What stops alpha, beta and gamma radiation?

Alpha: paper or a few cm of air. Beta: a few mm of aluminium. Gamma: reduced by thick lead or concrete, never completely stopped.

What is a beta-minus particle?

A fast electron emitted from the nucleus.

State the inverse-square law for gamma radiation.

I = k/x²: intensity is inversely proportional to the square of the distance from a point source.

Which graph is a straight line through the origin if the inverse-square law holds?

Corrected count rate against 1/x².

How is the background count eliminated from results?

Measure it with no source over a long time, then subtract it from every reading.

Give three origins of background radiation.

Radon gas, rocks and soil, cosmic rays, food and drink, medical sources, nuclear fallout.

Why is beta used to monitor the thickness of aluminium foil?

It is partly absorbed, so count rate changes noticeably with thickness. Alpha is fully absorbed and gamma passes through almost unchanged.

Why are long tongs used with radioactive sources?

They keep the hands further from the source, and the intensity falls with the square of the distance.

How does the result of absorption tests identify gamma radiation?

The count rate stays above background with aluminium but is reduced by thick lead.

Why does the benefit of radiotherapy outweigh its risk?

It destroys cancer cells in a life-threatening disease, while the risk of damage to healthy tissue is small and minimised by directing the dose at the tumour.

Why must background be measured over a long time?

Radioactive decay is random, so the count rate fluctuates, and a long average is more reliable.

Alpha, beta and gamma radiationAQA A-Level Physics: Flashcards

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What is an alpha particle?

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What is an alpha particle?: A helium nucleus of 2 protons and 2 neutrons, with charge +2e.
What stops alpha, beta and gamma radiation?: Alpha: paper or a few cm of air. Beta: a few mm of aluminium. Gamma: reduced by thick lead or concrete, never completely stopped.
Which radiation is most strongly ionising?: Alpha radiation.
What is a beta-minus particle?: A fast electron emitted from the nucleus.
State the inverse-square law for gamma radiation.: I = k/x²: intensity is inversely proportional to the square of the distance from a point source.
Which graph is a straight line through the origin if the inverse-square law holds?: Corrected count rate against 1/x².
How is the background count eliminated from results?: Measure it with no source over a long time, then subtract it from every reading.
Give three origins of background radiation.: Radon gas, rocks and soil, cosmic rays, food and drink, medical sources, nuclear fallout.
Why is beta used to monitor the thickness of aluminium foil?: It is partly absorbed, so count rate changes noticeably with thickness. Alpha is fully absorbed and gamma passes through almost unchanged.
Why are long tongs used with radioactive sources?: They keep the hands further from the source, and the intensity falls with the square of the distance.
How does the result of absorption tests identify gamma radiation?: The count rate stays above background with aluminium but is reduced by thick lead.
Why does the benefit of radiotherapy outweigh its risk?: It destroys cancer cells in a life-threatening disease, while the risk of damage to healthy tissue is small and minimised by directing the dose at the tumour.
Why must background be measured over a long time?: Radioactive decay is random, so the count rate fluctuates, and a long average is more reliable.

Exam questions on Alpha, beta and gamma radiation

A factory makes aluminium foil in a continuous strip. A radioactive source is placed on one side of the strip and a Geiger–Müller (GM) tube on the other side. The count rate at the detector is monitored by a computer, which adjusts the rollers that set the thickness of the foil.
Explain why a gamma source would not be suitable for monitoring the thickness of the aluminium foil.2 marks
A student investigates how the count rate from a gamma source varies with distance. With no source present, a Geiger–Müller (GM) tube records an average of 24 counts per minute. With the source 0.10 m from the tube, the GM tube records 664 counts per minute.
The student subtracts 24 counts per minute from every reading. Explain why, and give two sources of the radiation responsible for this count.2 marks
A hospital technician handles a sealed gamma source. At a distance of 0.50 m from the source, the gamma intensity is 80 μW m⁻². The source can be treated as a point source in air.
Calculate the intensity of the gamma radiation at a distance of 2.0 m from the source.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).