Inverse beta decay

Inverse beta decay, commonly abbreviated to IBD,[1] is a nuclear reaction involving electron antineutrino scattering off a proton, creating a positron and a neutron. This process is commonly used in the detection of electron antineutrinos in neutrino detectors, such as the first detection of antineutrinos in the Cowan–Reines neutrino experiment, or in neutrino experiments such as KamLAND, Borexino and SAGE (Soviet–American Gallium Experiment). It is an essential process to experiments involving low energy neutrinos (< 60 MeV)[2] such as those studying neutrino oscillation,[2] reactor neutrinos, sterile neutrinos, and geoneutrinos.[3]

Reaction

Inverse beta decay proceeds as

${\displaystyle {\bar {\nu }}_{e}+p\to e^{+}+n}$,[2][3][4]

where an electron antineutrino (${\displaystyle {\bar {\nu }}_{e}}$) interacts with a proton (p) to produce a positron (${\displaystyle e^{+}}$) and a neutron (n). The IBD reaction can only be initiated when the antineutrino possesses at least 1.806 MeV[3][4] 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[4] undergoes matter-antimatter annihilation after creation and yields a flash of light with energy calculated as

${\displaystyle E_{\text{vis}}=511{\text{ keV}}+511{\text{ keV}}+(E_{{\bar {\nu }}_{e}}-1806{\text{ keV}})=E_{{\bar {\nu }}_{e}}-782{\text{ keV}}}$,[4]

where 511 keV is the electron and positron rest mass, ${\displaystyle E_{\text{vis}}}$is the visible energy from the reaction, and ${\displaystyle E_{{\bar {\nu }}_{e}}}$ 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.[4] The timing of the delayed capture is 200–300 microseconds after IBD initiation (≈256 µs in the Borexino detector[4]). The timing and spatial coincidence between the prompt positron annihilation and delayed neutron capture provides a clear IBD signature in neutrino detectors.[4]

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.