Background Radiation (Oxford AQA IGCSE Physics)
Revision Note
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
What is background radiation?
Background radiation is defined as:
The radiation that exists around us all the time
Sources of background radiation
There are two types of background radiation:
Natural sources
Man-made sources
Composition of background radiation
Every second of the day there is some radiation emanating from natural sources within our environment and from space, such as:
Rocks
Cosmic rays from space
Foods
Man-made sources of radiation increase the background radiation levels, examples include:
Fallout from nuclear weapons testing and nuclear accidents
Exposure to medical testing and devices, e.g. X-ray machines
Corrected count rate
Background radiation must be accounted for when taking readings in a laboratory
This can be done by taking readings with no radioactive source present to find the background radiation and then subtracting this from readings with the source present
This is known as the corrected count rate
Variation in background radiation
The level of background radiation varies across the planet and depends on:
Altitude
Geology of the rocks
Proximity to nuclear power stations
Use of nuclear weapons
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
Levels of radon gas are generally very low and are not a health concern, but they can vary significantly from place to place
For example, some areas may contain rocks and soil which emit higher concentrations of radon gas, such as Wales and Cornwall
Radon concentration map of the UK
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 Chernobyl, 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
Worked Example
A student is using 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 here.
Results Table
Distance from Source (m) | Count Rate (counts per minute) |
---|---|
0.2 | 180 |
0.4 | 67 |
0.6 | 29 |
0.8 | 17 |
1.0 | 15 |
1.2 | 15 |
1.4 | 15 |
Graph
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
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