Neutron activation analysis in practice

Typically, the sample or object to be activated is placed in a container for irradiation. This may be aluminium foil (as aluminium cannot easily be measured by this technique) or silica glass tubes. Several samples and standards are placed in the same container for irradiation, to ensure the same conditions for samples and standards. For example, in the case of ceramics measured at the British Museum laboratories (Hughes et al. 1991), powdered 

As discussed above, the measurement of characteristic y rays is very similar to the methods used in EDXRF. Early studies used a scintillation counter, typically a crystal of sodium iodide containing a small amount of thallium (Tite 1972). y ray absorption by these counters produces visible light, which is converted into an electrical pulse using a photosensitive detector. More recently semiconductor detectors have been used, either a lithium drifted germanium crystal, or, more typically, a pure ( intrinsic ) 

Although quantification of the elements present in the y spectrum can in theory be achieved from first principles using the equation given above, in practice uncertainties in the neutron capture cross-section and variations in the neutron flux within the reactor mean that it is better to use standards. These standards must be included in each batch of samples irradiated in order to account for variations in neutron flux inside the reactor. For analysis of minor and trace elements calibration is easier than with other analytical methods provided that the major element composition remains reasonably constant, as the y ray intensity is proportional to concentration over a very wide range of concentrations. However, for analysis of major elements, e.g., silver in silver coins, the relationship between intensity and concentration is more complex, due to progressive absorption of neutrons as they pass through the specimen. In such cases y ray intensity will also depend on the thickness of the sample and therefore specialized calibration methods are required (Tite 1972 277). 

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