Internal report 662
Radium-226 (226Ra) is a radionuclide of importance in post-mining rehabilitation dose calculations. Gamma spectrometry is used to measure 226Ra in soils, sediments and biota. The principal gamma emission line for 226Ra is of low emission intensity and is overlaid with an emission line of uranium-235 (235U). Greater accuracy of 226Ra activity can be gained by measuring stronger gamma emissions from radionuclides further down the uranium-238 (238U) decay chain. However, this method relies on the retention of the noble gas radon-222 (222Rn) within the sample container, so that its progeny radionuclides, bismuth-214 (214Bi) and lead-214 (214Pb), can reach secular equilibrium with 226Ra. The acrylic sample containers in use at the Environmental Research Institute of the Supervising Scientist (eriss) have been tested for 222Rn diffusion to ensure that the reported 226Ra results are consistently accurate.
In the samples that we tested the diffusion of 222Rn was consistently low and ranged from 0.74–1.5% of the total 222Rn (or 226Ra) activity in the sample. This diffusion was found to not have a significant effect on reported 226Ra activity concentrations measured by gamma spectrometry, as calibration and counting uncertainties are typically larger than these 222Rn losses. Moreover, the 222Rn diffusion is comparable between detector calibration standards and environmental samples, further reducing any effect on the reported 226Ra results.
Tests for free 222Rn (i.e. 222Rn that has emanated from the sample material and is potentially free to escape the acrylic container if not properly sealed) were also conducted. The tests showed that a considerable percentage (>50% for some sample matrices) of the 222Rn in the containers is free, highlighting the need to carefully follow the existing procedure for gamma spectrometry sample preparation to avoid any excessive loss of 222Rn, identified here as ‘leakage’. This will ensure that high standards of accuracy and precision are maintained in measuring and reporting 226Ra activity concentrations in environmental samples analysed by gamma spectrometry.
If the seal between the acrylic container, lid and o-ring are not radon tight, leakage of 222Rn will be apparent in the gamma spectrometry results, where the 226Ra calculated from 214Bi and 214Pb will appear to be lower activity than the 226Ra measured directly at 186.2 keV. In these rare cases, the cause is often excessive sample which prevents the lid, o-ring and container contacting with sufficient pressure. This is remedied by the removal of excess sample material, with the appropriate adjustment to the weight used in calculations, and requires repacking, careful resealing and then recounting after reaching secular equilibrium.