ignore the volume correction for all volumes between 1 and 10 ml. 2.7.4 Position When a source is measured, removed from the chamber, replaced and measured again, there will be a possible difference in the two measurements arising from the inability to replace the source in an identical position. The response of the calibrator depends on both the horizontal and vertical position of the source relative to the ionisation chamber. With most systems, the source is placed at the point of maximum response on the vertical axis of the chamber using a dedicated sample holder and the change in response for a change in vertical height of a few millimetres is usually negligible. For horizontal displacement from the vertical axis of symmetry, the effect is usually also small. The spatial sensitivity of a typical ionisation chamber, mapped using a 99m Tc point source, is illustrated in Figure 2.4 [22]. Figure 2.4 The spatial sensitivity of a typical ionisation chamber to a point source. In practice, calibrators are generally used to assay a finite volume of solution in a container but the effects are very similar to those for a point source. Figure 2.5 shows the variations in response for the ionisation chamber of the NPL secondary standard radionuclide calibrator when the radial and axial positions are changed for a 5 ml ampoule containing 3 ml of solution.
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