Alpha, beta and gamma radiation, and nuclear equations

Background radiation

There is always a low level of background radiation, from radon gas, rocks and building materials, cosmic rays, food and drink, and medical uses. To find the count rate from a source, measure the background count rate (over a long time, with the source away) and subtract it from every reading.

Comparing the radiations

• Alpha (α): a helium nucleus (2 protons, 2 neutrons), charge +2e. Very strongly ionising; range a few cm in air; stopped by a sheet of paper.
• Beta-minus (β-): a fast electron from the nucleus, charge -e. Moderately ionising; range about a metre in air; stopped by a few mm of aluminium. (β+ is a positron.)
• Gamma (γ): high-energy electromagnetic radiation, no charge. Weakly ionising; spreads out in air (inverse square law); reduced, but not stopped, by several cm of lead.

The more strongly a radiation ionises, the faster it loses energy, and the shorter its range.

Nuclear equations

Nucleon numbers and proton numbers must balance on both sides:

• α decay: A falls by 4, Z falls by 2. Example: Po-210 → Pb-206 + He-4.
• β- decay: A unchanged, Z rises by 1 (a neutron becomes a proton); an electron antineutrino is also emitted.
• β+ decay: A unchanged, Z falls by 1; an electron neutrino is also emitted.
• γ emission: no change in A or Z; the nucleus loses energy.

Worked example

Thorium-234 (Z = 90) decays by β- emission. Write down the nucleon and proton numbers of the daughter nucleus.

1. β- decay: A stays 234
2. Z goes up by 1: 90 + 1 = 91 (protactinium)

Answer: Pa-234, with A = 234 and Z = 91

Check yourself

1. A detector records 520 counts in 2.0 minutes near a source. Without the source it records 180 counts in 10 minutes. Find the corrected count rate in counts per minute.
   Show answer

   242 counts/min. Measured rate = 260 counts/min; background = 18 counts/min; corrected = 260 - 18 = 242 counts/min.

2. Polonium-210 (Z = 84) decays by alpha emission. What is the nucleon number of the daughter nucleus?
   Show answer

   206. Alpha emission removes 4 nucleons: 210 - 4 = 206.

3. What is the proton number of the daughter nucleus when polonium-210 (Z = 84) emits an alpha particle?
   Show answer

   82. Alpha emission removes 2 protons: 84 - 2 = 82 (lead).

4. Carbon-14 (Z = 6) decays by β- emission. What is the proton number of the daughter nucleus?
   Show answer

   7. A neutron becomes a proton, so Z = 6 + 1 = 7 (nitrogen-14).

5. Which type of radiation is the most strongly ionising?
   • Beta-minus
   • Alpha
   • Gamma
   • Beta-plus
   Show answer

   Alpha. Alpha particles are large and doubly charged, so they ionise strongly and have a short range.

6. A radiation passes through paper but is stopped by 3 mm of aluminium. What is it?
   • Alpha
   • Gamma
   • Background
   • Beta
   Show answer

   Beta. Alpha is stopped by paper; gamma passes through aluminium.

7. What happens to the nucleon number and proton number of a nucleus when it emits a gamma ray?
   • A falls by 4 and Z by 2
   • Neither changes
   • Z rises by 1
   • A falls by 1
   Show answer

   Neither changes. A gamma ray is a photon; it carries away energy only.

8. How should background radiation be accounted for in an experiment?
   • Measure the background count rate and subtract it from each reading
   • Add the background count rate to each reading
   • Ignore it, because it is always zero
   • Shield the source with lead
   Show answer

   Measure the background count rate and subtract it from each reading. The detector counts background as well as the source.

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Sources

• Pearson Edexcel International Advanced Subsidiary/Advanced Level in Physics specification (XPH11/YPH11), Issue 3
• Pearson qualifications: International Advanced Levels Physics (2018)

Last updated 7 November 2026. Spotted a mistake? Tell us.