The neutron energy spectrum is affected by various factors, including reactor type, position in the reactor, and immediate surroundings, such as adjacent fuel, control rods, and empty surroundings. The overall shape of the neutron spectrum is influenced by the specific type of reactor. For reactors using mod­erators, such as heavy water, light water, or graphite, Figure 1.5a depicts ideal­ized curves for a normalized flux of neutrons as a function of neutron energy.

The neutron fission spectrum calculated by Watt is also superimposed on the graph for comparison. Convenient techniques such as assuming monoener­getic neutron flux and the arbitrary selection of neutron flux cutoff level (>1 MeV) are mostly general approximations. Remember that most neutron fluxes cited at irradiation damage studies are expressed in terms of >1 MeV. Figure 1.5b depicts the two flux spectra obtained from the Advanced Test Reactor (ATR). One spectrum is without the use of cadmium shroud and another one is with the cadmium shroud (of ~ 1.14 mm thickness). It is clear that fast (hard) spectrum is achieved with the use of cadmium shroud (i. e., irradiation jig wrapped into cadmium foil) due to its absorption of thermal neutrons, but not fast ones. Dosimetric experiments followed by calculations can generate the flux spectrum for a specific position in the reactor.

1.8