Neutron activation analysis - A nuclear technique used in the study and monitoring of environment

The neutron activation analysis (NAA) was used at INR Pitesti since 1987 first for investigating the content (in Rb, Ba, La, Ce. Nd, Sm, Eu, Tb, Yb, Lu, Hf, Ta, Th, U, etc) of geological sample and, later on, for different toxic elements (like Al, V, Mn, As, Br, I, Cl, Fe, Sb, K, Na) in environmental samples (urban dust, ashes from coal burning), moss and lichen and other biological tissues. Adequate standards were prepared for high accuracy quantitative determinations. The following steps are done in NAA analyses: - collecting and preparation of samples; - reference standard preparation; - gamma spectrometry measurements; data processing. The principle of the method is explained. The concentration of the investigated element is determined from the intensity of the gamma rays from the neutron activated nuclei in comparison with gamma ray intensity from the standard sample of known concentration. The measurements are made by the TRIGA-14 MW Reactor provided with pneumatic system for sample circulation and gamma spectrometric measuring station. As a function of the sample's matrix composition and elements to be determined, established are: - the irradiation time; - irradiation location in the reactor and accordingly the neutron flux intensity. For instance, inside the beryllium reflector and thermal column the thermal neutron flux is 1013 n/cm2.s and 109 n/cm2.s, respectively. The gamma spectrometric system is composed of a PGT or ORTEC gamma radiation detector, ORTEC preamplifier, amplifier and stabilized high voltage supply, CT analog-to-digital converter, multichannel analyzer, IN 96 or P III computer and peripherals (printer, magnetic disk units). The energy resolution of the gamma spectrometric system is 1.8 keV for the 1332.5 keV line of 60 Co. For gamma spectra processing the GANAAS or GENIE-2000 codes supplied by IAEA (for P III) or dedicated codes supplied by INTERTECHNIQUE-France for IN-96 were used. A table is given presenting the detection limits as sensitivity values ranging from 10-12 g to 10-5 g for a number of elements