Prompt vs Delayed NAA
Written and developed by Prof. Tor Bjørnstad (IFE/UiO)
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As mentioned earlier, the NAA technique can be categorized according to whether gamma rays are measured during neutron irradiation (PGNAA = Prompt-Gamma Neutron Activation Analysis) or at some time after the end of the irradiation (DGNAA = Delayed-Gamma Neutron Activation Analysis). The PGNAA technique is generally performed by using a beam of neutrons extracted through a reactor beam port. Fluxes on samples irradiated in beams are on the order of one million times lower than on samples inside a reactor but detectors can be placed very close to the sample compensating for much of the loss in sensitivity due to flux. The PGNAA technique is most applicable to elements with extremely high neutron capture cross-sections (B, Cd, Sm, and Gd); elements which decay too rapidly to be measured by DGNAA; elements that produce only stable isotopes; or elements with weak decay gamma-ray intensities.
DGNAA (sometimes called conventional NAA) is useful for the vast majority of elements that produce radioactive nuclides. The technique is flexible with respect to time such that the sensitivity for a long-lived radionuclide that suffers from interference by a shorter-lived radionuclide can be improved by waiting for the short-lived radionuclide to decay. This selectivity is a key advantage of DGNAA over other analytical methods.