In nuclear physics, beta decay is a type of particle decay in which beta radiation is emitted . It owes its name to the classification made by Emest Rutherford in 1899 of the radioactive emissions in alpha and beta, according to the capacity of penetration in the objects and the capacity of ionization.
Beta radiation has a greater range of penetration and greater ionization capacity than alpha radiation, but less than gamma radiation. We now know that beta radiation is made up of electrons or high-energy positrons with neutrinos. The study of beta decay was, in fact, the first physical evidence of the existence of neutrinos .
Definition of beta decay
Beta decay is defined as the radioactive nuclear decay in which beta particles and neutrinos are emitted . For each beta particle an associated neutrino is emitted. Beta particles at high speed form beta radiation .
The beta particle can be an electron (e – ) or a positron (e + ) . And the neutrino can be an electronic antineutrino or an electronic neutrino . If the beta particle is an electron, it will be accompanied by an electronic antineutrino. If the beta particle is a positron, it will be accompanied by an electronic neutrino.
This results in two types of beta decay , one in which an electron + antineutrino (? – ) is emitted, and another in which a positron + neutrino (? + ) is emitted :
- Decay ? – (beta minus or beta negative) : emits an electron and an electronic antineutrino. It occurs in nuclei with excess neutrons. A neutron is transformed into a proton that is integrated into the nucleus, and into an electron and an electronic antineutrino that are emitted in the form of radioactivity. The decay ? – produces a nucleus whose atomic number is increased by 1.
- Decayment ? + (beta plus or beta positive) : emits a positron and an electronic neutrino. It occurs in nuclei with excess protons. It is less common than ? – decay . A proton is transformed into a neutron and a positron and an electronic neutrino are emitted. The ? + decay produces a nucleus whose atomic number is reduced by 1.
Description and foundation
Beta decay is a radioactive phenomenon of nuclear disintegration in which an unstable atom acquires a more stable ratio between protons and neutrons. It can be by transforming a neutron into a proton, or by transforming a proton into a neutron. In this transformation, beta particles are emitted, either electrons (decay ? – ) or positrons (decay ? + ).
Beta decay is a consequence of the weak nuclear force , which produces relatively long decay times.
Each nucleon (proton or neutron) is made up of different combinations of type u (up) quarks and type d (down) quarks . Specifically, the proton consists of two quarks up and one down, while the neutron is formed by two quark down and one up.
The weak nuclear force allows a quark to change type ua type d by exchanging a W-boson ( responsible for the weak nuclear force ) and creating an electron / antineutrino pair or a positron / neutrino pair.
In other words, beta decay is produced by changing the type of quarks in a neutron or a proton, so that one transforms into another and emits radiation.
In beta decay, the mass number is retained (neutrons plus protons) but not the atomic number (number of protons). The nucleus resulting from beta decay has an atomic number different from the starting one and, therefore, is a different element from the starting one.
14 C 6 ? 14 N 7 + e – + antineutrino
Decay ? –
In beta minus decay or beta negative, a neutron transforms into a proton and emits an electron and an electronic antineutrino :
n ? p + e – + antineutrino
Beta decay occurs spontaneously in free neutrons. If it occurs within an atomic nucleus, it will result in the transmutation to the next element in atomic number, since the atomic number increases by one unit ( there is one more proton ):
Z X A ? Z and A + 1 + e – + antineutrino
Where X is the starting element and Y the final element; Z is the mass number and A is the atomic number.
Example : cesium 137 decays to Bario 137
137 Cs 55 ? 137 Ba 56 + e – + antineutrino
At the fundamental level, the process is produced by the conversion of a quark d (down), of negative charge (-1/3 e), into a quark u (up), of positive charge (+2/3 e), and the emission of a W – boson . The boson W – subsequently decays into an electron and an electron antineutrino.
Decayment ? + (positive beta)
Depletion ? + , beta positive or beta plus, is less common than negative beta decay. It is also known as positron emission . Among other applications, positron emission is used in tomography .
In positive beta decay, a proton is transformed into a neutron and emits a positron and an electronic neutrino .
p ? n + e + + neutrino
But unlike the negative beta decay that occurred spontaneously in free neutrons, this process does not occur in free protons . When it occurs in protons that are part of a nucleus, the resulting element will have the atomic number reduced by one unit ( there is one less proton ):
Z X A ? Z Y A-1 + e + + neutrino
Example : Sodium 22 decays to Neon 22
22 Na 11 ? 22 Ne 10 + e + + neutrino
In the positive beta decay process the exchanged boson is W + type .
Electronic capture is often described as another type of beta decay. In all cases in which the positive beta decay is energetically allowed, so is the electronic capture.
The result is similar to the positive beta decay and the atomic number is reduced by one unit ( there is one less proton ), but instead of emitting a positron and a neutrino, only one neutrino is emitted :
Z X A + e + ? Z Y A-1 + neutrino
In nuclei rich in protons with an energy difference between the initial and final state less than 2 m and c 2 , positive beta decay is not possible. These cores can only be disintegrated by electronic capture.
If the electron comes from the atom itself, from the so-called K-layer, then it is known as the K-capture . The electrons in the K-layer are the ones that are most likely to interact with the nucleus.
Example : sodium 22 can decay in neon 22 by positive beta decay, but it can also do so by electronic capture:
22 Na 11 ? 22 Ne 10 + e + + neutrino (decay ? + )
22 Na 11 + e + ? 22 Ne 10 + neutrino (electronic capture)
Example : Krypton 81 decays to Bromo 81 by electronic capture
81 Kr 36 + e – ? 81 Br 35 + neutrino