Solar neutrino unit (SNU) is a measure of the detection of neutrinos by neutrino detectors, consisting of one detection per 1036 of the detector's target atoms. Neutrino detectors work through the interactions between neutrinos with target atomic nuclei that comprise much of the detector. The interactions effectively convert a nucleon in the atom's nucleus from neutron to proton (with an electron emitted). The Sun's neutrino production is the typical target of neutrino detection. The SNU is naturally scaled to be a useful unit for detector experiments.
Detectors generally only detect neutrinos that possess more than some threshold energy level, i.e., they are sensitive to a subset of neutrinos consisting of those with sufficient energy. A particular atom used for detection requires a certain amount of neutrino energy for such a conversion. This energy, in turn, depends on the reaction that produced it (a proton-proton chain reaction in the Sun). Detection also depends upon a cross section associated with the particular atoms used in detection, which could be an atom that is converted into radioactive isotope that can be detected by its radioactivity. Thus, the number of SNUs expected in a detection experiment must take these two factors into account.