Detecting Radiation and Background

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 7.9 — Photographic film and the Geiger–Müller detector as detectors of ionising radiation
• 7.10 — The sources of background radiation from Earth and space

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

Radiation cannot be seen, heard, smelt or felt, so every detector works indirectly — by catching something the radiation DOES rather than the radiation itself. The two on this specification catch two different consequences of the same property: ionisation.

PHOTOGRAPHIC FILM was the first detector, and it is the one Becquerel used by accident. Ionising radiation affects a photographic emulsion in the same way light does, so the film darkens when it is developed, and the more radiation it has received the darker it goes. The film has to be kept wrapped in light-proof paper, which changes nothing about the radiation reaching it — that is precisely the point of Becquerel's experiment.

Its great advantage is that it gives a CUMULATIVE record: it adds up everything received over days or weeks. That is why people who work with radiation wear a FILM BADGE, developed at intervals to check the total dose received. Its disadvantage is the mirror image — it tells you nothing at the time, and you have to develop it to find out anything at all.

A GEIGER–MÜLLER TUBE does the opposite job. It is a sealed tube of low-pressure gas with a wire down the middle held at a high voltage. When ionising radiation enters, it ionises some of the gas; the freed electrons are accelerated towards the wire and ionise more atoms on the way, so a single entering particle produces an avalanche and a measurable pulse of current. That pulse is counted, and often made audible as a click.

Think of it like a smoke alarm against a diary

A smoke alarm tells you about a problem at the instant it happens and remembers nothing afterwards; a diary tells you nothing while things are happening but gives a complete record when you sit down and read it. The Geiger tube is the alarm — every particle gets its click, right now, and once counted it is gone. The film badge is the diary — silent all month and then, when developed, an honest total of everything received. Which you want depends on the question. 'Is it safe to walk in there?' needs the alarm. 'How much has this worker been exposed to this year?' needs the diary.

Now the fact that shapes every measurement in this topic. Switch on a Geiger counter in an ordinary room with no source anywhere near it, and it still clicks — a few times a minute, irregularly. This is BACKGROUND RADIATION, and it has always been there. It is not a fault in the counter and it is not contamination; it is the ionising radiation naturally present everywhere on Earth.

Where background radiation comes from — statement 7.10

1. RADON GAS seeping out of rocks, particularly granite. This is the largest single contributor in most of the United Kingdom, and it matters most in poorly ventilated buildings because the gas collects indoors.
2. ROCKS AND SOIL themselves, which contain small amounts of uranium, thorium and potassium-40, and so do the building materials made from them.
3. COSMIC RAYS from space, mostly from the Sun and from beyond the solar system. The atmosphere absorbs much of this, so the dose is higher at altitude and noticeably higher in an aircraft.
4. FOOD AND DRINK, because living things take up naturally radioactive potassium-40 and carbon-14 — every human body is measurably radioactive, and always has been.
5. MEDICAL sources, chiefly X-rays and radiotherapy, which is the largest artificial contribution.
6. Nuclear power and weapons testing contribute a very small proportion of the total — smaller than most people assume, and far smaller than radon.

The practical consequence is that background must be SUBTRACTED from every reading. A source measured at 500 counts per minute in a room with a background of 30 counts per minute is actually producing 470. The subtracted figure is called the CORRECTED COUNT RATE, and forgetting it is the commonest experimental error in this topic — most visibly when an alpha source behind a sheet of paper reads 30 rather than 0, and a learner concludes the paper did not stop it.

Correcting for background, and why it matters more for weak sources

Background is measured at 24 counts per minute. A source gives 624 counts per minute. (a) What is the corrected count rate? (b) A much weaker source gives 30 counts per minute. What is its corrected rate, and what does that show?

1. (a) Corrected count rate = measured − background = 624 − 24 = 600 counts per minute.
2. Background is only 4% of that reading, so ignoring it would give a small error.
3. (b) Corrected count rate = 30 − 24 = 6 counts per minute.
4. Here the background was 80% of what was measured. Ignoring it would have made the source appear five times more active than it is.
5. So the weaker the source, the more the correction matters — which is exactly the situation at the end of an absorption experiment.

Answer: (a) 600 counts per minute. (b) 6 counts per minute — and the correction changed the answer fivefold.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Learn this definition · Background radiation

The ionising radiation naturally present in the environment at all times, from rocks, radon gas, cosmic rays, food and drink, and medical sources. It is registered by a detector even with no source present.

Learn this definition · Corrected count rate

The measured count rate minus the background count rate. Every reading in a radioactivity experiment must be corrected in this way before it is used.

Photographic film against a Geiger–Müller tube

Statement 7.10 — the sources of background radiation

• RADON GAS from rocks, especially granite — the largest single source in most of the UK, and worst in poorly ventilated buildings
• ROCKS AND SOIL, containing uranium, thorium and potassium-40 — and building materials made from them
• COSMIC RAYS from the Sun and beyond — greater at high altitude and in aircraft
• FOOD AND DRINK, through naturally occurring potassium-40 and carbon-14
• MEDICAL sources such as X-rays and radiotherapy — the largest artificial contribution
• Nuclear power and weapons testing — a very small proportion of the total

Taking a corrected reading

1. Measure the count with NO source present, over a long time, and divide to get a background rate.
2. Bring the source to a fixed position and measure the count over the same length of time.
3. Subtract the background rate from the measured rate.
4. Use the corrected rate for every calculation and every graph.
5. Repeat and take a mean, because decay is random and successive counts differ.

