Neutron Activation Analysis (NAA)* 14 Neutron Activation Analysis (NAA) is a technique for trace analysis in bulk mate- rial. It involves two separate steps. The sample is initially irradiated by neutrons in a nuclear reactor to create the radio isotopes (which may take 1 to 7 days). This is fol- lowed by removal to a gamma-ray spectroscopy facility, where the gamma-rays given off during subsequent decay of the radio isotopes can be monitored. The gamma-ray energies emitted are characteristic of the atoms decaying and are determined using a solid state energy dispersive detector in an analogous manner to the X-ray analysis in EDS (see appendix 7), except that the energies are in the MeV range. The technique is a bulk one, since both neutrons and gamma-rays penetrate deeply. There is also no spatial resolution. Both the initial neutron capture probability (cross-section) to create the radio isotopes, and the half-lives for the subsequent decay process vary enormously, in no particular pattern, across the periodic table. In fact, only about two thirds of the ele- ments can produce radio isotopes this way. So trace element sensitivity varies enor- mously among elements, and also with the matrix, which may give off competing gamma-rays. For trace metals in Si or SiO 2 , ppb to ppt sensitivity is easily obtained, but other semiconductor matrices such as GaAs are less favorable. Trace elements in plastics and biological materials can be measured because the matrix elements of C, O, H and N all have low neutron capture cross-sections. The material may suffer radiation and heating damage in the process, however.