Inverse beta decay

Reaction

Inverse beta decay proceeds as

ν ¯ e + p → e + + n ,

where an electron antineutrino ( ν ¯ e ) interacts with a proton (p) to produce a positron ( e + ) and a neutron (n). The IBD reaction can only be initiated when the antineutrino possesses at least 1.806 MeV of kinetic energy (called the threshold energy). Most of the antineutrino energy is distributed to the positron due to its small mass relative to the neutron. The positron promptly undergoes matter-antimatter annihilation after creation and yields a flash of light with energy calculated as

E vis = 511  keV + 511  keV + ( E ν ¯ e − 1806  keV ) = E ν ¯ e − 782  keV ,

where 511 keV is the electron and positron rest mass, E vis is the visible energy from the reaction, and E ν ¯ e is the antineutrino kinetic energy. After the prompt positron annihilation, the neutron undergoes neutron capture on an element in the detector, producing a delayed flash of 2.22 MeV if captured on a proton. The timing of the delayed capture is 200–300 microseconds after IBD initiation (6996256000000000000♠≈256 µs in the Borexino detector). The timing and spatial coincidence between the prompt positron annihilation and delayed neutron capture provides a clear IBD signature in neutrino detectors.

Inverse beta decay may also sometimes refer to the interaction of an electron and proton, creating a neutrino and neutron, although this process is normally referred to as electron capture.