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
- Although most background radiation is natural, a small amount of it comes from artificial sources, such as medical procedures (including X-rays) and nuclear power stations
- The waste materials from these artificial sources are radioactive and some of them will remain radioactive for thousands of years
- Consequently, this waste must be stored and disposed of carefully
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 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
