Radiation detectors

The first device that comes to mind as a radiation detector (for use in schools) is probably the Geiger-Mueller (GM) tube. Although GM tubes can be used to detect all three types of ionising nuclear radiation (alpha, beta and gamma) they are subject to some important limitations. These shortcomings will be discussed below, together with other techniques for detecting particular types of ionising radiation.

Alpha radiation has very low penetrating power so the GM tube must have an end window that is transparent to alpha particles. Mica is used as it has low density and an appropriate level of mechanical strength. It is also impervious to gas molecules, even across very thin sheets, and provides a leak-proof seal.

But the transparency of the window is not the only consideration when attempting to use a GM tube to measure alpha radiation. Equally important is the fact that air absorbs alpha particles very quickly, so the GM tube must be held very close to the radiation source; probably within 10 cm and certainly no more than 20 cm away.

Beta radiation is more penetrating so can be detected at a greater distance from the source. There is a general rule of thumb that states the range of beta radiation in air is 3.7 m per MeV of particle energy. This means a 1 MeV beta particle (high-speed electron) could be detected by a GM tube positioned up to 3.7 m from the source.

That said, beta particles are released from a nuclear decay event with a spectrum of energies, not a single value. The distribution of beta energies is such that the peak (most common) energy is approximately one-third of the maximum value. Therefore, we would expect the highest count rate to occur at a distance that is about one-third of the predicted maximum range for beta penetration through the air.

To summarise so far: GM tubes, fitted with a mica window, can be used to detect alpha radiation when positioned very close to the source (within 10 – 20 cm) and will detect beta radiation at greater distances (let’s say 1 – 2 m, or maybe slightly further).

Unfortunately, alpha and beta radiation produce the same pulse signal in a GM tube so at very close range it is impossible to be certain about which type of radiation is being detected. The simplest way to resolve this problem is to place a shield (thin sheet of card) in front of the GM tube’s window. If the new reading is approximately the same as the original reading then it is beta radiation that is being detected; conversely, if there is a significant drop in the count rate then the source must be emitting primarily alpha radiation.

Gamma radiation can be detected using a GM tube that is constructed as a closed steel cylinder, without an end window. Unfortunately, because of its low ionising power, gamma radiation is not easily detected in the gas directly and the pulses recorded are mostly ionisation events due to gamma photons knocking electrons out of atoms in the steel housing.

GM tubes are inefficient detectors of gamma radiation as most photons pass straight through the steel walls and the gas inside. Nevertheless, the counter can be calibrated to determine overall gamma levels despite the GM tube detecting only a subset of the incident radiation.

It is also worth noting that some GM tubes are not “tubes” at all: instead they are constructed using a pancake design that can be sized according to the application’s requirements. Mr Tarrant owns Gamma Master II dosimeter watch containing a miniature GM “tube”, which you may have seen if you have attended his classes. A photograph of the watch is shown below and more details about this (now obsolete) device are available at https://www.berkeleynucleonics.com/model-pm1208m/.

Other detectors that can be used to detect and measure ionising radiation vary from the ultra-sophisticated (and expensive) down to the very simple (and cheap). Generally speaking, beta radiation is pretty well covered by GM tubes so only gamma and alpha radiation require further thought.

Gamma radiation is best measured using semiconductor devices based on high-purity germanium (HPGe) detectors. These are commonly used in medical and industrial settings but they aren’t covered in school curricula. A suggested source for more information is given in the Further Reading section below.

Alpha radiation is the trickiest to measure on account of its very short penetration distances, as mentioned above. Historically, alpha radiation was detected using chemicals that converted its energy into visible light. Rutherford’s gold-foil scattering experiment used a coating of zinc sulphide, which fluoresces to produce a visible flash when struck by an alpha particle. Geiger and Marsden, who conducted Rutherford’s experiment, used a microscope to detect these tiny flashes of light at different angles around the gold foil and thereby obtained evidence for the nuclear model of the atom.

A key feature of the experiment was the evacuated chamber in which the apparatus and the alpha source were contained. This overcame the problem of alpha particles having low penetration through air. There is more about this experiment in a previous post, https://physbang.com/2020/10/04/rutherfords-gold-foil-experiment/.

The most dramatic classroom demonstration for alpha particles is the spark detector, which can be used without an evacuated chamber. Like GM tubes, spark detectors use two electrodes with a high potential difference between them. In a spark detector, one electrode is a thin wire and the other is a strip of mesh that runs parallel to the wire. The voltage across the electrodes is set to give an electric field strength that nearly ionises the air – but not quite.

When an alpha source is brought close to the detector, the alpha particles cause ionisation and sparks are seen. Alpha radiation is highly ionising and produces around 10,000 ion pairs for each millimetre of travel through the air. Therefore, each alpha particle can produce a powerful spark that is easily seen with the naked eye. The activity of different sources can be compared by counting the number of sparks in a fixed time interval, assuming that the sources are placed in exactly the same position.

Modern radiation detectors are much more sophisticated but it is important to mention the electroscope as a final example. This is an old-fashioned (low-technology) device where the rate of loss of a static-electric charge can be used to indicate the presence of ionising radiation. A previous post explains how this equipment was once used to investigate radiation levels above the Earth’s surface (https://physbang.com/2023/09/24/radiation-and-altitude/) and I will write a separate post very shortly with more information about the apparatus itself.

Further reading

Both of the diagrams shown in this post are from a document created by physics teacher Kin Weng Teng. The full document is an easy read and contains lots of information that is relevant to school physics courses: see https://www.academia.edu/35600668/Chapter_10_Radioactivity

For more information about the relationship between particle energy and penetrating distance; a short statement is available at https://ionactive.co.uk/resource-hub/guidance/distance-travelled-by-beta-particles-in-air-and-other-materials and a longer (but very accessible) explanation can be downloaded from https://www.gla.ac.uk/media/Media_590605_smxx.pdf.

Information about the use of HPGe detectors tends to be quite specialised but there is a good open access (free-to-read) paper available at https://link.springer.com/article/10.1140/epjp/s13360-024-04903-y.