Neutron Activation Analysis - Variations

Variations

NAA can vary according to a number of experimental parameters. The kinetic energy of the neutrons used for irradiation will be a major experimental parameter. The above description is of activation by slow neutrons, slow neutrons are fully moderated within the reactor and have KE <0.5 eV. Medium KE neutrons may also be used for activation, these neutrons have been only partially moderated and have KE of 0.5 eV to 0.5 MeV, and are termed epithermal neutrons. Activation with epithermal neutrons is known as Epithermal NAA (ENAA). High KE neutrons are sometimes used for activation, these neutrons are unmoderated and consist of primary fission neutrons. High KE or fast neutrons have a KE >0.5 MeV. Activation with fast neutrons is termed Fast NAA (FNAA). Another major experimental parameter is whether nuclear decay products (gamma rays or particles) are measured during neutron irradiation (prompt gamma), or at some time after irradiation (delayed gamma). PGNAA is generally performed by using a neutron stream tapped off the nuclear reactor via a beam port. Neutron fluxes from beam ports are the order of 106 times weaker than inside a reactor. This is somewhat compensated for by placing the detector very close to the sample reducing the loss in sensitivity due to low flux. PGNAA is generally applied to elements with extremely high neutron capture cross-sections; elements which decay too rapidly to be measured by DGNAA; elements that produce only stable isotopes; or elements with weak decay gamma ray intensities. PGNAA is characterised by short irradiation times and short decay times, often in the order of seconds and minutes. DGNAA is applicable to the vast majority of elements that form artificial radioisotopes. DG analyses are often performed over days, weeks or even months. This improves sensitivity for long-lived radionuclides as it allows short-lived radionuclide to decay, effectively eliminating interference. DGNAA is characterised by long irradiation times and long decay times, often in the order of hours, weeks or longer.

If NAA is conducted directly on irradiated samples it is termed Instrumental Neutron Activation Analysis (INAA). In some cases irradiated samples are subjected to chemical separation to remove interfering species or to concentrate the radioisotope of interest, this technique is known as Radiochemical Neutron Activation Analysis (RNAA).