Beta Decay Example

During the last years of the nineteenth century, some physicists thought that all that had to be discovered in physics had already been discovered and all that remains is consolidation of facts. How wrong they were proved to be when in 1896, Becquerel discovered radioactivity! Radioactivity 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 decay, beta decay and gamma decay.

What is Beta Decay?
Radioactivity occurs due to the instability of atomic nucleus. 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. As described there are two types of radioactive decay which involve beta (electron emission) and positron decay (positron emission).

Actually in both types of radioactive 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 beta particles emitted 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.

Beta Decay Rules
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. 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 below.

• 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.

Examples of Beta Decay

Here are some examples that involve electron and positron emission.

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 beta decay in various radioactive elements.