13.1 INTRODUCTION
As well as in the nuclear industry, artificial sources of ionizing radiation are used extensively in a range of other industries. Examples of such use include: X-ray machines and sealed sources for industrial radiography (non-destructive testing) and for other industrial inspection purposes; sealed sources as gauges or measuring devices in the paper industry and in construction; and unsealed sources as radioactive tracers. Research and development facilities make use of a variety of radiation sources as do teaching institutions such as universities, colleges and schools. In order to carry out any of these activities safely an appropriate level of radiation protection is required. Some industrial activities, such as mining, may expose workers to significant amounts of naturally occurring radioactive material (NORM), which necessitates the implementation of radiation protection measures.
The medical and dental sectors are by far the biggest users of radiation machines, especially X-ray machines, and the relevant radiation protection procedures are covered in the next chapter. However, it should be noted that veterinary surgeons make similar use of X-ray machines for diagnostic purposes.
13.2 X-RAYS 13.2.1 General
X-rays were discovered by the German physicist Wilhelm Conrad Roentgen in 1895. Soon after, he found that he could photograph the interior of objects (including human body parts) by passing X-rays through them. Such photographs became known as radiographs.
The medical implications of this discovery were immediately appreciated and, within a few months, physicians in many parts of the world were using X-rays as an aid to diagnosis.
X-rays are now widely used in medicine, and this topic is dealt with in Chapter 14. X-rays also have many other applications in industry and in research.
The most important method of producing X-rays depends on a process known as bremsstrahlung, which is German for braking radiation. Bremsstrahlung X-rays are produced when charged particles, usually electrons, moving with a very high velocity are slowed down rapidly by striking a target – for example, when b-particles from a radioactive substance impinge upon a shielding material.
The efficiency of X-ray production by this means is dependent on the atomic number (Z) of the target material, with high-Z materials giving a much higher yield than low-Z materials. (This is the reason for using low-Z materials, such as Perspex, for shielding
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beta sources.) In any case the intensity of X-rays produced by b-particles from radioactive substances is too low for most applications. The method used to produce X-rays for medical and industrial purposes is based on an electrical discharge tube and is similar to the method developed by Roentgen, though modern equipment is much safer and more efficient.
Like light, radio waves and g-rays, X-rays belong to the electromagnetic group of radiations. They have no mass or charge, but have a wavelength which depends on their energy. They differ from g-rays in two important respects. First, g-rays originate within atomic nuclei whereas X-rays originate from changes in the electron orbits. Second, g-rays from a given source have definite discrete energies but X-rays from an X-ray generator usually have a broad range or spectrum of energies up to some characteristic maximum value.
13.2.2 X-ray equipment
X-rays are produced when electrons moving with high velocity are suddenly stopped by a material of high atomic number, and so an X-ray generator requires a source of electrons, a means of accelerating them to a high velocity and a target at which they are directed. An X-ray set consists of a tube and various electrical circuits, which are usually in a separate control unit.
The modern type of X-ray tube, shown diagrammatically in Figure 13.1, consists of a cathode and an anode inside a glass tube evacuated to an extremely low pressure.
The cathode is the source of the electrons and consists of a tungsten filament heated to incandescence by an electric current which ‘boils out’ electrons. The electrons are accelerated to the target by a high voltage applied between the anode and cathode.
The target is part of the anode assembly and is constructed of a material of high atomic number to achieve the best possible efficiency of X-ray production. However, even when the efficiency is as high as practicable, less than 1 per cent of the energy of the electrons appears as X-rays. The remainder appears as heat and so the target must have a high melting point and be able to dissipate the heat. This is achieved by constructing the anode of copper, which has a high thermal conductivity, with a tungsten target insert facing the cathode.
