Section 6.2.1.2.1. Thermal moderators

Neutrons thermalized within a `semi-infinite' moderator/reflector typical of a steady-state reactor source establish an equilibrium Maxwellian energy distribution characterized by the temperature (T) of the moderator (Fig. 6.2.1.2). The wavelength, , at which the above distribution has a maximum is given by Depending on the width of the moderator and its composition, the Maxwellian distribution merges with the slowing-down distribution from the reactor core to give a total distribution at the beam-tube entry.

Figure 6.2.1.2 | top | pdf | Neutron wavelength distributions for a thermal (310 K) and a cold (30 K) neutron moderator in a `typical' dedicated beam reactor. The Maxwellian distribution merges with a slowing-down distribution at shorter wavelengths. A wavelength distribution for a monochromatic beam application on a thermal source is illustrated. It should be noted that, depending on the value of the mean wavelength for the monochromatic beam, harmonic contamination may be significant.

Clearly, the neutron wavelength distribution will depend on the local equilibrium conditions. Since steady-state reactors typically operate with moderator/reflector temperatures in the range 308–323 K, the corresponding is about 1.4 Å (Fig. 6.2.1.2). However, it is possible to alter the neutron distribution by re-thermalizing the neutrons in special moderator regions, which are either cooled significantly below or heated significantly above the average moderator temperature. One such device of prime importance is the cold moderator in the form of a cold source.