Beta decay is a type of radioactive decay that releases an energic electron or positron (the antimatter version of an electron). The process occurs when an atomic nucleus is unstable because it has too many protons or neutrons. In beta minus decay (β−), a neutron decays into a proton, an antineutrino, and an electron. In beta plus decay (β+), a neutron decays into a proton, a neutrino (ν), and a positron. In beta decay, the total number of nucleons remains unchanged. The emitted electron or positron has high speed and high energy, so it is called a beta particle, beta ray, or beta radiation to distinguish it from the normal particles. Beta particles are a form of ionizing radiation that have a range of around one meter in air and an energy of 0.5 MeV.
β− Decay or Electron Emission
Beta minus emission is the more common process on Earth because it usually results from neutron-rich nuclei resulting from fission or alpha decay. It is common in fission nuclear reactors. In beta minus decay, a neutron (n) converts into a proton (p), electron (e–) and electron antineutrino (the neutrino antiparticle):
n → p + e–+ νe (usually written with a bar over the neutrino, indicating the antiparticle)
In beta minus decay, the atomic number increases by 1, while the number of neutrons decreases by 1.
ZXA → ZYA+1 + e– + antineutrino
The weak interaction mediates the process. Technically, the neutron emits a virtual W– boson, turning a down quark into an up quark. A neutron contains one up quark and two down quarks, while a proton has two up quarks and one down quark. Then, the W– boson decays into an electron and antineutrino.
An example of beta minus decay is the decay of carbon-14 into nitrogen-14.
614C → 714N + e–+ νe
Other examples of beta emitters include strontium-90, tritium, phosphorus-32, and nickel-63
β+ Decay or Positron Emission
While less common on Earth, beta plus decay occurs in stars when fusion produces neutron-deficient nuclei. Here, a proton converts into a neutron, positron (e+), and electron neutrino (νe):
p → n + e++ νe
In beta plus decay, the atomic number decreases by 1, while the number of neutrons increases by 1.
ZXA → ZYA-1 + e+ + neutrino
An example of beta plus decay is the decay of carbon-10 into boron-10:
610C → 510B + e++ ν
Another example is the decay of sodium-22 into neon-22.
Beta Radiation Properties
In comparison to alpha and gamma radiation, beta radiation has intermediate ionizing and penetrating power. A few millimeters of aluminum stops most beta particles. However, that doesn’t mean thin shielding is completely effective. This is because beta electrons emit secondary gamma rays as they slow down in matter. The best shielding materials consists of atoms with low atomic weights because then the beta electrons produce lower energy gamma radiation. Beta deceleration may give off bremsstrahlung x-rays. The water of a nuclear reactor often glows blue because the beta radiation from fission products is faster than the speed of light in water. The Cherenkov radiation glows blue.
Beta Decay Health Effects
Because beta particles are ionizing radiation, they penetrate into living tissue and can cause spontaneous DNA mutations. These mutations can kill cells or cause cancer.
However, beta sources also find use as tracers in medical diagnostic tests and in cancer treatment. Strontium-90 is a common isotope that produces beta particles used in treating bone and eye cancer.
- Jung, M.; et al. (1992). “First observation of bound-state β− decay”. Physical Review Letters. 69 (15): 2164–2167. doi:10.1103/PhysRevLett.69.2164
- Krane, K.S. (1988). Introductory Nuclear Physics. John Wiley & Sons Inc. ISBN 978-0-471-80553-3.
- L’Annunziata, Michael F. (2007). Radioactivity: Introduction and History. Amsterdam, Netherlands: Elsevier Science. ISBN 9780080548883.
- Martin, B.R. (2011). Nuclear and Particle Physics: An Introduction (2nd ed.). John Wiley & Sons. ISBN 978-1-1199-6511-4.
- Petrucci, Ralph H.; Harwood, William S.; Herring, F. Geoffrey (2002). General Chemistry (8th ed.). Prentice Hall. ISBN 0-13-014329-4.