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Beta

Radiation and the atom · level 5, lesson 3 of 6

A question to start

There are no electrons in a nucleus. Yet some nuclei shoot out fast electrons. Where do they come from?

The idea

In beta decay, a neutron inside the nucleus turns into a proton, and a fast electron is shot out. That electron is the beta particle.

  • The nucleus gains a proton: the atomic number goes up by 1.
  • It has one neutron fewer and one proton more, so the mass number stays the same.
\[{}^{14}_{6}\text{C} \;\to\; {}^{14}_{7}\text{N} + \text{electron}\]

(A tiny, almost undetectable particle called an antineutrino comes out too.)

A beta particle is about 7,000 times lighter than an alpha particle. It travels further, up to a few metres in air, and can get into the skin. A sheet of plastic or a few millimetres of aluminium stops it. Tritium, carbon-14 and strontium-90 are beta sources.

+ neutronprotonfast electron

Worked examples

1. Carbon-14. \(Z\): \(6 + 1 = 7\), which is nitrogen. \(A\) stays 14. So carbon-14 becomes nitrogen-14.

2. Tritium. Hydrogen-3 has 1 proton and 2 neutrons. After beta decay it has 2 protons and 1 neutron: helium-3.

3. Potassium-40. Most potassium-40 nuclei decay by beta: \(Z\) goes from 19 to 20, which is calcium. So it becomes calcium-40.

Try it

  1. What is a beta particle?
  2. What happens inside the nucleus in beta decay?
  3. Strontium-90 has 38 protons. What does it become after beta decay?
  4. What would stop beta particles?
Answers
  1. A fast electron shot out of a nucleus.
  2. A neutron turns into a proton.
  3. \(Z = 39\), yttrium, with \(A = 90\): yttrium-90.
  4. A sheet of plastic or a few millimetres of aluminium.

Common mistake

"Beta decay takes an electron from around the atom." The beta particle is made in the nucleus, when a neutron turns into a proton. The electrons around the atom are not involved.

Where it leads