Detecting Radiation (Edexcel IGCSE Science)

Spec point — 8.9 · know that photographic film or a Geiger−Müller detector can detect ionising radiations

Detecting radiation

• Ionising radiation can be detected using

  • photographic film
  • a Geiger–Müller tube

Photographic film

• Photographic films detect radiation by becoming darker when it absorbs radiation, similar to when it absorbs visible light

  • The more radiation the film absorbs, the darker it is when it is developed

• People who work with radiation, such as radiographers, wear film badges which are checked regularly to monitor the levels of radiation absorbed

• To get an accurate measure of the dose received, the badge contains different materials that the radiation must penetrate to reach the film

  • These materials may include aluminium, copper, paper, lead and plastic

• The diagram shows what a typical radiation badge looks like:

A badge containing photographic film can be used to monitor a person’s exposure to radiation

Geiger-Müller tube

• The Geiger-Müller tube is the most common device used to measure and detect radiation

• Each time it absorbs radiation, it transmits an electrical pulse to a counting machine

• This makes a clicking sound or displays the count rate

• The greater the frequency of clicks, or the higher the count rate, the more radiation the Geiger-Müller tube is absorbing

  • Therefore, it matters how close the tube is to the radiation source
  • The further away from the source, the lower the count rate detected

A Geiger-Müller tube (or Geiger counter) is a common type of radiation detector

Worked Example A Geiger-Müller tube is used to detect radiation in a particular location. If it counts 16,000 decays in 1 hour, what is the count rate?

Answer:

Step 1: Identify the different variables

• The number of decays is 16 000
• The time is 1 hour

Step 2: Determine the time period in seconds

• 1 hour is equal to 60 minutes, and 1 minute is equal to 60 seconds

Time period = 1 × 60 × 60 = 3600 seconds

Step 3: Divide the total counts by the time period in seconds

Counts ÷ Time period = 16 000 ÷ 3600 = 4.5

• Therefore, it detects 4.5 decays per second

Exam Hint If asked to name a device for detecting radiation, the Geiger-Müller tube is a good example to give. You can also refer to it as a GM tube, a GM detector, GM counter, Geiger counter etc. (The examiners will allow some level of misspelling, providing it is readable). Don’t, however, refer to it as a ‘radiation detector’ as this is too vague and may simply restate what was asked for in the question.

Background radiation

Spec point — 8.10 · explain the sources of background (ionising) radiation from Earth and space

• It is important to remember that radiation is a natural phenomenon

• Radioactive elements have always existed on Earth and in outer space

• However, human activity has added to the amount of radiation that humans are exposed to on Earth

• Background radiation is defined as:

The radiation that exists around us all the time

• Every second of the day there is some radiation emanating from natural sources such as:

  • Rocks
  • Cosmic rays from space
  • Foods

Chart of Background Radiation Sources

Background radiation is the radiation that is present all around in the environment. Radon gas is given off from some types of rock

• There are two types of background radiation:

  • Natural sources
  • Artificial (man-made) sources

Natural Sources of Background Radiation

Radon gas from rocks and buildings

• Airborne radon gas comes from rocks in the ground, as well as building materials e.g. stone and brick

• This is due to the presence of radioactive elements, such as uranium, which occur naturally in small amounts in all rocks and soils

  • Uranium decays into radon gas, which is an alpha emitter
  • This is particularly dangerous if inhaled into the lungs in large quantities

• Radon gas is tasteless, colourless and odourless so it can only be detected using a Geiger counter

• Levels of radon gas are generally very low and are not a health concern, but they can vary significantly from place to place

Cosmic rays from space

• The sun emits an enormous number of protons every second
• Some of these enter the Earth’s atmosphere at high speeds
• When they collide with molecules in the air, this leads to the production of gamma radiation
• Other sources of cosmic rays are supernovae and other high energy cosmic events

Carbon-14 in biological material

• All organic matter contains a tiny amount of carbon-14
• Living plants and animals constantly replace the supply of carbon in their systems hence the amount of carbon-14 in the system stays almost constant

Radioactive material in food and drink

• Naturally occurring radioactive elements can get into food and water since they are in contact with rocks and soil containing these elements
• Some foods contain higher amounts such as potassium-40 in bananas
• However, the amount of radioactive material is minuscule and is not a cause for concern

Artificial Sources of Background Radiation

Nuclear medicine

• In medical settings, nuclear radiation is utilised all the time
• For example, X-rays, CT scans, radioactive tracers, and radiation therapy all use radiation

Nuclear waste

• While nuclear waste itself does not contribute much to background radiation, it can be dangerous for the people handling it

Nuclear fallout from nuclear weapons

• Fallout is the residue radioactive material that is thrown into the air after a nuclear explosion, such as the bomb that exploded at Hiroshima
• While the amount of fallout in the environment is presently very low, it would increase significantly in areas where nuclear weapons are tested

Nuclear accidents

• Nuclear accidents, such as the incident at Chornobyl, contribute a large dose of radiation to the environment
• While these accidents are now extremely rare, they can be catastrophic and render areas devastated for centuries

Accounting for background radiation

• Background radiation must be accounted for when taking readings in a laboratory
• This can be done by taking readings with no radioactive source present and then subtracting this from readings with the source present
• This is known as the corrected count rate

Measuring background count rate

The background count rate can be measured using a Geiger-Müller (GM) tube with no source present

• For example, if a Geiger counter records 24 counts in 1 minute when no source is present, the background radiation count rate would be:

  • 24 counts per minute(cpm)
  • 24/60 = 0.4 counts per second(cps)

Measuring the corrected count rate of a source

The corrected count rate can be determined by measuring the count rate of a source and subtracting the background count rate

• Then, if the Geiger counter records, for example, 285 counts in 1 minute when a source is present, the corrected count rate would be:

  • 285 − 24 = 261 counts per minute(cpm)
  • 261/60 = 4.35 counts per second(cps)

• When measuring count rates, the accuracy of results can be improved by:

  • Repeating readings and taking averages
  • Taking readings over a long period of time

Worked Example A student uses a Geiger counter to measure the counts per minute at different distances from a source of radiation. Their results and a graph of the results are shown below.

Determine the background radiation count.

Answer:

Step 1: Determine the point at which the source radiation stops being detected

• The background radiation is the amount of radiation received all the time
• When the source is moved back far enough it is all absorbed by the air before reaching the Geiger counter
• Results after 1 metre do not change
• Therefore, the amount after 1 metre is only due to background radiation

Step 2: State the background radiation count

• The background radiation count is 15 counts per minute

Exam Hint The sources that make the most significant contribution are the natural sources:

• Radon gas from rocks and buildings
• Food and drink
• Cosmic rays

Make sure you remember these for your exam!