GCSE · Physics · Edexcel · Spec 1PH0

Detecting radioactivity (photographic film, GM tube)

In 1896 a wrapped lump of uranium ore left its own picture on photographic film. Radiation is invisible, so how did it give itself away?

Two detectors, one job

Film or Geiger counter?

Radiation is invisible, so we need detectors. Tick the detector (or both) that each feature belongs to. Go with your gut first, then check.

  • A Photographic film
  • B Geiger counter
  1. Detects invisible radiation
  2. Darkens where radiation reaches it
  3. Clicks for each individual bit of radiation and records the total
  4. Was how Becquerel first spotted radiation in 1896
  5. Can show which of two rocks is more radioactive
  6. Clicks even with no source nearby, because of background radiation

Becquerel’s surprise, 1896

Commit to a prediction first. Then see what he actually found.

Henri Becquerel stored a wrapped sample of uranium ore on photographic film. What do you think the film showed?

Where background radiation comes from

Natural or artificial?

Pick a source, then pick where it belongs. Natural, or artificial?

Still to sort

Natural (0)

Part of the world around us.

Where the line is: Natural sources are there without any human activity causing them.

Artificial (0)

Down to something people do or build.

Where the line is: Artificial sources come from human activity, such as medical procedures, weapons testing and power generation.

7 of 7 still to sort.

Background radiation is caused by unstable nuclei that are around us all of the time. But where do they come from?

Which idea is closest to yours?

Why did it click?

A Geiger counter is switched on. There’s no obvious radioactive source nearby. It clicks occasionally.

Which of these is closest to what you think right now?
How sure are you?

Which rock is more radioactive?

Step through it. At each step, you’ll see what to do with film and with a Geiger counter. You only need one detector.

  1. Choose a detectorEither photographic film or a Geiger counter can be used to test which of two rocks is more radioactive.
  2. Set up a fair testFilm: put each rock near its own piece of film, kept well apart so each film is affected by only one sample. Geiger counter: plan to count the clicks near each rock for the same fixed time.
  3. Collect the evidenceFilm: each film is exposed to its own rock. Geiger counter: count the clicks near each rock for that same fixed time.
  4. CompareFilm: compare the darkening of the two films. Geiger counter: the counter produces more clicks in a fixed time near the rock that emits more radiation, and that rock is the more radioactive one.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Film darkens. A Geiger counter clicks. And sometimes it clicks when nothing is there.

What you need to know

  • Radioactive substances give out radiation: tiny particles or high energy electromagnetic waves from unstable atoms.
  • Radiation is invisible to the naked eye, can darken photographic film, and can pass through some solid materials such as thick paper.
  • Have a goYour mate Sam says: “If that rock were radioactive, I’d see it glowing.” Using what you just read, what’s wrong with that?

    Radiation is invisible to the naked eye, so you can’t see it.

    Not being able to see radiation is exactly why we need detectors like photographic film.

  • In 1896, Henri Becquerel found that a wrapped sample of uranium ore had left a matching image on photographic film.
  • A Geiger counter (a GM tube and counter) makes a click for each bit of radiation and records the total.
  • Have a goA Geiger counter makes 8 clicks in one go. What does each click stand for, and what else does the counter do?

    Each click is one bit of radiation, and the counter records the total number.

    A Geiger counter clicks once for each individual bit of radiation and keeps a record of how many there were.

  • Switched on with no obvious source nearby, a Geiger counter still clicks occasionally. That’s background radiation.
  • Background radiation is caused by unstable nuclei around us all the time, so it’s present just about everywhere.
  • Natural sources: rocks (igneous rocks often contain more radioactive atoms), cosmic rays from the Sun and other stars, food and radon gas.
  • Have a goYou pick up an igneous rock and a rock of another type. Which would you guess is more radioactive, and is that guess guaranteed?

    The igneous rock, but it isn’t guaranteed.

    Igneous rocks often contain more radioactive atoms than other rock types, which makes it a good guess rather than a certainty.

  • Artificial sources: medical X-rays and radiotherapy, nuclear weapons testing and nuclear power.
  • Levels differ by place: usually higher where there’s a lot of igneous rock, and on mountain tops, where cosmic rays are stronger.
  • To compare two rocks, use film kept well apart, or count Geiger counter clicks for the same fixed time.

The big picture

Radiation can’t be seen, but it can be detected. It darkens photographic film, and a Geiger counter (a GM tube and counter) clicks for each bit of radiation it detects and records the total. A switched-on counter still clicks occasionally with no obvious source nearby, because background radiation is present just about everywhere, from natural and artificial sources, at levels that differ from place to place. Either detector can be used to compare which of two rocks is more radioactive, as long as the test is fair.

Key points

1Radiation is invisible, but it darkens photographic film, and a Geiger counter clicks for each bit of it.
2A Geiger counter can click with no source nearby, because background radiation is present just about everywhere.
3Background radiation comes from natural and artificial sources, and its level differs between places.
4To compare two rocks fairly, keep each film affected by only one sample, or use the same fixed time on the Geiger counter.

Worked example

Problem

Maya switches on a Geiger counter in an empty classroom. There is no radioactive source anywhere near it, but it clicks now and then. Explain why.

⚠ Watch out

Thinking that any click means a radioactive source is close by. A Geiger counter still clicks occasionally with no obvious source nearby, because background radiation is present just about everywhere, from natural as well as artificial sources.

🧠

Memory hook

Film darkens, counter clicks, and the background is everywhere. Natural and artificial, high on mountains and igneous rock.

✓

Check yourself

Cover the page: what does film do, what does a Geiger counter do, and why might the counter click with nothing radioactive nearby?

Flashcards

(13)
What is radiation (in this context)?
Tiny particles or high energy electromagnetic waves emitted from unstable atoms. Substances that emit it are called radioactive substances.
Name three things radiation can do or be.
It is invisible to the naked eye, it can expose (darken) photographic film, and it can pass through some solid materials such as thick paper.
What did Becquerel find in 1896?
A wrapped sample of uranium ore stored on photographic film produced an exposed image matching the sample. He concluded the uranium gave off invisible rays that passed through the wrapping.
How does photographic film detect radiation?
It darkens when it is exposed to radiation.
What is a Geiger counter made of, and what does it do?
A GM tube and a counter. It detects individual bits of radiation, makes a click for each one and records the total number.
Which detector counts individual bits of radiation: film or Geiger counter?
The Geiger counter. Film darkens where radiation reaches it, but a Geiger counter clicks for each bit.
What is background radiation?
Radiation caused by unstable nuclei that are around us all of the time. It is present just about everywhere, from natural and artificial sources.
Name four natural sources of background radiation.
Rocks, cosmic rays from the Sun and other stars, food and radon gas.
Name three artificial sources of background radiation.
Medical procedures (X-rays and radiotherapy), nuclear weapons testing and nuclear power.
Which type of rock often contains more radioactive atoms than other rock types?
Igneous rock.
Where is background radiation usually higher?
In areas with a lot of igneous rock and at the tops of mountains, where there is more exposure to cosmic rays. An airline pilot is exposed to higher levels than someone at ground level.
How can you use film to test which of two rocks is more radioactive?
Put each rock near its own piece of film, kept well apart so each film is affected by only one sample, then compare the darkening.
How can you use a Geiger counter to test which of two rocks is more radioactive?
Count the clicks near each rock for the same fixed time. It produces more clicks near the rock that emits more radiation.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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