2 x 106 eV
Reactor Theory (Neutron Characteristics)
Since represents the average logarithmic energy loss per collision, the total number of collisions
necessary for a neutron to lose a given amount of energy may be determined by dividing into
the difference of the natural logarithms of the energy range in question. The number of collisions
(N) to travel from any energy, E
, to any lower energy, E
, can be calculated as shown below.
How many collisions are required to slow a neutron from an energy of 2 MeV to a
thermal energy of 0.025 eV, using water as the moderator? Water has a value of 0.948
Sometimes it is convenient, based upon information known, to work with an average fractional
energy loss per collision as opposed to an average logarithmic fraction. If the initial neutron
energy level and the average fractional energy loss per collision are known, the final energy level
for a given number of collisions may be computed using the following formula.