Measuring amounts of radiation
Radioactivity can be detected using a Geiger-Muller tube connected to a counter.
When alpha particles, beta particles or gamma rays enter the GM-tube the counter clicks and the count is displayed on the screen.
The number of counts per second or per minute is called the count rate or activity of the source.
The simplest unit of activity is the becquerel (Bq).
It is the number of counts per second.
A source that emits one particle per second has an activity of one Bq.
Since radioactive decay is a random process, it is always good practice to determine the average count rate rather than to measure the counts that occur in just one second or one minute.
Background radiation
Radioactive materials occur naturally and, as a result, everyone is exposed to a low-level of radiation every day.
Background radiation is the name given to the radiation that is always present in our surroundings.
It is released by soil, rocks and cosmic rays and is always in the environment.
Most radioactive background activity comes from natural sources such as:
- Carbon-14, found in carbon dioxide in the air and in the cells of all living organisms.
- Soils and rocks containing uranium which is radioactive. These may be used for building materials. When uranium decays, radon, a radioactive gas, is released.
- Cosmic rays - radiation reaching the Earth from outer space.
Human behaviour adds slightly to the background activity that we are exposed to through medical X-rays, radioactive waste from nuclear power plants and the radioactive fallout from nuclear weapons testing.
As a result, gas, living things and plants absorb radioactive materials from the soil, which are then passed along the food chain.
For example, by eating a banana which contains radioactive potassium.
As it passes along the food chain the concentration of radioactivity will increase.
The actual amount of radiation that a person is exposed to depends on where they live, what job they do and many other things.
There is little we can do about natural background radiation, although people who live in areas with a high background due to radon gas require homes to be well ventilated to remove the gas.
Measuring the background radiation
- Remove all known sources of radioactivity from the room.
- Set the counter to zero.
- Switch on and start a stop clock.
- After 20 minutes switch off. Record the count.
- Divide the count by 20 to calculate the count rate per minute.
The background count rate is measured over a period of 20 minutes because of the random nature of radioactive decay.
Dividing by 20 enables the average count rate per minute to be determined.
Background count rate is typically 18 counts per minute which does not present a serious health risk to humans.