Elastic Scattering and Neutron Moderators
To be an effective moderator, the probability of elastic reaction between neutron and the nucleus must be high. In terms of cross-sections, the elastic scattering cross-section of a moderator’s nucleus must be high. Therefore, a high elastic scattering cross-section is important but does not describe the comprehensive capabilities of moderators. To describe the capabilities of a material to slow down neutrons, three new material variables must be defined:
- high cross-section for neutron scattering
- high energy loss per collision
- low cross-section for absorption
- high melting and boiling point
- high thermal conductivity
- high specific heat capacity
- low viscosity
- low activity
- low corrosive
- cheap
Macroscopic Slowing Down Power – MSDP
We have defined the probability of elastic scattering reaction and the average energy loss during the reaction. The product of these variables (the logarithmic energy decrement and the macroscopic cross-section for scattering in the material) is the macroscopic slowing down power (MSDP).
MSDP = ξ . Σs
The MSDP describes the ability of a given material to slow down neutrons and indicates how rapidly a neutron will slow down in the material. Still, it does not fully reflect the effectiveness of the material as a moderator. In fact, the material with high MSDP can slow down neutrons with high efficiency, but it can be a poor moderator because of its high probability of absorbing neutrons. It is typical, for example, for boron, which has a high slowing down power but is absolutely inappropriate as a moderator. The complete measure of the effectiveness of a moderator is the Moderating Ratio (MR), where:
MR = ξ . Σs/Σa
