Background radiation

Background radiation is ionising radiation that is always present in the environment, even when no particular radioactive source is being investigated. Ionising radiation can remove electrons from atoms, forming ions.

Background radiation comes from several sources:

• Radon gas in the air, which is released from the ground.
• Radioactive substances in rocks and building materials, which emit radiation as they decay.
• Radioactive substances in food and drink, which can enter the body when consumed.
• Cosmic rays, which reach Earth from space.

The level of background radiation varies with location and circumstances. For example, areas with different rocks can release different amounts of radon, and radon can build up inside buildings. Exposure to cosmic rays is generally greater at higher altitudes.

Radiation detection and count rate

Ionising nuclear radiation can be measured using a detector connected to a counter. When radiation reaches the detector, the detector responds and the counter records a count. The number of counts is recorded over a measured time interval.

Calculating count rate

The count rate is the number of counts recorded per unit time:

If time is measured in seconds, the count rate is given in counts per second. If time is measured in minutes, it is given in counts per minute.

For example, if a counter records $$120$$ counts in $$60$$ seconds, the count rate is $$120 \div 60 = 2$$ counts per second. This is also $$120$$ counts per minute.

Background radiation and counts

A detector can also record counts caused by background radiation: ionising radiation present in the surroundings. Counts recorded while measuring a source can therefore include counts from background radiation as well as from the source.

Background-corrected count rate

A detector records background radiation as well as radiation from a source. The count rate measured with the source present therefore includes counts from both. To estimate the source’s count rate alone, first measure the background count rate without the source, then subtract it from the rate measured with the source present.

Both rates must use the same units, such as counts per second or counts per minute. For example, if the measured rate is 50 counts per minute and the background rate is 20 counts per minute, the corrected rate is 30 counts per minute.

If the detector readings are given as counts rather than count rates, subtract them directly only if they were recorded over equal time intervals. Otherwise, convert each reading to a count rate in the same units before subtracting.