Inverse beta decay

Inverse beta decay proceeds as^[2][3][4]

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^[3][4] of kinetic energy (called the threshold energy). This threshold energy is due to a difference in mass between the products (
e⁺
and
n
) and the reactants (
ν
_e and
p
) and also slightly due to a relativistic mass effect on the antineutrino. 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^[5]

$${\displaystyle {\begin{aligned}E_{\text{vis}}&=511{\text{ keV}}+511{\text{ keV}}+E_{\rm {\,{\overline {\nu }}_{e}}}-1806{\text{ keV}}\\[2pt]&=E_{\rm {\,{\overline {\nu }}_{e}}}-784{\text{ keV}}\end{aligned}}}$$ 

where 511 keV is the electron and positron rest energy, E_vis is the visible energy from the reaction, and ⁠${\displaystyle E_{\rm {\,{\overline {\nu }}_{e}}}}$ ⁠ 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, allowing for discrimination from background.^[4] The IBD cross section is dependent on antineutrino energy and capturing element, although is generally on the order of 10⁻⁴⁴ cm² (~ attobarns).^[6]

Another kind of inverse beta decay is the reaction

ν
_e +
n
→
e⁻
+
p

The Homestake experiment used the reaction

${\displaystyle \mathrm {\nu _{e}+\ ^{37}Cl\longrightarrow \ ^{37}Ar+e^{-}} }$ 

to detect solar neutrinos.

During the formation of neutron stars, or in radioactive isotopes capable of electron capture, neutrons are created by electron capture:

p
+
e⁻
→
n
+
ν
_e.

This is similar to the inverse beta reaction in that a proton is changed to a neutron, but is induced by the capture of an electron instead of an antineutrino.

• Kamioka Liquid Scintillator Antineutrino Detector