Alpha, Beta & Gamma Radiation (OCR A Level Physics)

Alpha, Beta & Gamma Particles

• Some elements have nuclei that are unstable

  • This tends to be when the number of nucleons does not balance

• In order to become more stable, they emit particles and/or electromagnetic radiation

  • These nuclei are said to be radioactive

• There are three different types of radioactive emission: Alpha, Beta and Gamma

Alpha Particles

• Alpha (α) particles are high energy particles made up of 2 protons and 2 neutrons (the same as a helium nucleus)

• They are usually emitted from nuclei that are too large

• Alpha is a low penetrating type of radiation

  • Alpha particles have a range of a few cm in air

Beta Particles

• Beta (β⁻) particles are high energy electrons emitted from the nucleus

  • β⁻ particles are emitted by nuclei that have too many neutrons

• Beta is a moderately ionising type of radiation

  • This is due to it having a charge of +1e

  • This means it is able to do some slight damage to cells (less than alpha but more than gamma)

• Beta is a moderately penetrating type of radiation

  • Beta particles have a range of around 20 cm - 3 m in air, depending on their energy

• Beta can be stopped by a few millimetres of aluminium foil

Gamma Rays

• Gamma (γ) rays are high energy electromagnetic waves

• They are emitted by nuclei that need to lose some energy

• Gamma is a highly penetrating type of radiation

  • Gamma particles have a range of around 1- 10 cm in lead or several metres in concrete

• If these particles hit other atoms, they can knock out electrons, ionising the atom

• This can cause chemical changes in materials and can damage or kill living cells

When radiation passes close to atoms, it can knock out electrons, ionising the atom

• The properties of the different types of radiation are summarised in the table below

• u is the atomic mass unit (see “Atomic Mass Unit (u)”)

• e is the charge of the electron: 1.60 × 10^-19 C

• c is the speed of light: 3 × 10⁸ m s^-1

Investigating the Absorption of Alpha, Beta & Gamma Radiation

Aim of the Experiment

•  The aim of this experiment is to investigate the penetration powers of different types of radiation using either radioactive sources or simulations

Variables:

• Independent variable = Absorber material

• Dependent variable = Count rate

• Control variables:

  • Radioactive source

  • Distance of GM tube to source

  • Location / background radiation

Method

Investigating radiation apparatus

1. Connect the Geiger-Müller tube to the counter and, without any sources present, measure background radiation over a one minute period

2. Repeat this three times, and take an average

3. Now place a radioactive source a fixed distance of 3 cm away from the tube and take another reading over a one minute interval

4. Now take a set of absorbers: some paper, several different thicknesses of aluminium (increasing in 0.5mm intervals) and different thickness of lead

5. One at a time, place these absorbers between the source and the tube and take another reading over a one minute interval

6. Repeat the above experiment for other radioactive sources

Results

• Alpha radiation will be absorbed by the paper

• Beta radiation will be absorbed by the aluminium foil

• Some gamma radiation will be absorbed by the thick lead

 Penetrating power of alpha, beta and gamma radiation

Safety Considerations

• When not using a source, keep it in a lead lined container

• When in use, try and keep a good distance (a metre or so) between yourself and the source

• When handling the source, do so using tweezers (or tongs) and point the source away from you