The nex quantity is the average logarithmic energy loss or average logarithmic energy decriment. In practice the following quantity is very useful for the slowing down process description. The average logarithmic energy loss is denoted as the lowercase ξ. This quantity can be obtained from the formula for an average function by using the elastic scattering law. A benefit of the function is the independency on the energy of incident neutrons. The function depends only on the atomic mass of the nucleus. For hydrogen the average logarithmic energy loss equals 1, for heavy nucleus the function tends to zero. The next important quantity is the lethargy. Similarly to the energy, the lethargy is the variable for a description of the slowing down process. Lethargy is denoted as U and it equals to the logarithm of the ratio of reference energy and the current value of energy corresponding to the value of u. The reference energy — the maximal possible neutron energy in a system (source energy) ~10 MeV. Through the slowing down, the lethargy increases in contrast to the energy. The benefit of the variable is that in terms of the lethargy the average increment of lethargy doesn’t depend on the current value of incident neutron energy in contrast to the energy variable. By using thisquantity it is very easy to calculate the average number of collisions for a neutron to nucleus to slow down. From for example E1 = 1 MeV to E2 = 1 eV. The result depicted on this slide. One can see that the fastest slowing down occurs from the scattering on the hydrogen. Can we say that hydrogen is the best moderator? So, first of all, we have to introduce three new quantities. The first us the average logarithmic loss of energy for mixture of nuclei. The mixture means molecules or a homogeneous medium which is composed of different nuclei. The second, slowing down power determines the moderator which better moderates. The third, the moderation ratio or quality factor defines the best moderator and emphasizes the fact, that the best moderator has to do both, to better slow down the neutron and worse to absorb the neutrons. By the last criterion, as it is presented in the table, the best moderator is the heavy water.