A measurement of the nuclear material content or of the element or isotopic concentration of an item without producing significant physical or chemical changes in the item. It is generally carried out by observing the radiometric emission or response from the item and by comparing that emission or response with a calibration based on essentially similar items whose contents have been determined through destructive analysis. There are two broad categories of NDA:

• Passive analysis (assay), in which the measurement refers to spontaneous emissions of neutrons or gamma rays or to the total decay energy;
• Active analysis (assay), in which the measurement refers to a stimulated emission (e.g. neutron or photon induced fission).