Experimental difficulties and standardisation needs in measuring natural radioactivity by gamma-ray spectrometry

In the last twenty years the interest of the international scientific community on naturally occurring radioactive materials (NORM) in the environment and industry has been ever growing. In particular, the use of gamma-ray spectrometry techniques for activity measurements of gamma emitters belonging to uranium and thorium decay series greatly contributed to these investigations.
On the other hand the physical and chemical properties of naturally occurring radionuclides greatly increase the complexity of the experimental procedures compared to the traditional measurements of radionuclides of non-natural origin.
The main difficulties and limitations are due to a number of factors such as: accuracy of available nuclear and atomic data; reference nuclides used to express and verify radiation protection limits;photon emissions used for activity determination of each nuclide;deviation from radioactive secular equilibrium within each natural series;means and procedures for detector calibration;
corrections for sample self-attenuation and coincident-summing;procedures for sampling and sample treatment.
During the last years the National Institute of Health (NIH) and the National Institute for Metrology of Ionising Radiations (INMRI-ENEA) in Italy have been conducting a scientific collaboration for the identification of the main problems encountered in gamma-ray spectrometry of NORM and for the improvement of these measurements, also with the aim to develop a national measurement protocol on this subject. An experimental work has also been carried out on several specific topics such as: secular equilibrium in sample containers, self-attenuation and energy resolution for different matrices, spectral interference. The paper gives a summary of this experience and suggests practical solutions experimentally tested. Finally, the importance of an international measurement protocol and of comparisons on this matter are strongly supported, in order to make NORM measurements actually comparable. This is particularly urgent in view of the incoming regulations or recommendation (i.e. on building materials) under development by the European Union.