Detecting Radioactivity
 
 

 

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Radioactivity is invisible, has no smell, makes no sound - in fact it cannot be detected by any of our senses.

However, because radioactivity affects the atoms that it passes, we can easily monitor it using a variety of methods:

Geiger-Müller tube  
Photographic Film
Gold Leaf Electroscope 
Spark Counter   
Cloud Chamber 
Bubble Chamber
Modern Detectors

Geiger-Müller tube ("GM tube")

Most people have heard of a "Geiger Counter" for measuring radioactivity. This is actually a Geiger-Müller tube with some form of counter attached, which usually tells us the number of particles detected per minute ("counts per minute").

GM tubes work using the ionising effect of radioactivity. This means that they are best at detecting alpha particles, because -particles ionise strongly.

Different models of GM tubes are available for
detecting , and radiation.

Geiger-Muller tube and counter
How it Works How a Geiger-Muller tube works

You can see how the tube works in the animation.
The tube is filled with Argon gas, and around +400 Volts is applied to the thin wire in the middle. When a particle enters the tube, it pulls an electron from an Argon atom. The electron is attracted to the central wire, and as it rushes towards the wire, the electron will knock other electrons from Argon atoms, causing an "avalanche". Thus one single incoming particle will cause many electrons to arrive at the wire, creating a pulse which can be amplified and counted. This gives us a very sensitive detector.


Photographic Film

In 1896, Henri Becquerel, working in Paris, discovered that Uranium compounds would darken a photographic plate, even if the plate were wrapped up so that no light could get in.

Radioactivity will darken ("fog") photographic film, and we can use this effect to measure how much radiation has struck the film.

Workers in the nuclear industry wear "film badges" which are sent to a laboratory to be developed, just like your photographs. This allows us to measure the dose that each worker has received (usually each month).

Film badge

The badges have "windows" made of different materials, so that we can see how much of the radiation was alpha particles, or beta particles, or gamma rays.


The Gold Leaf Electroscope
 The electroscope leaf falls

Dry air is normally a good insulator, so a charged electroscope will stay that way, as the charge cannot escape.

When an electroscope is charged, the gold leaf sticks out, because the charges on the gold repel the charges on the metal stalk.

When a radioactive source comes near, the air is ionised, and starts to conduct electricity. This means that the charge can "leak" away, the electroscope discharges and the gold leaf falls.


The Spark Counter

An early form of detector, the Spark Counter is another instrument that uses the ionising effect of radioactivity, and for this reason it works best with particles.

A high voltage is applied between the gauze and the wire, and adjusted until it is just below the voltage required to produce sparks.

When a radioactive source is brought near, the air between the gauze and the wire is ionised, and sparks jump where particles pass.

 Spark Counter

The Cloud Chamber


There are two types of cloud chamber: the "expansion" type and the "diffusion" type.

In both types, alpha or beta particles leave trails in the vapour in the chamber, rather like high-altitude aircraft leave trails in the sky.

The chamber contains a supersaturated vapour (e.g. methylated spirits), which condenses into droplets when disturbed and ionised by the passage of a particle (alpha particles are best for this).

You can clearly see the direction and energy of the particles (low energy particles only leave short trails).
Occasionally, a particle collides with an air molecule and changes direction.



A cloud chamber shows the randomness of radioactive emissions clearly.

 

Expansion cloud chambers use a vacuum pump to briefly produce the right conditions for trails to form, whilst the Diffusion type shown in the video uses solid Carbon Dioxide to cool the bottom of the chamber and produce a temperature gradient in which trails can be seen continuously.

Video Clip: Diffusion Cloud Chamber



The Bubble Chamber

A similar idea to an expansion cloud chamber, particles leave trails of tiny bubbles in a liquid. This used to be the main instrument for tracking the results of collisions in particle accelerators.


The chamber would be surrounded by powerful magnets, so any charged particles passing though the chamber would move in curved paths. The shapes of the curves tell us about the charge, mass and speed of each particle, so we can work out what they are - otherwise one line of bubbles looks pretty much like another.


Particle tracks in a bubble chamber


Technicians at CERN working on a bubble chamber in 1970

Modern Detectors

Modern instruments are much more sensitive than the others listed on this page.

"Scintillation Detectors" work by the radiation striking a suitable material (such as Sodium Iodide), and producing a tiny flash of light. This is amplified by a "photomultiplier tube" which results in a burst of electrons large enough to be detected. Scintillation detectors form the basis of the hand-held instruments used to monitor contamination in nuclear power stations. They can recognise the difference between a, b and g radiation, and make different noises (such as bleeps or clicks) accordingly.

"Solid-State Detectors" are the most up-to-date instruments. They are used in particle-accelerator laboratories to show the results of high-energy collisions, with banks of them clustered around the collision site, feeding data into huge computers. The way they work is way beyond what we need for GCSEs, but basically they are similar to the CCD Silicon chips used in video cameras.


Find out about the units we use to measure radioactivity:

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