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3.9.4 Gross alpha and gross beta measurement

For health physic purposes will, in principle, a gross alpha and gross beta measurement (typically referred to as ‘gross alpha/beta’) be sufficient to characterise the total radioactivity of the sample. This is especially the case for analysis of environmental water samples, where accurate and precise detection to less than 0.1 kBq.m-3 can be achieved. For assessment purposes the water analyses are compared to the Guideline Values produced by the World Health Organisation for radioactivity in drinking water.

In practice, gross alpha/beta analysis of soil samples is a screening technique, which enables distinction to be made between uncontaminated samples and those samples contaminated to levels of a few kBq.kg-1 or more. The intervening region is more difficult to characterise because:

  • The soil sample required for analysis is very small (< 1 g) and sub-sampling errors (arising from sample heterogeneity) may be significant.
  • The typical sample preparation technique involves using the fine-grained (< 200 Qm portion of the soil. This can introduce a systematic bias in the result, because any radiation contamination tends to be associated with the fine fraction.

A more accurate measurement of gross alpha/beta activity in soil can be obtained if a 100 gram-sized sample of soil is homogenised and crushed so that there is no size separation prior to analysis.

Gross beta analysis does not detect weak beta-emitters such as emitted by 3H, 14C, 35S, 129I and so on. If these isotopes are potential contaminants in the soil or water samples, then additional isotope-specific analysis will be required.