Fission gammas and neutrons interact with surrounding material in many ways Three interaction processes are of special interest m reactor power measurement, nuclear reactions, recoils or collisions, and ionization The inter­action of any single gamma photon or neutron may involve more than one of these three basic processes (For details on these interactions, see Refs 3 and 4 )

A nuclear reaction results from sufficiently energetic collisions of a specific radiation with the nuclei of a specific material The consequence of a nuclear reaction is a nuclear excitation or transmutation or the formation of a new

material The reaction may be described symbolically as A (a b) C where A is the target nucleus, the first symbol, a within the parentheses denotes the radiation causing the reaction, the second symbol, b, within the parentheses denotes the effect or secondary radiation, and C is the nucleus that remains after the secondary radiation, b has been emitted An example of this symbolism is the nuclear reaction 1 03Rh(w,7)’ 04Rh, in this reaction 103Rh has been converted to 104Rh by the capture of a neutron, n and the emission of a capture gamma, 7

If the principal interest lies not in 104Rh but in its decay product, the expression may be expanded to 103Rh (я,7)’ 04Rh — Д. 104Pd Here the arrow with j3 superposed indicates radioactive decay by beta particle emission to 104Pd

Usually there are other radiations associated with the nuclear reaction, but the symbolism is restricted to the principal reaction or the reaction of interest Reactions of interest m nuclear instrumentation are neutron induced and result in the emission of fission fragments and of alpha particles, namely, (n, f) and (n, a) reactions

Most fission-neutron interactions with the nuclei of material m and around a nuclear reactor core are capture reactions [(я,7) reactions] and elastic collisions [(и, я) reactions] Some interactions are transmutations of the (n p) type, and some are inelastic collisions {n n ) reac tions] The products of these interactions, 1 e, the neu trons, gammas, and protons, also interact with the nuclei of material in and around the reactor core

Fission gammas and the gammas produced in neutron capture or -scattering reactions interact with the electrons in surrounding material, usually creating energetic electrons (Compton or photo effect)

Energetic particulate radiations (protons or nuclei recoiling after a nuclear reaction), if not stopped in nuclear reactions or nuclear collisions, ultimately lose their energy by ionizing atoms Ionization results in a “cloud” of free electrons and positive 10ns The cloud or track of ionization is sharply defined by the trajectory of the initiating particle and usually has a definite length, or range, that depends on the particle energy and the density of the medium A heavy, highly charged particle, such as a fission fragment has a short, densely ionized range An electron, or beta, has a longer range with less dense ionization

The neutral radiations (neutrons and gamma rays) associated with fission travel much farther than the charged particles and have poorly defined ranges They can penetrate thick layers of matter Neutrons usually terminate in some nuclear reaction, gamma rays produce secondary electrons, which, in turn, are stopped by ionizing other atoms The difference in the penetrating power of neutron and gamma rays makes possible a partially selective detection process in monitoring nuclear reactors

All the interactions of the radiations accompanying fission produce heat Thus the heat generated is a direct measure of the power of a nuclear reactor (It is also a nuisance in instruments used to detect radiation ) Unfortunately, heat is a very slow indicator (because of thermal inertia) In the steady state, the reactor heat or thermal power can be accurately measured and used in calibrating the nuclear instrumentation