Some sources name only a handful, but that might refer to the use of optical film. Others state more than 70. I would like to know which elemets or isotopes can be detectet/imaged by NAA.
NAA depends on the capture of a neutron by a nucleus which results in the production of a heavier radioactive nucleus. Many nuclei will become radioactive after capturing a single neutron. Some nuclei may require 2 or more extra neutrons before becoming radioactive. The probability of producing a radioactive nucleus will depend on the nuclear neutron capture cross-section. The probability of a neutron being captured is small. If a nucleus requires 2 or more neutrons, the probability is even smaller.
When a radioactive nucleus is produced it will undergo gamma decay. By measuring the energy of the gamma ray and the half-life of the nucleus one can determine the nucleus that decayed and hence the element.
Some nuclei will have a neutron capture cross-section that is so small that measurements aren't sensitive enough to detect. Others require more than two neutron captures and hence production of these are too small to measure. And some produce radioactive nuclei which decay almost instantly. these require a different technique to measure the gamma rays and half lives. About 70% of elements can be measured using the normal methods of NAA. (Reference 1)
(1) Overview of Neutron Activation Analysis. University of Missouri Archaeometry Laboratory