Detection of Ionising Radiation

Ionisation: the key to detection

Alpha, beta and gamma radiation all carry enough energy to knock electrons out of atoms. An atom that loses an electron becomes a positive ion, and the electron is set free. This is called ionisation. Every detector in this section works by spotting the ions and electrons that radiation leaves behind.

Alpha particles ionise most strongly (large charge of +2e, large mass and slow, so they ionise very densely and are stopped within a very short path). Gamma rays ionise least but penetrate furthest.

Three detectors

The Geiger-Müller (GM) tube

A GM tube is a sealed metal tube filled with a low-pressure gas, with a thin wire (the anode) along its axis at a high positive voltage relative to the wall. A thin mica window at one end lets alpha (and beta) particles in.

When radiation enters, it ionises the gas. The electrons are pulled strongly towards the anode and gain so much energy that they ionise more atoms, causing an avalanche of charge. This gives a short pulse of current for each particle. A counter or ratemeter (or a loudspeaker that clicks) records the pulses.

So the GM tube detects each ionising particle as one pulse.

The cloud chamber

A cloud chamber contains air saturated with alcohol vapour, cooled so that it is supersaturated. When a charged particle passes through, it ionises the air. The vapour condenses on the ions, forming a thin line of tiny droplets: a visible track.

• Alpha: short, thick, straight tracks (dense ionisation).

• Beta: thin, longer, often wiggly tracks.

• Gamma: very faint tracks, produced only by electrons it knocks free.

The gold-leaf electroscope

A charged electroscope has its leaf held away from the rod by repulsion. Ionising radiation near the cap makes the air conductive: ions of opposite sign are attracted to the cap and neutralise its charge. The leaf falls as the charge leaks away. The faster it falls, the more intense the radiation.

Background radiation

Even with no source nearby, a Geiger counter clicks. This is background radiation: ionising radiation that is always present in the environment.

Radon is a radioactive gas produced in the decay of uranium in rock. It is an alpha emitter. Ireland's average annual dose from background is about 4 mSv, and radon makes up more than half of it. Levels are higher in parts of Galway, Mayo, Clare and Wicklow because of the underlying granite and limestone.

Counting and correcting for background

The count rate is the number of counts divided by the time taken:

Because background is always present, we measure it first with no source, and subtract it from every reading with the source:

Distance and the inverse-square law

For a small source that emits equally in all directions, the radiation spreads over the surface of a sphere of area $4\pi d^2$. So the count rate (corrected) is proportional to $1/d^2$. Doubling the distance cuts the corrected count rate to a quarter.

Random fluctuations

Radioactive decay is random: we cannot say when a given nucleus will decay, only the probability. So repeated counts over equal times differ even when the source is unchanged. Two causes of fluctuation: the random nature of decay, and the random nature of background.

Counting for longer reduces the percentage uncertainty. If a count is $N$, the typical fluctuation is about $\sqrt{N}$, so the percentage uncertainty is about $\sqrt{N}/N = 1/\sqrt{N}$, which falls as $N$ increases.

Experiment: measuring background

Set up the GM tube with no source in the room (or all sources in a lead container far away). Record the counts in equal periods, e.g. six readings of 60 s each. Find the mean and the range. Repeating the readings and averaging improves reliability, as does counting for longer.

Identifying radiation with absorbers

Different radiations are stopped by different materials. Place absorbers one at a time between the source and the GM tube and watch the corrected count rate.

Dose and radiation protection

The dose measures the effect of radiation on the body, in sieverts (Sv); everyday values are in millisieverts (mSv). A radiographer wears a film badge or dosimeter to record the dose received over a month. An annual limit applies to those who work with radiation. This occupational limit (20 mSv per year) is an extra dose in addition to natural background, which is about 4 mSv per year in Ireland, so the extra allowed dose is about five times the background dose.

The three principles of protection

These principles follow the ALARA idea: doses should be kept as low as reasonably achievable, even when they are below the legal limit.

• Time: spend as little time as possible near a source. Example: a nurse leaves the room during a scan.

• Distance: stay as far away as possible. Radiation falls with the inverse square of the distance, so doubling it cuts the rate to a quarter. Example: handle sources with tongs.

• Shielding: put absorbing material in the way. Example: lead aprons or lead-lined walls in an X-ray room.

Background radiation can be shown to exist with a GM tube and counter and no source present: repeated equal-time counts always give a non-zero mean. Other detectors, such as an electroscope or a cloud chamber, show it too. Background radiation matters when judging risk: it is a baseline everyone receives, so any extra dose from a hospital or workplace should be compared with it, and it must be subtracted when measuring a source.

Vuelve al principio: a Geiger counter clicks with no source because it detects ionising background radiation, mostly from radon, and an electroscope or a radon detector works the same way: by the ions radiation leaves behind.

Summary

• Ionising radiation removes electrons from atoms; all detectors rely on the resulting ions.

• A GM tube gives a pulse per particle, a cloud chamber shows tracks of condensed vapour, a gold-leaf electroscope discharges, and a film badge records total dose.

• Background radiation (mostly radon in Ireland, about 4 mSv per year) is always present: measure it and subtract it to get the corrected count rate.

• Counts fluctuate randomly; counting longer reduces the percentage uncertainty (about $1/\sqrt{N}$).

• Absorbers identify radiation: paper stops alpha, aluminium stops beta, lead reduces gamma.

• Protect yourself with time, distance and shielding.