Detecting Radiation (Edexcel IGCSE Physics)
Revision Note
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
Examiner Tips and Tricks
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
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
Examiner Tips and Tricks
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!
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