From the large number of analytical methods, activation analysis techniques are the only ones which are based upon nuclear reaction. The material sample studied is exposed to high‐energy radiation which can be partly absorbed by a nucleus in the sample. Thus the nucleus is excited to a high energy level which can decay through quasi‐prompt emission of a nuclear particle or photon. The product nuclide produced is mostly radioactive, and so emits delayed radiation. Both this and the aforementioned prompt radiation can be measured using appropriate radiation detectors. By evaluating the energy and the count rate of the particles detected, qualitative and quantitative analyses of the target material under study can be performed. Thus it is clear that elements only, not chemical species, can be determined directly. A large variety of particles can be used for activation, namely uncharged ones (neutrons, photons) or charged particles like protons, deuterons, tritons and even heavier ones. Mostly thermal neutrons from nuclear research reactors are used since this technique offers the highest average analytical sensitivity. During photon activation, the target nucleus is activated by photonuclear reaction. This is induced to “normal” material at high energies, usually not below about 10 MeV. The photonuclear reaction data of the elements suggest an activation energy around 30 MeV with respect to analytical sensitivity and interfering reactions, respectively. This energy is best achievable with bremsstrahlung sources like high‐power linear accelerators or microtrons. Favorable irradiation parameters are: 30 MeV electron energy at 100–150 µA mean electron beam current. With the help of suitable radiation spectrometers, e.g. high‐resolution germanium detectors connected to appropriate pulse processing electronics, photon (γ or characteristic X‐ray) spectra can be taken by which simultaneous multicomponent analyses can be carried out without chemical separations, sometimes even nondestructively. Moreover, partly extreme sensitivities can be achieved, and some elements can be analyzed whose determinations are difficult or impossible using other techniques, e.g. light elements like carbon, nitrogen, oxygen and fluorine. A further advantage is the relative freedom from blanks in many cases; after bremsstrahlung exposure, undesirable surface contaminants can be removed from the sample, and the recontamination that eventually occurs is inactive, and thus can be disregarded. Since the activation and measuring process is independent of the chemical status of the component studied, a large variety of matrices can be analysed. Photon activation has been applied in several areas including:
geo‐ and cosmochemistry;
environmental, biological and medical science;
industrial product and high‐purity material analysis;
archaeological and forensic science;
certification of reference materials.
The disadvantages of the method are common to all activation analysis techniques, e.g. the instrumental equipment costs. The cost of a high‐performance germanium spectrometer is about US $30 000, and this does not include the permanent costs of maintenance and liquid nitrogen supply. Also, additional personnel qualifications are required for radioactive laboratory work. Finally, the handling of radioactive waste unavoidably produced during activation analysis might be problematic in some cases.
