How do you correct a count rate for background radiation?

Measure the background count rate over a long time with the source removed and subtract it.

What is an alpha particle?

A helium nucleus (2 protons and 2 neutrons), charge +2e.

What is a beta-minus particle?

A fast electron emitted from the nucleus when a neutron changes into a proton.

What is gamma radiation?

A high-energy photon of electromagnetic radiation with no charge.

Which absorber stops alpha radiation?

A sheet of paper (and a few centimetres of air).

Which absorber stops beta-minus radiation?

A few millimetres of aluminium.

How is gamma radiation reduced?

By thick lead, but it is never completely stopped, since its intensity falls exponentially.

How are ionising ability and range related?

The more strongly ionising the radiation, the shorter its range and the less it penetrates.

What changes in a nucleus in alpha decay?

Nucleon number falls by 4 and proton number falls by 2.

What changes in a nucleus in beta-minus decay?

The nucleon number is unchanged and the proton number rises by 1; an electron and an antineutrino are emitted.

Why does count rate fall exponentially as lead thickness increases?

Each equal thickness of lead absorbs the same fraction of the gamma photons, giving a constant half-thickness.

Types of nuclear radiationEdexcel International A Level Physics: Flashcards

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Question

Name three natural sources of background radiation.

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Name three natural sources of background radiation.: Radon gas from rocks, cosmic rays, and radioactive isotopes in rocks, soil, food and drink.
How do you correct a count rate for background radiation?: Measure the background count rate over a long time with the source removed and subtract it.
What is an alpha particle?: A helium nucleus (2 protons and 2 neutrons), charge +2e.
What is a beta-minus particle?: A fast electron emitted from the nucleus when a neutron changes into a proton.
What is gamma radiation?: A high-energy photon of electromagnetic radiation with no charge.
Which absorber stops alpha radiation?: A sheet of paper (and a few centimetres of air).
Which absorber stops beta-minus radiation?: A few millimetres of aluminium.
How is gamma radiation reduced?: By thick lead, but it is never completely stopped, since its intensity falls exponentially.
Which radiation is the most strongly ionising?: Alpha.
How are ionising ability and range related?: The more strongly ionising the radiation, the shorter its range and the less it penetrates.
What changes in a nucleus in alpha decay?: Nucleon number falls by 4 and proton number falls by 2.
What changes in a nucleus in beta-minus decay?: The nucleon number is unchanged and the proton number rises by 1; an electron and an antineutrino are emitted.
Why does count rate fall exponentially as lead thickness increases?: Each equal thickness of lead absorbs the same fraction of the gamma photons, giving a constant half-thickness.

Exam questions on Types of nuclear radiation

A student investigates a radioactive source in a school laboratory using a Geiger–Müller tube connected to a counter. Before bringing the source out of its store, she records 120 counts in 10 minutes. With the source placed 5 cm from the tube she then records 1890 counts in 5.0 minutes.
Explain why the student measures the background count over a long period and what she does with it.2 marks
A technician is testing two unlabelled sources, X and Y, using a Geiger–Müller tube about 2 cm away and subtracting the background count rate each time. For source X, the corrected count rate falls to zero when a single sheet of paper is placed in front of the tube. For source Y, a sheet of paper makes no difference, but the corrected count rate falls to zero when a 4 mm thick aluminium plate is placed in front of the tube.
Explain why the radiation from source X is stopped by paper but the radiation from source Y is not.2 marks
Radium-226 (proton number 88) is a naturally occurring alpha emitter that decays to an isotope of radon (Rn). Carbon-14 (proton number 6) is a beta-minus emitter that is used in radiocarbon dating and decays to an isotope of nitrogen (N).
Complete the nuclear equation for the alpha decay of radium-226: $^{226}_{88}\text{Ra} \rightarrow \ ?\ + \ ?$ 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).