AQA A Level Physics

Revision Notes

8.1.3 Alpha, Beta & Gamma Radiation

Alpha, Beta & Gamma Radiation

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 particle, downloadable AS & A Level Physics revision notes

  • Alpha is the most ionising type of radiation
    • This is due to it having the highest charge of +2e
    • This means it produces the greatest number of ion pairs per mm in air
    • This also means it is able to do more damage to cells than the other types of radiation
  • Alpha is the least penetrating type of radiation
    • This means it travels the shortest distance in air before being absorbed
    • Alpha particles have a range of around 3-7 cm in air
  • Alpha can be stopped by a single piece of paper

Beta Particles

  • Beta (β) particles are high energy electrons emitted from the nucleus
  • Beta (β+) particles are high energy positrons (antimatter of electrons) also emitted from the nucleus
    • β particles are emitted by nuclei that have too many neutrons
    • β+ particles are emitted by nuclei that have too many protons

Beta particle, downloadable AS & A Level Physics revision notes

  • 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 Radiation

  • Gamma (γ) rays are high energy electromagnetic waves
  • They are emitted by nuclei that need to lose some energy

Gamma particle, downloadable AS & A Level Physics revision notes

  • 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

Ionisation, downloadable AS & A Level Physics revision notes

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

  • Gamma is the least ionising type of radiation
    • This is because it is an electromagnetic wave with no charge
    • This means it produces the least number of ion pairs per mm in air
    • It can still cause damage to cells, but not as much as alpha or beta radiation. This is why it is used for cancer radiotherapy
  • Gamma is the most penetrating type of radiation
    • This means it travels the furthest distance in air before being absorbed
    • Gamma radiation has an infinite range and follows an inverse square law
  • Gamma can be stopped by several metres of concrete or several centimetres of lead

Comparing Alpha, Beta & Gamma

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

 

 

Worked Example

WE - Alpha beta gamma particles question image, downloadable AS & A Level Physics revision notes

ANSWER:     D

Worked example - alpha beta gamma particles (2), downloadable AS & A Level Physics revision notes

Applications of Alpha, Beta & Gamma

Smoke Detectors

  • Smoke detectors contain a small amount of Americium-241, which is a weak alpha source
  • Within the detector, alpha particles are emitted and cause the ionisation of nitrogen and oxygen molecules in the air
  • These ionised molecules enable the air to conduct electricity and hence a small current can flow
  • If smoke enters the alarm, it absorbs the alpha particles, hence reducing the current which causes the alarm to sound
  • Am-241 has a half-life of 460 years, meaning over the course of a lifetime, the activity of the source will not decrease significantly and it will not have to be replaced

Thickness Controls

  • Beta radiation can be used to determine the thickness of aluminium foil, paper, plastic, and steel
  • The thickness can be controlled by measuring how much beta radiation passes through the material to a Geiger counter
  • Beta radiation must be used, because:
    • Alpha particles would be absorbed by all the materials
    • Gamma radiation would pass through undetected through the materials
  • The Geiger counter controls the pressure of the rollers to maintain the correct thickness
  • A source with a long half-life must be chosen so that it does not need to be replaced often

Worked Example

Below are listed four radioactive sources, together with the type of radiation they emit

A         Americium-241                        Alpha (α)

B         Strontium-90                           Beta Minus (β)

C         Cobalt-60                                Beta Minus (β) & Gamma (γ)

D         Fluorine-18                              Beta Plus (β+)

 

Which isotope is suitable for the purpose of:

a) Sterilising hospital equipment sealed inside plastic bags?

b) Discharging static electricity that has built up in the manufacture of polythene?

c) Monitoring the thickness of a thin metal being produced in a factory?

d) A smoke detector?

a) ANSWER: C

  • Alpha and low energy beta radiation would most likely be absorbed by the bag
  • Therefore, gamma radiation, or very high energy beta particles, would be needed to penetrate the bag
  • This would be best suited to Cobalt-60

 

b) ANSWER: D

  • Static electricity is an imbalance of electric charges on the surface of the polythene and is generally composed of negatively charged electrons
  • In order to get rid of the static charge, it will need to be neutralised
  • Beta-plus particles, or positrons, are the antimatter counterpart of the electron, and hence, are oppositely charged
  • When the positrons are directed at the surface of the polythene, the electrons will be attracted to them and become neutralised as the particles annihilate as they collide
  • Therefore, the beta-plus emitter, Fluorine-18, would be best suited to this job

 

c) ANSWER: B

  • Alpha particles would not be suitable for measuring the thickness of metal as they can be stopped by a thin sheet of paper
  • Gamma rays are the most penetrating of the radiations and hence would not be suitable where thickness monitoring is up to a few millimetres as they would all pass through
  • Beta particles are ideally suited as they have enough energy to pass through thin sheets of metal and any changes in thickness would be easily detected
  • Therefore, the beta-minus emitter Strontium-90 would be the most suitable isotope

 

d) ANSWER: A

  • Since smoke detectors are present inside homes and other buildings, they must pose no hazard to residents
  • This means the smoke detector must contain a very small amount of the radioactive material
  • Also, the radiation should not be too penetrating and should only be able to travel a few centimetres
  • Therefore, an alpha source should be selected – this means Americium-241 would be the most suitable isotope
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