Radioactive count rate
An environmental factor that has sprung into prominence in the last few decades is the radioactive count level, and this figure has been used in such a way as to prove that a little learning can be a remarkably dangerous
thing. The quantity that is measured in these readings is the number of ions produced per second by all types of radiation and by particles from radioactive materials.
Since the whole Earth consists of materials that are to some extent radio- active, and is constantly bombarded with radiation from the Sun and other stars, there is no place that is free of radiation. In addition, the normal level of radioactivity varies very considerably from one place to another, and is particularly high where there are old granite rocks, or in the presence of deep-mined materials. One very revealing test is to measure the radiation count downwind of a coal-fired power station or on moors of granite rock. Most detectors can be driven completely off-scale by the radiation from any pre-war luminous watch, because these used radium for the luminescence.
The basis of many counters of ionizing radiation or particles is the Geiger- Muller tube (see Figure 7.6). This consists of a tube that contains a mixture of gases (usually the inert gases of the air, such as krypton) at a low pressure, about l/80 of atmospheric pressure. The electrodes in the tube are maintained with a voltage difference of about 400 V, and in the presence of radiation or ionizing particles, the gas in the tube is itself ionized, allowing a brief pulse of current to pass. The current is brief because the gas mixture contains traces of bromine or iodine that have the effect of neutralizing the ions rapidly so that the gas does not continue to conduct after the cause of the ionization has passed.
The output from the tube is taken across a load resistor and, when the particles do not arrive in great profusion, consists of a pulse for each particle. The pulses can be amplified and used to operate a counter, a rate- meter, or a loudspeaker to give the clicking noise beloved of films on radio- activity. The count rate is obtained by passing the pulses into an integrating circuit whose DC voltage is then proportional to the rate at which the pulses arrive. The GM counter, however, detects only the ionization caused by radiation or particles – it does not indicate the cause of the ionization, nor is it equally sensitive to all causes of ionization.
Natural sources of radioactivity can produce ionization from three causes. The first cause is the alpha particle, which is an ionized nucleus of the gas helium. This has a comparatively large mass and very strong ionizing effect (and a correspondingly large effect on living cells), but is absorbed strongly in all materials, including air, so that its range from its source is usually only a fraction of a millimetre. Only GM tubes with a very thin entry window (usually mica) can detect alpha particles, and only when the window end of the tube is held against the source of the particles.
The second type of particle is the beta particle, which is the familiar electron. This particle ionizes materials quite efficiently but is very much smaller than the alpha particle so that its effects on living cells are much less. The range in air can be several cm, but a sheet of paper is enough to stop electrons from all but a very active source. Electrons are very efficiently detected by the GM counter.
The third type of radiation is the short-wavelength ray, such as the gamma rays from radioactive materials and the cosmic rays from outer space. These are only weakly ionizing, but are immensely penetrating and require several feet of concrete or several inches of lead for screening. Because these rays have a rather weak ionizing effect, they are not efficiently detected by the GM counter. These rays, however, are the most dangerous to life.
As well as these natural sources of radiation from the Earth itself and from outer space, the use of natural radioactive materials in concentrated form (as in nuclear power sources) gives rise to other particles. The main particle of concern is the neutron, which has virtually no ionizing effect, since it is electrically uncharged, and which therefore does not affect the GM counter unless the bombardment is dense. The alternative is to use a GM tube that contains a vapour that will absorb neutrons and give out electrons. Neutrons are very penetrating and harmful, but difficult to detect. Shielding can make use of water, paraffin wax or other low-density materials.
The main alternative to the GM type of counter is the scintillation counter. The basis of this is a crystal that will give off a faint flash of light when affected by an ionizing radiation. The crystal is kept in a dark space and the faint light is detected by a photomultiplier (see Chapter 3) so that the pulses from the photomultiplier can be counted. Several types of sensitive crystals can be used, so that one type of radiation can be detected rather than another. This makes it possible to estimate the relative contribution that different types of radiation make to the whole.
One of the main problems of using detectors like the GM counter is to determine what amount of the reading is due to the presence of an unwanted radioactive material. The continual bombardment of ionizing particles and radiation from the Earth and from space produces a back-ground count, and this count has to be determined, often over a long period, and subtracted from the count obtained in the presence of suspected radio- active material. Low-level radioactive material, such as laboratory glassware and clothing used in the radiological industries, have a count level so close to the natural background level that it is difficult to establish the level of contamination. Given the choice of having a nuclear dump or a heap of manure next to your house, it’s better to plump for the nuclear waste!