A phenomenon exploited for the purpose of detecting and quantifying the flow of water in or around a borehole based on oxygen activation. Oxygen (¹⁶O) can be activated by high-energy neutrons to produce an isotope of nitrogen (¹⁶N), which decays back to oxygen with a half-life of 7. 1 seconds and emits an easily detected gamma ray of 6. 13 MeV. The neutrons are supplied by the generator of a pulsed-neutron spectroscopy tool. The gamma rays are counted in detectors placed above the tool, for upward flow, or below it for downward flow.

Various techniques have been developed to analyze the count rates in terms of water velocity, flow rate and distance from tool. The earliest methods were based of the ratio of the counts from two detectors. Stationary oxygen and other background signals are eliminated by calibration in a known zone of zero flow, by counting only in a window near 6. 13 MeV, or by optimizing the detector spacings. More recently, impulse-activation techniques have been introduced. While they require the tool to be stationary, they are generally more accurate.