The copper anode is sometimes in solid form and has a finned radiator extending outside the tube to assist cooling. In higher power sets the anode is hollow and is cooled
Rotating anode Stator of induction motor
Bearings
Figure 13.1 A typical rotating anode X-ray tube.
by circulating oil or water through it. In applications such as radiography it is important, in the interests of good definition, that the source of X-rays is very small. The filament is therefore mounted in a concave cup that focuses the electrons onto a small area of the target. Special measures are then necessary to prevent overheating of the target and the anode may consist of a rotating disc. The effective target area is then still small but the heated area is greatly increased and the tube may be heavily loaded without melting the target. This type of tube is used in medical X-ray sets, in which very high intensities and short exposure times are used to minimize difficulties caused by body movement.
The electrical supplies required for the operation of an X-ray tube are a low-voltage supply to the filament and a very high voltage supply applied between anode and cathode.
The supplies are usually derived from mains alternating current (a.c.). In the case of the filament supply, a step-down transformer is used to provide a voltage of about 12 V a.c.
at a current which can be varied up to a few amperes. The tube voltage is provided by a high-voltage transformer which steps up the mains voltage (230 V a.c. in the UK) to the level required for operation of the tube. This is normally in the range from 5000 V up to some millions of volts, depending on the application. As it is derived from a.c. mains, the high voltage between the anode and cathode, if unmodified, would be also alternating, or at least self-rectified to ‘half-wave’ by the inherent action of the X-ray tube. This is a very inefficient way to generate X-rays. A typical power supply is illustrated in Figure 13.2.
V
Figure 13.2 Simplified power supply for X-ray set.
Most modern X-ray generators, especially those used for medical applications, are based on a ‘high-frequency’ or ‘constant potential’ design. The generator can be thought of as three sub-circuits:
(a) initial rectification and smoothing of the incoming a.c. supply;
(b) a frequency multiplying circuit; and
(c) a high-voltage transformer, rectification and smoothing circuit.
This converts the incoming mains a.c. into a high-voltage waveform which is almost constant with a ripple of less than 1 per cent.
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13.2.3 Quality and intensity of X-rays – the X-ray spectrum
The quality or energy of X-rays depends on the voltage waveform applied to the anode of the tube. If the peak voltage is 200 000 V, this is expressed as 200 kV peak or 200 kVp. The maximum energy of the X-rays produced is 200 keV, but only a very small fraction will have this value and most of the X-rays will be of lower energy. The quality of the X-rays is, however, defined in terms of this peak energy and they are said to be 200-kVp X-rays. The penetrating power of X-rays is highly dependent on their energy. For example, the quality of X-rays used to radiograph a person’s hand would be much too low to radiograph a 10 mm steel plate. The voltage on the tube is therefore set to give the appropriate quality of X-rays for each application. The spectrum of X-ray photons produced by a typical X-ray tube and generator combination is shown in Figure 13.3. A few examples of suitable operating voltages and exposures for medical and industrial radiographic applications are given in Table 13.1.
While the voltage on the tube controls the quality of the X-rays, the intensity is also governed by the current flowing in the tube, that is, between the anode and cathode. This current, expressed in milliamperes (mA), is limited by the number of electrons ejected from
Photon energy (keV) Maximum photon energy Characteristic radiation Relative intensity
of X-rays
0 50 100 150
Bremsstrahlung
Figure 13.3 The spectrum of X-ray photons from a typical X-ray tube and generator.
Table 13.1 Typical operating voltages for radiography
kVp Distance (m) Milliampere- seconds (mAs) Medical
diagnosis
Dental intra-oral 70 1 10
Computed tomography abdomen
80 1 100
Chest 80 2 3
Pelvis 120 1 30
Industrial 6 mm steel 120 0.5 10
25 mm steel 160 0.5 200
the cathode. This is controlled by the temperature of the filament, which in turn depends on the current flowing in the low-voltage filament circuit. As the dose rate depends on the tube current, the total dose in a particular case depends on the tube current multiplied by the time of exposure. At a fixed tube voltage, the same dose would be received from an exposure of 10 mA for 1 s as for 1 mA for 10 s. In both cases the exposure is 10 milliampere-seconds or 10 mAs.
