neutron log alone should be undertaken with a realization of the uncertainties involved. Effect of light hydrocarbon and gas The quantitative response of neutron tool to gas or light hydrocarbon depends primarily on hydrogen index and "excavation effect". The hydrogen index can be estimated from the composition and density of the hydrocarbons Given a fixed volume, gas has considerably lower hydrogen concentration. When pore spaces in the rock are excavated and replaced with gas, the formation has smaller neutron-slowing characteristic, hence the terms "Excavation Effect". If this effect is ignored, a neutron log will show a low porosity value. This characteristic allows a neutron porosity log to be used with other porosity logs (such as a density log) to detect gas zones and identify gas-liquid contacts. Measurement technique 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. 3He + 1n → 1H + 3H + 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. See also Neutron temperature Effective porosity Gas porosity References John T. Dewan, "Open-Hole Nuclear Logging - State of the Art" - SPWLA Twenty-Seventh Annual Logging Symposium, June 9–13, 1986. Radioactivity Well logging