Neutron tools are based on the measurement of a neutron cloud of different energy levels within the investigated volume. Epithermal-neutron tools measure epithermal neutron density with energy levels between 100eV and 0.1eV in the formation. Thermal-neutron tools only measure the population of neutrons with a thermal energy level, and Neutron-gamma tools measure the intensity of gamma flux generated by thermal neutron capture. The tools usually have two detectors (or more) with different spacings from the source to produce ratio of count rates, which theoretically reduce borehole effects.
A Helium-3 (He-3) filled proportional counter is the most common epithermal and thermal neutron detector. Helium has a high neutron capture cross section and produces the following reaction when interacting with a neutron.
    He + n  →   H + H + 764keV energy
To boost the charge produced by the interaction between helium and a neutron, a high voltage is applied to the anode of the counter. A high operating voltage is chosen to give enough gain for counting purposes. Most helium-3 counters use a quench gas to stabilize high voltage performance and prevent run-away.