2. Small samples versus large samples

The usual procedure in NAA is to analyse a sample whose mass is lower than 500 mg, considering it as a geometrical point source. This entails a number of simplifications during irradiation and gamma spectrometry [25, 26]. This way, several simplifications can be made such as disregarding the neutron self-shielding, neutron-flux gradients over the sample and selfattenuation of gamma rays. The impact to the accuracy of the results is negligible.

On the other hand, there is a growing demand for the NAA established at CDTN to explore its potential in order to overcome the main limitations when analysing point samples, which are: to reach lower detection limits than those currently in use (for instance in food samples, plants, medicines and lichens) and to carry on analysis at lower cost, that is, to analyse a smaller number of samples and shorter time of analysis. For example, instead of analysing 20 small samples, a single about 4 g composite sample could be analysed; to provide greater representativeness of samples of non-homogeneous materials, for instance, industrial waste materials; to enable the analysis of whole parts in which it is not possible or permitted to remove an aliquot for analysis, for example, of archaeological ceramics. In addition, the low neutron flux of a low-power reactor can be compensated by increasing the amount of sample to be exposed to irradiation.

A possibility to overcome these problems is to analyse larger samples—samples of more than 0.5 g [25–32]. In order to obtain reliable analysis results, some parameters should be determined: (i) detector efficiency evaluation over the volume source, (ii) neutron flux depression due to absorption and scattering and (iii) the relative attenuation of gamma rays originating from different positions within the sample.

During the irradiation, the neutron field is perturbed during absorption and scattering inside the sample. It is called neutron self-shielding. This can be overcome by experimentally determining the neutron flux distribution in real samples in a defined volume for a matrix [6, 33, 34]. The degree of gamma self-attenuation depends on a number of factors such as sample geometry, linear attenuation coefficient, material density, sample composition and photon energy [35].

The laboratories that have been applying the neutron activation to large samples (LS-INAA) analyse samples in a range of kilogrammes, and for this procedure, special facilities are required, for the activation as well as for the detection. For instance, in Delft, The Netherlands, a facility was built to irradiate and measure samples from 2 to 50 kg [3, 26–31].
