Physics 6.12 - 6.14 - Background and detection

Background from Earth and space

A detector therefore normally records some events before a test source is brought near it. The background count is the detector reading caused by this environmental radiation during a stated time. Background radiation is the physical radiation; background count is the measured response.

Background is not exactly the same everywhere. It can vary with local geology and building materials, while the cosmic contribution can vary with altitude. A fluctuating GM count is also expected because individual emissions and detections are random; one short reading is therefore a poor estimate of the mean background.

Film and Geiger-Muller detection

We detect radioactivity through the effects of emitted radiation on matter. Photographic film records cumulative exposure; a Geiger–Müller tube gives immediate electrical counts.

Photographic film

Ionising radiation changes radiation-sensitive photographic film. When the film is developed, exposed regions are darker or more fogged; greater accumulated exposure generally produces more darkening. Film can therefore reveal that radiation was present and give a cumulative record, as in a film badge, but it does not provide an immediate sequence of clicks.

Geiger-Muller tube

A Geiger-Muller (GM) tube contains gas and is connected to a high-voltage supply and counter or ratemeter. Radiation entering the tube ionises the gas. The resulting movement of charge produces an electrical pulse, which the counter registers as one count or click.

The GM system can measure the number of detected events in a time interval or display a count rate in counts per second. It does not detect every emission from the source, and a count alone does not identify the radiation's type or energy. The instrument count rate should not be called the source's activity in becquerels here.

Worked example. A GM counter records a mean total of $276\,\mathrm{counts}$ in $90\,\mathrm{s}$ with a source present. With the source stored away, it records a mean background of $42\,\mathrm{counts}$ in the same $90\,\mathrm{s}$.

1. Both counts cover the same time, so subtract the background: $276\,-\,42\,=\,234\,\mathrm{counts}$ due to the source above background.
2. Divide by the counting time: $R=\,234\,\div\,90\,=\,2.6\,\mathrm{counts}\,\mathrm{s}^{-1}$.
3. Report $2.6\,\mathrm{counts}\,\mathrm{s}^{-1}$; extra decimal places would suggest unjustified precision for fluctuating count data.
4. Sense-check: this is positive and lower than the uncorrected rate, $276\,\div\,90\,=\,3.07\,\mathrm{counts}\,\mathrm{s}^{-1}$, as subtracting background requires.