MRsaReactor Theory (Neutron Characteristics)DOE-HDBK-1019/1-93NEUTRON MODERATIONRev. 0Page 27NP-02ModeratingRatioMacroscopic slowing down power indicates how rapidly a neutron will slow down in the materialin question, but it still does not fully explain the effectiveness of the material as a moderator. Anelement 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. Themoderating ratiois the ratio of the macroscopic slowing down power to the macroscopic crosssection for absorption. The higher the moderating ratio, the more effectively the materialperforms as a moderator. Equation (2-11) shows how to calculate the moderating ratio of amaterial.(2-11)Moderating properties of different materials are compared in Table 2.TABLE 2Moderating Properties of MaterialsMaterialCollisions toSlowing DownNumber ofMacroscopicThermalizePowerModeratingRatioH O0.927191.425622D O0.510350.17748302Helium0.427429 x 1051-6Beryllium0.207860.154126Boron0.1711050.0920.00086Carbon0.1581140.083216