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Reactor Theory (Neutron Characteristics)
DOE-HDBK-1019/1-93
NEUTRON MODERATION
Rev. 0
Page 27
NP-02
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.
(2-11)
Moderating properties of different materials are compared in Table 2.
TABLE 2
Moderating Properties of Materials
Material
Collisions to
Slowing Down
Number of
Macroscopic
Thermalize
Power
Moderating
Ratio
H O
0.927
19
1.425
62
2
D O
0.510
35
0.177
4830
2
Helium
0.427
42
9 x 10
51
-6
Beryllium
0.207
86
0.154
126
Boron
0.171
105
0.092
0.00086
Carbon
0.158
114
0.083
216
