MR s a 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