Chapter 15. pdf

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Petrophysics MSc Course Notes The Neutron Log
Dr. Paul Glover Page 152
The initially fast neutrons (>0.5 MeV) quickly lose their energy and become slower (Fig. 15.3),
passing through stages called 
intermediate neutrons
(10
2
to 10
5
eV), 
epithermal neutrons
(0.1 to 100
eV), and finally 
thermal neutrons
(<0.1 eV). In solid materials containing reasonable amounts of low
atomic mass elements, this process can happen very quickly for a given neutron (of the order of micro-
seconds). However, the time taken to slow down to a given energy will vary from neutron to neutron,
depending on the chance collisions with nuclei. Thermal neutrons are so called because they have
energies which are those that a particle has as a result of it existing a room temperatures. In other
words, they only have the small energies associated with the random kinetic motion associated with
room temperatures.
Figure 15.3
The slowing of fast neutrons with time by elastic collision with formation nuclei.
When the neutrons attain epithermal or thermal energies, collisions occur much less frequently
because the neutrons are moving from nucleus to nucleus much more slowly. Within a few
microseconds of being exposed to the fast neutron source, the formation has slowed the incoming
neutrons down to epithermal and thermal levels, and a cloud of these thermal neutrons exists in the
formation surrounding the source. Collisions continue resulting in little further loss of energy and the
slow diffusion of the thermal neutrons from the zone around the detector. During this process the
neutrons are absorbed by the formation nuclei.