Inverse beta decay, commonly abbreviated to IBD, is a
nuclear reaction involving an
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 and
Borexino
Borexino is a particle physics experiment to study low energy (sub-MeV) solar neutrinos. The detector is the world's most radio-pure liquid scintillator calorimeter. It is placed within a stainless steel sphere which holds the photomultiplier tu ...
. It is an essential process to experiments involving low-energy neutrinos (< 60
MeV)
such as those studying
neutrino oscillation,
,
sterile neutrinos, and
geoneutrinos. The IBD reaction can only be used to detect antineutrinos (rather than normal matter neutrinos, such as from the Sun) due to
lepton conservation.
Reactions
Antineutrino induced
Inverse beta decay proceeds as
+ → + ,
where an
electron antineutrino () interacts with a
proton () to produce a
positron () and a
neutron (). The IBD reaction can only be initiated when the antineutrino possesses at least 1.806 MeV
of kinetic energy (called the
threshold energy). This threshold energy is due to a difference in mass between the products ( and ) and the reactants ( and ) 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
undergoes matter–antimatter
annihilation
In particle physics, annihilation is the process that occurs when a subatomic particle collides with its respective antiparticle to produce other particles, such as an electron colliding with a positron to produce two photons. The total ener ...
after creation and yields a flash of light with energy calculated as
,
where 511 keV is the
electron and positron rest energy, is the visible energy from the reaction, and 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 ( in the
Borexino
Borexino is a particle physics experiment to study low energy (sub-MeV) solar neutrinos. The detector is the world's most radio-pure liquid scintillator calorimeter. It is placed within a stainless steel sphere which holds the photomultiplier tu ...
detector
). 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.
The IBD
cross section is dependent on antineutrino energy and capturing element, although is generally on the order of 10
−44 cm
2 (∼
attobarns).
Neutrino induced
Another kind of inverse beta decay is the reaction
+ → +
The
Homestake experiment used the reaction
:
to detect solar neutrinos.
Electron induced
During the formation of
neutron stars, or in radioactive isotopes capable of
electron capture, neutrons are created by electron capture:
+ → + .
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.
See also
*
Kamioka Liquid Scintillator Antineutrino Detector
References
{{Reflist
Radioactivity