Sources, neutron photoneutron

Where it is desirable to have a monoenergetic source, i.e., one where all the neutrons are emitted with the same energy, use can be made of the (y, n) reaction, in which the target nucleus is excited by the capture of a y ray, and subsequently decays by the emission of a neutron photoneutron source). The only practical targets are beryllium or deuterium (in the form of heavy water), in both of which the neutron binding energy has a particularly low value. The reactions are... 

The radioisotope program has been supplemented lately by the stable isotope program and by the facilities for neutron irradiation either in one of the reactors or, by means of a photoneutron source, in any properly equipped laboratory. 

Photoneutrons are produced at considerable flux densities in the heavy metal of the bremsstrahlung converter. For example, in the linear accelerator used by the authors, a thermal neutron flux density of some 10 °cm s has been produced in the tantalum converter during normal operation (30MeV, 150 pA). This appears a relatively poor flux density compared to that of a standard nuclear research reactor. Nevertheless, in advantageous cases trace analyses can be performed. For example, routine analyses of several elements in air-dust samples were carried out by activation with photoneutrons (see below). Normally, however, the photoneutron flux is analytically negligible, whether as an analytical tool or as a source of interference. 

Secondary Source Rods Gamma and neutron The photoneutron somce material used in the secondary somce rods is an equal volume mixture of antimony and beryllium (Sb-Be). Extensive shielding is provided for refuelling activities. 

II-8. After shutdown of the plant, the main radiation source in the vicinity of the vessel is the gamma radiation from the fission products and activation products created in the vessel, in the metallic parts of the insulation and in any material that has been exposed for a sufficiently long time to the neutron flux. For some designs of HWR, neutrons produced by subcritical multiplication of the photoneutron source give rise to a significant power level accompanied by gamma radiation for a short period of time (about 24 h). 

One of the most common photoneutron sources, which produces neutrons of relatively low energy (26 keV) is a combination of antimony-124 and beryllium (Sb-Be source). In contrast to the (a, n) source, where the short range of the a particles necessitates intimate mixing of the reactants, the antimony can be separate from the beryllium, and the source can be switched off by separating the two. The half-life of Sb " is rather short (61 d), but the source can be reactivated by exposure to the high neutron flux in a reactor, when neutron capture in Sb leads to the formation of the active isotope. 

---