The dose rate from an X-ray set is very high compared with dose rates from typical sealed g sources. The output is usually expressed in terms of the absorbed dose rate in mGy/min to air at 1 m from the set for a tube current of 1 mA. Some typical outputs are shown in Table 13.2.
Table 13.2 Typical output of X-ray sets
Equipment and filtration mGy/min/mA at 1 m
50 kVp beryllium window tube 100
100 kVp 3 mm aluminium (external to tube) 30
200 kVp 2 mm copper + 1 mm aluminium (external to tube) 20 300 kVp 3 mm copper + 1 mm aluminium (external to tube) 10 500 kVp 3 mm copper + 1 mm aluminium (external to tube) 25
The significance of the beryllium window mentioned in Table 13.2 is that at low voltages the penetrating power of X-rays is so low that a high proportion would be absorbed by a glass bulb. The use of a thin beryllium window minimizes this loss of output.
At higher voltages additional absorbers, or filters as they are known, are provided in the X-ray beam. It has been mentioned that X-rays of all energies up to the peak voltage are produced. Only the small fraction with the higher energies is useful, the remainder being undesirable in many cases. For example, in medical radiography, the low-energy fraction would not contribute to the radiograph, but would result in unnecessary dose to the patient. The use of filters, usually an appropriate thickness of aluminium, selectively absorbs the low-energy or soft radiation without significantly affecting the useful beam.
13.2.4 Protection against X-rays – general principles
Unlike radioisotopes, which emit radiation continuously, X-ray sets can be switched on or off at will. During operation the dose rate from the set may be very much higher than from small sealed sources. The equipment must be run in such a way that the operator does not expose any part of his or her body to the direct beam and no other person should be inadvertently exposed. The general principles applied to the protection of personnel are as follows:
1. adequate training of all personnel who operate or use X-ray equipment in the correct operating procedures and in the hazards involved;
2. limitation of the beam size to the minimum necessary by the provision of shielding and having collimators built into the set;
3. the use of suitable filtration to remove unwanted soft radiation;
4. operation of X-ray equipment in a shielded room whenever possible. The controls are located in a shielded position either inside or outside the room and, depending upon the application, an interlock circuit may be used to prevent
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operation of the equipment while the door is open. A notable exception is where X-ray machines are used ‘in the field’ for mobile radiography;
5. provision of automatic visible and audible warnings that the X-ray set is operating or about to operate;
6. confirmation of the effectiveness of the control measures by means of a system of personal and area monitoring.
The measures applied in any given case depend very much on the type of work and the local conditions. The main applications of X-rays are in industrial and medical radiography.
This chapter deals with protection in industrial applications and research, and medical uses are considered in detail in Chapter 14.
13.2.5 Monitoring of X-ray installations
An important part of the commissioning procedure of any radiographic installation, or other facility which produces ionizing radiation, is a thorough radiation survey. Particular attention is paid to possible weaknesses in shielding, such as joints in the shielding material, viewing windows, doors and holes or ducts for services. The survey, which is usually performed at maximum tube voltage and current, is made under normal operating modes and then under other possible operating modes. Consider, for example, the case of a facility in which the X-ray beam is intended to operate in the horizontal plane and adjacent areas are shielded by thick walls. If the orientation of the set is changed and the beam operated in a vertical direction, would unacceptable levels of radiation occur in the areas above or below the facility? It should be borne in mind that if such a change is possible it is quite likely that, one day, it will be made. Even if the areas above or below the cell are unoccupied, high dose rates can occur outside shielding because of radiation scattered from the object being radiographed, the walls, floor or ceiling of the room, or even the air (which is often referred to as skyshine). If it is found that excessive radiation levels could possibly occur in adjacent areas, measures must be taken to prevent, or at least to give warning of, the situation. This can be done by mechanically preventing the beam from being operated outside set directions, by the provision of additional shielding, or by the installation of area radiation monitoring equipment with warning signals. In general, one of the first two methods is preferable.
