Cambridge IGCSE Physics 0625 – Detection of Radioactivity: Background Radiation
5.2.1 Detection of Radioactivity
Objective
Know what is meant by background radiation.
What is Background Radiation?
Background radiation is the ionising radiation that is present in the environment even when no artificial radioactive source is nearby. It originates from natural and man‑made sources and is constantly detected by radiation‑measuring instruments.
Natural Sources of Background Radiation
Cosmic rays from outer space.
Terrestrial radiation from radioactive isotopes in the Earth’s crust (e.g., uranium, thorium, radon).
Internal radiation from radionuclides naturally present in the human body (e.g., \$^{40}\$K, \$^{14}\$C).
Residual fallout from nuclear weapons testing and nuclear accidents.
Typical Levels of Background Radiation
Background radiation is usually expressed as an equivalent dose rate. Typical values are:
Source
Typical Dose Rate
Units
Cosmic rays at sea level
0.03 – 0.04
\$µSv \, h^{-1}\$
Terrestrial (soil, rocks)
0.02 – 0.05
\$µSv \, h^{-1}\$
Indoor radon (average home)
0.05 – 0.10
\$µSv \, h^{-1}\$
Average worldwide background
\overline{0}.10
\$µSv \, h^{-1}\$
Why is Background Radiation Important in Experiments?
It contributes to the total count recorded by a detector even when the source under investigation is absent.
Accurate measurements require the background count to be subtracted from the total count to obtain the net activity of the sample.
Understanding background levels helps in assessing the safety of a laboratory environment.
Measuring Background Radiation
To determine the background radiation level:
Set up the detector (e.g., Geiger‑Müller tube, scintillation counter) as you would for any measurement.
Ensure that no known radioactive source is present near the detector.
Record the number of counts for a fixed time interval (commonly 60 s).
Repeat the measurement several times and calculate the average count rate.
Express the result as counts per minute (cpm) or convert to dose rate using the instrument’s calibration factor.
Reducing the Effect of Background Radiation
Use shielding (e.g., lead bricks) around the detector to block external radiation.
Perform measurements in a low‑background environment, such as a basement or a purpose‑built laboratory.
Allow sufficient counting time so that statistical fluctuations in the background become relatively small.
Subtract the measured background count from the total count to obtain the net count from the source.
Suggested diagram: Sketch of a typical radiation detection setup showing the detector, shielding, and a source, with a separate measurement of background radiation (no source present).
Key Points to Remember
Background radiation is ever‑present ionising radiation from natural and man‑made origins.
Typical background dose rates are around \$0.1 \, µSv \, h^{-1}\$.
Always measure and subtract background counts to obtain accurate activity values.
Understanding background radiation is essential for both accurate experimental results and radiation safety.