Study of the representativity of uranium and thorium assays in soil and sediment samples by alpha spectrometry.

The activity concentrations of soil and sediment samples are determined by alpha spectrometry with a sample size usually < 1 g. Hence, a systematic and careful procedure must be followed in the sampling and pre-treatment of the sample in order to ensure that the aliquot analysed represents the activity concentration of the entire sample. Statistical analyses of the activity concentrations of uranium and thorium isotopes in soil and sediment samples were performed to study the activity variation due to the lack of the representativity of the sample. These studies showed that, although some uncertainty in the determination of the sample activity concentration may be due to the lack of representativity (about 10%), the procedure followed for the pre-treatment of the sample can be considered adequate, providing an acceptable level of representativity.