an energy range kT = 0.04 eV to 0.05 eV), these scattering nuclei are considered to be unbound and also stationary with respect to epithermal neutrons. In this case, the relative speed is the absolute speed of the neutron. As long as neutron speeds are above a few electron volts, nuclear motion does not affect the neutron slowing down process. All scattering collisions between epithermal neutrons and stationary nuclei will result in a decrease in neutron energy. The absorption of epithermal neutrons by nuclei having large resonance cross sections cannot be treated by assuming that the absorbing nuclei are at rest. At the maximum solid core fuel temperatures of about 30001K (corresponding to an energy kT = 0.26 eV), the translational motion of the nuclei is large enough to strongly influence resonance absorption through the Doppler effect. Resonance absorption will be discussed later in this chapter.

        When neutrons are slowed down from the epithermal energy range, into the thermal energy range, the neutron speeds are comparable to nuclear speeds and the nuclear motion cannot be ignored. The effects of nuclear motion

are important in two aspects of neutron interactions. The first involves the treatment of thermal neutron scattering reactions and the second deals with the specifications of low energy absorption cross sections. The energy distributions of nuclei and thermal neutrons can be described by


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