This review deals with applications of activation analysis to geological samples : ores, minerals and rocks. The various activation sources suitable for this purpose are discussed. Applications using isotopic neutron sources are re viewed in some detail, those using neutron generators are discussed on a more gene ral basis. Present trends in instrumental neutron-activation techniques are brief ly indicated, in particular Ge(Li) gamma-ray spectrometry, coincidence counting, delayed neutron counting, prompt gamma counting and observation of nuclear tracks. The application of photon and charged particle activation to geological samples is also considered.R.H. GIJBELS and J. HERTOGEN An unmoderated isotopic neutron source (e.g. 2^1 Am/Ee) is perhaps the best choice for deter mining the fluorine content of fluorine ores and concentrates via the reaction 19 F(n,a) 16 N (threshold energy ^ 2 MeV), without interference from the 16 0(n,p) 16 N reaction. For the precise instrumental determination of manganese in pyrolusite ores, via the sensitive reac tion 5 5 Mn(n,y)^*>Mn, a relatively modest isotopic source of moderated neutrons is ideally suited. Nuclear reactors, accelerators and isotopic neutron sources are so different in neu tron production and neutron energy, that only for some applications they can be considered competitors.Other criteria to be considered are, of course, cost of the neutron source, possibility for transportation, dimension of source plus shielding, the necessity of having highly special ized personnel, etc. In mobile installations, for geophysical prospection, radioisotope sources with yields of less than 10^n/s, with or without moderator, have mostly been used, mainly in the U.S.S.R. (Ref. 7). The neutron flux in the irradiation zone with the above source strength is usually not more than lO^n.cm" 2^" " 1 , depending upon the design. Experi ments in the U.S.A. and in the U.S.S.R. have shown that deuteron accelerators, (pumped and sealed tube), producing 14 MeV, 3 MeV or moderated neutrons are also transportable .In stationary conditions, heavier shielding is only a cost, not a transportation problem, hence more powerful neutron sources can be used than in field conditions (i.e. isotopic neu tron sources of greater strength; laboratory neutron generators and so-called neutron multi pliers) .Reactor neutron activation analysis,charged particle and gamma activation analysis (Bremsstrahlung of varying energy, 3-40 MeV, obtained from electrostatic, linear wave-guide and cyclic electron accelerators : betatron and synchrotons) is practically restricted to sta tionary applications. The main trends of the use of various neutron and Bremsstrahlung sour ces for the study of rocks and ores have been described by Savosin et al. (15) .
APPLICATIONS USING ISOTOPIC NEUTRON SOURCES AND NEUTRON MULTIPLIERS.The use of isotopic neutron sources and neutron multipliers in ore and rock analysis has been discussed in some detail by Gijbels (16) . One can use the original neutron spectrum with neutron energies up to 4...
