Beta Decay Example

In 1896, Becquerel discovered radioactivity, which opened up a Pandora's box of problems, related to the microcosm of the atomic nucleus. Subsequent development of the special theory of relativity and quantum physics, opened up even bigger problems, that would keep physicists busy for more than a century.

One of the important findings of Becquerel was the discovery of the three types of radioactive decay, which are alpha, beta, and gamma decay.

Radioactivity occurs due to the instability of certain atomic nuclei. It is the spontaneous emission of ionizing radiation by atoms of certain elements, that have unstable nuclei. Beta decay involves the emission of an electron (e^-) or a positron (e⁺, antiparticle of electron) from an atomic nucleus, causing its transformation into another element.

Actually, in both types of this decay, a neutrino is also emitted, along with the beta particles. Neutrinos are neutral particles with very little mass, that travel close to the speed of light. The energy of emitted beta particles varies according to how the energy is shared between them and the emitted neutrinos.

That is why, the energy of emitted beta particles varies over a spectrum that extends up to a maximum energy value. Typical energies of beta particles are recorded to be from a few kilo electron volts to around 10 mega electron volts. High energy beta particles are spitted out of the nucleus at relativistic speeds.

In such a decay reaction, involving emission of an electron, an antineutrino is also emitted, while in positron decay, a neutrino is emitted.

When studied at a deeper level, it was realized that beta decay was a result of proton and neutron decay at the nuclear level. A decay involving electron emission (also known as beta minus decay) occurs, when a neutron inside the nucleus decays to a proton, emitting an electron and an antineutrino in the process.

On the other hand, a positron emission (beta plus decay) is the result of a proton decaying into a neutron and it is accompanied by the emission of a neutrino.

Based on these facts, here are the governing decay rules, which you can verify through the examples provided here.

✦ When an electron is emitted by a radioactive nucleus, the resulting transmuted nucleus has an atomic number greater than 1 (as it converts a neutron into a proton), while the atomic weight remains the same.

✦ When a positron is emitted, the resulting transmuted nucleus has its atomic number reduced by 1 (as it converts a proton into a neutron), while the atomic weight remains the same.

¹³⁷Cs₅₅ -> ¹³⁷Ba₅₆ + e^- + antineutrino

²²Na₁₁ -> ²²Ne₁₀ + e⁺ + neutrino

⁶⁰Co₂₇ -> ⁶⁰Ni₂₈ + e^- + antineutrino

³H₁ -> ³He₂ + e^- + antineutrino

¹⁴C₆ -> ¹⁴N₇ + e^- + antineutrino

¹⁰C₆ -> ¹⁰B₅ + e⁺ + neutrino

Alpha decays are much more likely than beta decays. In some rare cases, a process called double beta decay may also occur, which involves the emission of two beta particles simultaneously.

In your physics lab course, you may conduct experiments using a Geiger counter to study this decay type, in various radioactive elements.