Beta decay
Beta decay changes the proton-neutron balance of a nucleus through the weak interaction. Unlike alpha decay, the mass number can remain unchanged while the atomic number changes by one. The emitted electron or positron is created in the decay; it is not an electron that was stored inside the nucleus.
Beta minus changes a neutron into a proton
In beta-minus decay, a neutron becomes a proton while an electron and an electron antineutrino are produced. At nuclear level, A stays the same and Z increases by one.
beta minus: neutron → proton + electron + antineutrino
Whole nucleus: (A, Z) → (A, Z + 1) + electron + antineutrino
Charge, energy, momentum and angular momentum are conserved in the complete process.
Beta plus and electron capture
Other weak processes move the nucleus in the opposite direction. In beta-plus decay, a proton is converted into a neutron while a positron and an electron neutrino are produced, so Z decreases by one. In electron capture, the nucleus captures an atomic electron and a proton becomes a neutron while a neutrino is emitted. These processes share the same basic goal: changing the neutron-proton balance.
The continuous spectrum and the neutrino
If beta decay produced only a daughter nucleus and an electron, a fixed nuclear transition would give the electron a nearly fixed energy. Experiments instead show a continuous spectrum. The missing share is carried by the neutrino or antineutrino, with a small amount in nuclear recoil. This is a direct reason the neutrino belongs in the energy and momentum bookkeeping.
What beta particles do in matter
Beta particles are much lighter than alpha particles and typically travel farther in matter, but their paths are easily deflected by electric fields of atomic nuclei and by collisions with electrons. Their penetration and energy loss depend on energy and material. As with every radiation type, activity, particle energy, geometry and exposure conditions should not be collapsed into one vague idea of “strength.”
Worked examples
1. Find the daughter in beta-minus decay
Solution
Carbon-14 has A = 14 and Z = 6. Beta-minus decay leaves A = 14 and raises Z to 7, giving nitrogen-14.
2. Distinguish beta plus
Solution
If a nucleus undergoes beta-plus decay, A is unchanged while Z decreases by one. A parent with A = 22 and Z = 11 therefore produces a daughter with A = 22, Z = 10.
3. Explain the spectrum
Solution
Two beta electrons from identical nuclear transitions can have different kinetic energies because the available energy is shared differently among the electron, neutrino and recoil in each event.