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3.9.3 Gamma spectrometry

Gamma spectrometry is the most used analysis method for quantifying nuclide specific radioactivity in samples. This method makes use of emitted gamma radiation, by a sample, which is produced during the decay of radionuclides. Gamma spectrometry provides no information on potential radioactive contaminants that emit only beta- or alpha radiation and whose presence cannot be inferred from short-lived gamma-emitting daughter radionuclides, such as 90Sr.

The required sample size is in the range of grams to several kilograms. It is evident, that how smaller the sample size how larger the increase of the measuring time for getting the same analysis accuracy.

The sample size is in general larger than that required for gross alpha/beta analysis; hence sub-sampling errors will be smaller and results will probably be more representative of in-situ conditions. In particular, activities of common man-made radionuclides, such as 137Cs and 60Co, and of natural series decay chains (headed by 235U, 238U and 232Th) can be measured or inferred. In the case of samples contaminated with natural series, e.g., phosphate ores, zirconium, etc. attention has to be given that for accurate measurements decay chains are in equilibrium.

However, because not all radionuclides will be detected using gamma spectrometry, e.g., radionuclides emitting very low gamma fluxes and/or pure alpha- and/or beta radiation. Therefore, it is advised that the technique should not be used in isolation unless the radionuclide fingerprint of the contaminated site is well understood and there is confidence that total levels of radioactive contamination can be derived from the gamma spectrometry data.

In waters, gamma spectrometry is typically used to provide more detailed analysis of contaminated samples. This is principally because gross alpha/beta analysis of waters provides accurate and precise measurement of detection limits below activity levels that are of radiological concern.

The detection limits of gamma spectrometry typically exceed those of the gross/alpha/beta technique.

Gamma spectrometers will usually be screened against background radiation to improve the minimum detectable activity of measurements [Velzen-2004]. (The level of radon daughters inside a shield can be reduced further by allowing the liquid nitrogen Dewar usually used to cool high resolution gamma spectrometers to vent into the shield, thereby purging room air and replacing it with clean gas.)