Detection Methods

Counting the radioactive atoms

When harnessed, radioactivity has proven itself to be an unparalleled tool for the exploration of the world around us. The functioning of living bodies and of our environment can all be examined with tremendous accuracy by radiation detection methods.

Sensitive detectors
The extreme sensitivity of radiation detectors is illustrated by this ‘energy spectrum’; the gamma photons emitted by a sample of radioactive matter. Modern germanium-based detectors are capable of measuring these energies with great precision, thus revealing the ‘characteristic rays’ emitted by different elements. Given the extremely high counts that are measured, one can discover the characteristic rays of elements present in very trace quantities, such as caesium 134.

These methods are constantly improving, allowing us to produce equipment sensitive enough to detect the decay of an individual nucleus. The reasons that allow us to detect such an unimaginably small event are twofold.

Firstly, the radiation that is emitted by a decaying nucleus is capable of ‘electrifying’ many of the atoms it passes. Alpha and beta particles are capable, owing to their electrical charge, of direct “ionisation”, while the neutral gamma rays can collide and put in motion charged particles, thus achieving an indirect ionisation. The hundreds of thousands of atoms that are electrified (ionised) as a result of the decaying nucleus serve as an excellent amplifier of the event.

The effects of this primary ionisation are then further amplified by electronics with high gain values, creating a signal large enough to be detected.

A third important amplifying factor is the tremendous quantity of atoms present in a given sample of matter. In 18 grams of water – less than a mouthful – there are over 600 million billion billion molecules (the exact figure, 6.02 x 1023, is known as Avogadro’s number). The sheer size of the sample means that even with a comparatively low proportion of radioactive atoms, the actual number present will be extremely large. The counting rate of the detector, which is directly related to the number of disintegrations per second, will remain high enough to be measurable.

Wire Chamber
This detector, comprised of an array of wires set at very high tensions, allows for accurate detection of the position of the particles which go through. This model is a great improvement on the early Geiger counters, which were only able to indicate whether a particle had gone by or not. The development of the wire chamber got the French physicist Georges Charpak the 1992 Nobel Prize.
The exceedingly high value of Avogadro’s number means that even tiny amounts of radioactive atoms (one part per million billion) can be detected. By comparison, a lethal poison (such as arsenic) requires a concentration billion times more abundant to be detected chemically.

Ironically, while our ability to detect radioactivity is extremely well-developed, it is precisely the fact that we can detect radiation everywhere that has people worried.

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