Clearly, questions of this sort should be considered at the design stage as part of a prior risk assessment, but it is essential to confirm the safety of the facility by direct measurement. Surveys should be repeated periodically, particularly when any changes in operating procedure are introduced.
Care is necessary in the selection of instruments for monitoring X-rays. The major problem is that of energy response. Many instruments that are satisfactory for g-rays and higher energy X-ray work seriously underestimate the dose rate from X-rays of below about 100 kVp. For low-energy work, instruments incorporating thin-window ionization chambers are probably the most suitable, although they are sometimes lacking in sensitivity.
Another possible problem when pulse-type instruments (e.g. Geiger–Müller tubes) are used is that the instrument may saturate in high X-ray dose rates and yet still appear to be working satisfactorily. This is due to the pulsed nature of the X-rays, which allows the instrument to recover between pulses. The instrument might then record the X-ray pulse rate rather than the average dose rate. This problem is fortunately rare in equipment of modern design but may arise as a result of a fault.
The safety of a facility is ultimately judged by the radiation doses received by operators and other persons working in the vicinity. These are normally measured by thermoluminescent dosimeter (TLD) badges, although many establishments still use film-based dosimeters. It is often worthwhile using a few TLD badges or other dosimeters to monitor fixed locations around the area on a routine basis. It should be borne in mind that personal monitors are small in area and X-ray beams, particularly in crystallography, may also be small in cross-section. It is quite possible for a beam to miss a personal monitor but nevertheless to irradiate the worker.
13.2.6 Protection in industrial radiography
The general principles that apply to the control of hazards from industrial radiography are as follows:
1. Non-destructive testing using ionizing radiation should be used only where it offers a clear advantage over other methods, in other words, the use of radiation is justified.
2. Whenever practicable, radiography or any other process using ionizing radiation from machines or sealed sources should be carried out within a shielded enclosure.
3. The control panel for the apparatus should be located outside the enclosure and devices should be provided to ensure that, if any door of the enclosure is opened while the apparatus is energized, the apparatus is automatically de-energized.
4. For the protection of persons accidentally shut inside the enclosure, a means of communication is required to enable them to summon help. In addition, one or more of the following facilities should be provided for such persons:
(a) means of exit;
(b) means of de-energizing the apparatus; or (c) a shielded area.
5. Audible or visible signals (or both) should be given when the apparatus is about to be energized, and a different signal while the apparatus is energized.
Where a g-ray source is used, ‘energized’ means that the source is out of its shielded storage location and ‘de-energized’ means that it has been returned to its storage location.
The main industrial radiographic procedure is the application of X-rays in the non-destructive testing of products, process plant and civil engineering structures. In other applications, particularly where the size of the object requires more penetrating radiation, radiography is undertaken using sealed g-ray sources, normally cobalt-60 or iridium-192.
Radiographic testing of products is normally part of the production process and is usually undertaken in purpose-designed enclosures with adequate shielding and appropriate safety systems to protect operating personnel. In other situations, such as the testing/inspection of process plant or civil engineering structures, radiography may need to be carried out in conditions that are far from ideal, such as on a construction site.
Historically, the main problems have occurred when radiography is undertaken under site conditions, usually by contractors or subcontractors. Doses to radiographers have been quite high, and there have been cases of inadvertent exposure of other workers. There have also been incidents involving lost or broken sources. Recent changes in regulations
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have been aimed at ensuring that on-site radiography is undertaken only when it is impracticable to move the item into a proper shielded enclosure and that where on-site work does need to be undertaken it is properly planned and controlled. Before any radiography is performed under site conditions, a risk assessment needs to be undertaken to identify all the risks associated with the proposed work, including non-radiological risks.
All reasonable measures need to be taken to protect others on the site, such as applying local shielding and physical restrictions on access. The possibility of accidents that could
All reasonable measures need to be taken to protect others on the site, such as applying local shielding and physical restrictions on access. The possibility of accidents that could