The rate at which neutrinos pass through a unit area, often used to measure a source's neutrino emission.Neutrino flux is the rate at which neutrinos pass through a unit area per unit time, typically expressed in neutrinos per square centimeter per second. It quantifies how many of these nearly massless, weakly interacting particles stream from a source like the Sun, a supernova, or a nuclear reactor. Because neutrinos barely interact with matter, measuring their flux provides a direct probe of processes deep inside stars or the early universe that are otherwise invisible.
The neutrino was proposed by Wolfgang Pauli in 1930 and detected by Clyde Cowan and Frederick Reines in 1956. The first solar neutrino flux measurement by Ray Davis (1960s-70s) revealed a deficit that led to the discovery of neutrino oscillations, earning a Nobel Prize in 2002.
Neutrinos are produced in nuclear reactions (e.g., fusion in the Sun) and travel almost unimpeded through space. To measure flux, detectors use large volumes of material — water, ice, or liquid scintillator — where a rare neutrino interaction creates a detectable signal, such as a flash of light (Cherenkov radiation) or a charged particle track. By counting events and accounting for detector efficiency and size, physicists infer the incoming flux. The low interaction probability means detectors must be enormous and shielded from background radiation.
Neutrino flux measurements monitor nuclear reactors for nonproliferation, study solar fusion, and detect supernovae — as with the 1987 event that confirmed core-collapse models.
About 100 billion solar neutrinos pass through every square centimeter of your body each second, yet you never feel a thing — they interact so weakly that only about one per lifetime would be absorbed.
