Detecting radiation, background and uses of isotopes

Last time

Before we start: last time, in Alpha, beta and gamma radiation and decay equations, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

A click you cannot hear

A Geiger counter clicks even in an empty room. Nobody has put a source near it. We are always surrounded by a low level of radiation, called background radiation. To measure a source properly, you must be able to detect radiation, and subtract the background. Then you can put radioactive isotopes to work.

Detecting radiation

A Geiger-Müller tube contains gas at low pressure. Radiation entering the tube ionises the gas, and a tiny pulse of current flows. A counter records the pulses, or a loudspeaker clicks for each one. The count rate is the number of counts each second or each minute. It shows how much radiation reaches the tube.

Other detectors

Photographic film goes dark when radiation hits it. Workers who handle radiation wear badges with film in them. A spark counter and a cloud chamber show the tracks of ionising radiation too. All of these rely on ionisation or its effect on film.

Background radiation

Radon gas seeps out of rocks, especially granite, and is the largest natural source. Cosmic rays arrive from space. Food, drink and our own bodies contain tiny amounts of radioactive isotopes such as carbon-14 and potassium. There are artificial sources too: medical X-rays, nuclear waste and fallout from old weapons tests.

Correcting for background

First measure the background count rate with no source present. Then measure the count rate with the source in place. This includes the background. The corrected count rate is the measured rate minus the background rate.