Neutron activation analysis (NAA) is an analytical technique based on the principles of neutron capture. When a free neutron collides with a target nucleus, the nucleus captures the neutron, producing an excited state (i.e., activation). In order to return to the ground state, the target nucleus immediately emits prompt gamma rays. The resultant nucleus remains unstable and continues to emit gamma rays and other forms of ionizing radiation through radioactive decay.
Both prompt and delayed gamma rays have distinct energies for every radioisotope and therefore identify the chemical element of the target nucleus. Furthermore, the number of gamma rays emitted from a particular radionuclide is proportional to the amount of the parent isotope in an irradiated sample. Because NAA is based on nuclear principles and is a well-established analytical technique, it is highly accurate at characterizing the elemental composition of samples and determining the concentration of those elements.
Beyond its high level of accuracy, NAA is also characterized by high sensitivity for many elements. This high sensitivity is facilitated by the technique’s essentially blank-free nature. Because NAA is a non-destructive technique with samples typically requiring little or no preparation, samples often can be analyzed repeatedly, even subsequently by other methods if needed. This simplicity in sample preparation also greatly reduces the potential for sample contamination.
In delayed gamma NAA, the irradiation, decay and sample counting times can all be customized to increase the sensitivity of particular analytes. Consequently, matrix effects are negligible and, when present, often can be accounted for. Moreover, NAA has multi-element capabilities and, because of this feature, it has been an important and consistent analytical method for the certification of Standard Reference Materials (SRMs) at the National Institute of Standards and Technology (NIST). Where NAA is the preferred analytical method of choice, it is often cheaper and faster than other analytical techniques.
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