Although the logarithmic energy decrement is a convenient measure of the ability of a material to slow neutrons, it does not measure all necessary properties of a moderator. A better measure of the capabilities of a material is the macroscopic slowing down power. The macroscopic slowing down power (MSDP) is the product of the logarithmic energy decrement and the macroscopic cross section for scattering in the material. Equation (2-10) illustrates how to calculate the macroscopic slowing down power.

Moderating Ratio

Macroscopic slowing down power indicates how rapidly a neutron will slow down in the material in question, but it still does not fully explain the effectiveness of the material as a moderator. An element such as boron has a high logarithmic energy decrement and a good slowing down power, but it is a poor moderator because of its high probability of absorbing neutrons.

The most complete measure of the effectiveness of a moderator is the moderating ratio. The moderating ratio is the ratio of the macroscopic slowing down power to the macroscopic cross section for absorption. The higher the moderating ratio, the more effectively the material performs as a moderator. Equation (2-11) shows how to calculate the moderating ratio of a material.

Moderating properties of different materials are compared in Table 2.

Summary

The important information in this chapter is summarized below.

Neutron Moderation Summary

Thermalization is the process of reducing the energy level of a neutron from the energy level at which it is produced to an energy level in the thermal range.

The moderator is the reactor material that is present for the purpose of thermalizing neutrons.

Moderating ratio is the ratio of the macroscopic slowing down power to the macroscopic cross section for absorption.

The average logarithmic energy decrement (~) is the average change in the logarithm of neutron energy per collision.

Macroscopic slowing down power is the product of the average logarithmic energy decrement and the macroscopic cross section for scattering.

There are three desirable characteristics of a moderator.

1. large scattering cross section

2. small absorption cross section

3. large energy loss per collision

The energy loss after a specified number of collisions can be calculated using the equation below.