7.4 More About Alpha, Beta and Gamma Radiation

A free GCSE Physics lesson for AQA (8463) from OBA School: penetration, range in air and ionising power; testing absorbers; irradiation and contamination; dangers, safety and uses. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Compare alpha, beta and gamma radiation in terms of penetrating power, range in air and ionising power
• Describe how to use a Geiger–Müller tube and absorbers to identify the radiation from a source
• Explain the difference between irradiation and contamination
• Explain why ionising radiation is dangerous and how people protect themselves
• Choose the right type of radiation for a use and explain the choice

Key words

• Penetrating power: How easily radiation passes through materials.
• Range: How far radiation can travel through a material, such as air, before it is all absorbed.
• Ionisation: Knocking electrons out of atoms, turning them into positive ions.
• Ionising power: How strongly radiation ionises the atoms it passes. Alpha is the most ionising, gamma the least.
• Irradiation: Being exposed to radiation from a source outside the object. The object does not become radioactive.
• Contamination: Unwanted radioactive atoms getting on or into an object, so it keeps receiving radiation.

Penetrating power and range

The three types of radiation are absorbed by different materials. The more strongly a radiation ionises, the faster it loses energy, so the shorter its range.

• What it is; Alpha α helium nucleus (2p + 2n); Beta β fast electron from the nucleus; Gamma γ electromagnetic wave
• Charge; Alpha α +2; Beta β −1; Gamma γ 0
• Stopped by; Alpha α paper, skin, a few cm of air; Beta β a few mm of aluminium; Gamma γ reduced by thick lead or concrete
• Range in air; Alpha α about 3 to 7 cm; Beta β about a metre; Gamma γ very long; spreads out and weakens
• Ionising power; Alpha α strong; Beta β medium; Gamma γ weak

Key fact: Ionising power and penetrating power go opposite ways. Alpha ionises most and so penetrates least; gamma ionises least and penetrates most.

Deflection by electric and magnetic fields

Because alpha and beta radiation are charged, they are pushed sideways by electric and magnetic fields. They bend in opposite directions because their charges are opposite. Beta bends much more, because a beta particle has far less mass. Gamma is uncharged, so it is not deflected.

Key fact: This shows that alpha particles are positive, beta particles are negative and gamma rays carry no charge.

Identifying radiation with absorbers

Measure the background count rate first. Then fix the source a short distance (about 2 cm) from a GM tube and record the count rate with no absorber, then with paper, a few millimetres of aluminium and a few centimetres of lead between them. Subtract the background each time. A large drop when an absorber is added shows that radiation of that type was present.

Worked example: what does the source emit?

Measured: none 520, paper 300, 4 mm aluminium 140, 3 cm lead 80 counts per minute; background 20.
paper removes 520 − 300 = 220 counts per minute, so alpha is present
aluminium removes 300 − 140 = 160 counts per minute more, so beta is present
after aluminium, 140 − 20 = 120 counts per minute remain and lead only reduces this, so gamma is present

Answer: the source emits alpha, beta and gamma radiation

To find the range in air, move the GM tube away from the source in small steps. The range is the distance at which the count rate falls to the background level. Without the source the count does not fall to zero, because of background radiation.

Irradiation, contamination and dangers

Ionising radiation can damage or kill living cells. Damage to DNA can cause a cell to divide out of control, which can lead to cancer, and damaged genes can be passed on when cells divide.

• Irradiation: the object is exposed to radiation from outside. It stops when the source is removed or shielded, and the object does not become radioactive. (Food and medical equipment are irradiated with gamma rays to kill bacteria.)
• Contamination: radioactive atoms are on or inside the object, for example dust breathed in. The object keeps receiving radiation until the atoms decay or are removed, so contamination is often more dangerous.

• Alpha; Source outside the body least dangerous: stopped by the dead outer layer of skin; Source inside the body most dangerous: all its energy is absorbed by nearby cells, causing heavy ionisation
• Beta and gamma; Source outside the body more dangerous: can reach and damage organs; Source inside the body dangerous, but much of the radiation passes out of the body

Exam tip: People who work with radiation reduce their dose by: keeping their distance (long tongs, remote handling), keeping the time of exposure short, using shielding (lead, concrete, lead-lined boxes), and wearing a radiation monitor badge. Gloves and masks stop contamination.

Choosing radiation for a use

• Smoke alarms use an alpha source. Alpha strongly ionises the air in a small gap, so a current flows. Smoke absorbs the alpha particles, the current falls and the alarm sounds. Alpha is safe here because it cannot get out of the alarm case.
• Thickness control of paper, plastic or metal sheet uses beta. Some beta passes through the sheet and the amount depends on the thickness. Alpha would be stopped completely; gamma would pass through almost unchanged whatever the thickness.
• Sterilising uses gamma, because it penetrates packaging and kills bacteria inside.

Worked example: thickness control

The detector reading rises from 300 to 340 counts per second.
More beta is getting through, so the paper has become thinner.
The control system reduces the pressure of the rollers until the reading returns to 300.

Answer: sheet too thin, so rollers ease off

Common mistakes

• Saying gamma is “stopped” by lead. Thick lead greatly reduces gamma but never stops all of it.
• Saying irradiated food becomes radioactive. Irradiation does not make objects radioactive.
• Saying alpha is the most dangerous in all cases. Outside the body it is the least dangerous; inside it is the most.
• Forgetting background radiation: with absorbers the count rate falls to background, not to zero.
• Mixing up penetrating power and ionising power – they go in opposite orders.

Revision questions

Try these, then check your answers below each one.

Question 1. Which type of radiation is the most strongly ionising? (Choose from: alpha, beta, gamma, all are equally ionising.)

Answer: alpha

Working: Alpha is the most ionising. This is why it is stopped by paper and has a range of only a few cm in air.

Question 2. True or false? Fruit that has been irradiated with gamma rays becomes radioactive.

Answer: False

Working: False. Irradiation exposes the fruit to radiation but does not leave radioactive atoms in it, so it does not become radioactive.

Question 3. A source gives (counts per minute): background 30, no absorber 830, paper 828, 3 mm aluminium 31. What does the source emit? (Choose from: beta only, alpha only, alpha and beta, beta and gamma.)

Answer: beta only

Working: Paper hardly changes the count, so there is no alpha. Aluminium brings the count down to background, so there is beta but no gamma.

Related lessons

• Previous lesson: 7.3 Changes in the Nucleus
• Next lesson: 7.5 Activity and Half-life
• In this chapter: 7.1 Atoms and Radiation
• In this chapter: 7.2 The Discovery of the Nucleus
• In this chapter: 7.6 Nuclear Radiation in Medicine

Continue in the full course

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