Model answer [3 marks]

Explain why a Geiger–Müller tube registers a count rate even when no radioactive source is present. [3]

The reading is due to background radiation, which is the ionising radiation naturally present in the environment at all times. It comes from sources such as radon gas released from rocks, the uranium and potassium-40 in rocks and soil, cosmic rays from space, and naturally radioactive isotopes in food and drink. This radiation ionises the gas in the tube in exactly the same way a source would, so it produces counts, and it must be measured and subtracted from every reading taken with a source.

Model answer [3 marks]

A worker is monitored with a film badge rather than a Geiger–Müller tube. Explain why a film badge is the appropriate detector for this purpose. [3]

A film badge gives a cumulative record, because the darkening of the film builds up in proportion to the total amount of radiation received over the whole period it is worn. This is exactly what is needed for monitoring a worker, since the concern is the total dose accumulated over weeks or months rather than the rate at any one instant. A Geiger–Müller tube gives only the count rate at the moment of measurement and keeps no record, so it could not show how much radiation a worker had received in total.

Not this: Background radiation comes mostly from nuclear power stations and weapons testing.

This: Those contribute a very small proportion of the total. The largest single source in most of the UK is RADON GAS seeping naturally out of rocks, and background radiation existed long before anyone built a reactor.

Mark-losing trap. ALWAYS subtract the background before using a reading. Corrected = measured − background.

Mark-losing trap. An alpha source behind paper reads BACKGROUND, not zero — that is a pass, not a failure.

Mark-losing trap. Film gives a TOTAL after developing; a Geiger tube gives a RATE right now. Match the detector to the question.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · Calculate [2 marks] — A Geiger counter records 380 counts per minute near a source. The background rate is 25 counts per minute. What is the corrected count rate?
2. Grade 7 · Identify [1 mark] — Which of these is the largest single contributor to background radiation in most of the United Kingdom?
3. Grade 8 · Explain [3 marks] — A student places an alpha source behind a sheet of paper and records 28 counts per minute. The background rate is 27 counts per minute. They conclude that the paper failed to stop the alpha radiation. Explain whether they are correct.
4. Grade 9 · Compare [6 marks] — Select every statement that belongs in a full-mark comparison of photographic film and a Geiger–Müller tube as detectors of ionising radiation.
5. 9+ · Analyse [6 marks] — Two students measure the same weak source. One counts for 10 seconds and finds 9 counts, giving 54 counts per minute. The other counts for 10 minutes and finds 486 counts, giving 48.6 counts per minute. The background rate is 24 counts per minute. They disagree about the source's activity. Select every statement that belongs in a full-mark analysis.

The people behind this science

Two ways into the same idea — the one who measured radiation before anyone knew what it was, and the one whose laboratory counted flashes by eye in the dark. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Marie Curie — the one who measured radiation before anyone knew what it was

Curie's decisive work depended on measuring, not on detecting, and the instrument matters. She used an electrometer built by her husband Pierre and his brother, which measured how well the radiation made air CONDUCT — the same ionisation this page's detectors rely on, quantified rather than merely noticed. That let her compare compounds numerically, and the numbers are what revealed that pitchblende was more active than its uranium content could explain, which is what sent her through tonnes of ore to radium. She is also the person to ask about the cost: her notebooks are still too radioactive to handle without precautions.

• “How did you measure radiation before there were counters?”
• “What did making the air conduct tell you?”
• “How did a number in a table lead you to a new element?”
• “Did you know the danger you were working in?”
• “Why are your notebooks still radioactive?”

Ernest Rutherford — the one whose laboratory counted flashes by eye in the dark

Before the Geiger–Müller tube, single alpha particles were detected by letting them strike a zinc sulphide screen and counting the faint flashes through a microscope, by eye, in a completely darkened room after half an hour of letting the eyes adjust. Observers could manage only a few minutes at a time before their counting became unreliable. Hans Geiger did enormous amounts of this in Rutherford's laboratory, and his later development of an electrical counter was a direct response to how bad a detector a tired human being is. Rutherford is the person to ask how you trust a measurement whose instrument is somebody's eye.

• “How did you actually count individual particles?”
• “How do you know the counting was reliable?”
• “Why did Geiger want to build an electrical counter?”
• “What could go wrong with counting flashes by eye?”
• “How long could anyone do that work for?”

Then defend it

On Incandio a lesson is not finished when the questions come out right. You teach the idea back to Ember, an AI apprentice who asks the awkward question, and then you argue it against Henri Becquerel in a structured debate marked against descriptors you can read before you enter. Learn it, teach it, then defend it — all three happen on this page once the app loads.

Carry on through the course

• Previous lesson: Nuclear Equations
• Next lesson: Half-Life
• Atomic Structure and Isotopes — A nucleus ten thousand times smaller than the atom around it, and why two atoms of the same element can weigh different amounts.
• Alpha, Beta and Gamma — Three emissions from unstable nuclei, arriving at random — and why the one that is easiest to stop is the one to be most careful with.
• Nuclear Equations — What each kind of decay does to the numbers — and why losing a negative particle makes the charge go up.
• Half-Life — Half of what is left, every time — which is why a radioactive sample never quite runs out.
• Uses and Dangers of Radiation — Choosing a source by its half-life and its penetration — and the difference between being irradiated and being contaminated.
• All of Physics · Incandio Science