Neutron Activation Analysis

Neutron Activation Analysis measures the bulk elemental composition of samples by irradiating them with neutrons from a research reactor. The elements form radioactive isotopes that then decay with emission of characteristic gamma rays. The gamma rays are detected and analyzed using a gamma ray spectrometer. The energy of the gamma ray identifies the element (e.g., 1332 keV gamma ray shows presence of Co). The intensity allows quantification (ppb to ppt level) using a known reference material for calibration.

Neutron Activation Analysis is particularly useful for analyzing materials that are difficult to dissolve in acid (e.g., poly silicon used in photovoltaic solar cells). One advantage of NAA for a silicon matrix is that, while Si converts into radioactive 31Si, it has a short half-life (only 2.6 hours). However the half-lives of the impurities are long (e.g., Au is 2.7 days). Therefore a very low background and high sensitivity measurement of the elements can be realized after the decay of the 31Si radioactivity. Additionally, organic samples such as polymer, tissue and biological materials can be analyzed for trace elements with equally low detection limits.

NAA can analyze nearly 2/3 of known elements, the most common being Ag, As, Au, Ba, Br, Ce, Co, Cr, Cu, Fe, Ir, K, La, Mo, Na, Ni, Sb, W, Zn.