4. APPLICATIONS, DEVICES AND ASSOCIATED SEALED SOURCES
4.2. Devices and Category 2 sources
Gamma ray radiography is one of a number of technologies used in industry for safety assessment and quality control purposes. In particular, it is widely used in the chemical, petrochemical, and building industries for radiographic inspection of pipes, boilers and structures where the economic and safety consequences of failure can be severe.
Industrial gamma radiography projectors are used for the radiography of engineered structures. They contain a single source attached to a flexible cable which can be exposed near the object that is being investigated. A radiographic film is attached behind the object, and the penetrating gamma rays expose the film. Variations in the density of the item being radiographed are shown in the image of the film. The devices are often also referred to as radiography cameras (Fig. 18).
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FIG. 18. A typical gamma radiography system.
Most industrial radiography equipment consists of a radiography unit, which may contain depleted uranium as shielding material, and one or more sealed sources (Fig. 19). Currently, sources containing 192Ir or 60Co, or 169Yb,
170Tm or 75Se, are used in gamma radiography. The sources are usually doubly encapsulated in stainless steel, and contain one or more pellets of active material in metal form. The source is held in a flexible assembly, sometimes called a source holder, pencil, shuttle or pig tail (see Figs 20–22). Portable, industrial radiography devices are generally small in size, although they are relatively heavy due to the shielding.
FIG. 19. Typical industrial radiography projectors.
When not in use, the source is located in the centre of the source container. During use, the source is pushed down into the desired position by a remote controlled device. In heavy industries, such as steel foundries or
fabrication plants, portable, mobile (on wheels) or fixed radiographic equipment, containing 192Ir, 60Co or 137Cs, may be installed in purpose built enclosures. Because mobile or fixed installations incorporate heavier shielding than portable source housings, they are less susceptible to theft and more difficult to dismantle.
FIG. 20. Typical old gamma radiography source/pigtail assemblies.
FIG. 21. A typical modern gamma radiography source/pigtail assembly.
FIG. 22. A typical gamma radiography inner source capsule prior to encapsulation in the pigtail.
The housings of portable sources contain several tens of kilograms of shielding material, such as depleted uranium, lead or tungsten, which may be perceived as being potentially valuable. Also relevant is the fact that the portable nature of most equipment allows it to be used almost anywhere. Often this is in remote locations or under extreme working conditions. This situation, coupled with limited or non-existent supervision, provides the potential for entire containers with their sources to be lost or stolen. They can end up in the metals recycling industry or remain in the public domain. These are similar problems to those for disused teletherapy sources, and while the activity levels for industrial radiography are lower, they are still sufficient to produce lethal effects. Perhaps the most significant threat comes from loss of the unshielded source. The large numbers, work environment, activity level and portability/mobility of most industrial radiography sources make them prime targets for the deliberate acquisition for malevolent purposes.
4.2.2. High/medium dose rate brachytherapy machines
Brachytherapy (therapy at a short distance) is a term that is used to describe the interstitial, or intra-cavity, application of radioactive sources by placing them directly in the tumour (breast, prostate), in moulds (skin, rectum), or in special applicators (vagina, cervix). Brachytherapy applications are of two slightly different varieties. These are generally referred to as HDR brachytherapy (Category 2) and LDR brachytherapy (Category 4 or 5). HDR sources, and some LDR sources, may be in the form of a long wire attached to a device (a remote afterloading device).
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Afterloading devices may be heavy due to the shielding for the sources when they are not in use. The device may be on wheels for transport within a facility. The remote afterloading device may also contain electrical and electronic components for its operation (Fig. 23). When using these devices, catheters are first inserted into the body and the sources, attached to cables, are then introduced by remote control.
Historically, 226Ra was used for brachytherapy. The sources were encapsulated in platinum in either needles or tubes of a few mm in width and up to 5 cm in length. Currently, most high and medium dose rate brachytherapy is performed with 192Ir, but 60Co and 137Cs are also used. Sources are manufactured in different sizes and shapes, including wires or ribbons.
FIG. 23. An LDR brachytherapy machine with 192Ir sources and a 192Ir source.
When not in use, brachytherapy sources are normally stored in lead shielded safes or containers, but there have been cases when the sources were improperly kept loaded in applicators in transport carts. Similarly, sources past their useful life have been left in safes or transport containers. If the cable of a remote afterloader breaks, the sources may become detached. Failure to recognize these problems may pose significant risks.
4.2.3. Calibration systems
Calibration systems use high activity radioactive sources (approximately 15 to 82 TBq (400 to 2200 Ci)) to produce radiation fields of known intensity for calibration of radiation monitoring equipment and dosimeters, whereby the equipment and dosimeters can be evaluated for accurate operation. A source of measured activity is required to calibrate instruments and dosimeters to accepted standards. Figure 24 shows a diagram and a photograph of a typical gamma beam calibration source.
FIG. 24. Typical gamma calibrator configuration for survey instrument calibration.
The system usually consists of radioactive sources, radiation shielding, a mechanism for positioning the source, and a track or internal chamber for positioning the items to be calibrated. Modern calibration systems may contain a computer controller and safety systems, such as video monitoring, radiation monitors, warning lights and indicators, and a safety interlock system.
There are a large number of radioactive sources that are used for instrument and other calibration purposes.
Because a wide range of radionuclides and activities are used, these sources cannot be assigned to any one particular category; however, the larger 60Co and 137Cs calibration sources generally fall into Category 2. Some sources could belong to Categories 3 and 4, but most of them fall into Category 5.
Some calibration sources, especially those of higher activity, are in specifically designed, shielded and collimated devices within large shielded facilities. Others are simply individual sources that might be used for a variety of purposes, such in the nuclear industry, environmental protection, and research and educational institutions. A wide range of radionuclides, or a combination of radionuclides, is used in calibration sources and a variety of source designs and shapes are available (Fig. 25).
FIG. 25. Examples of low activity 60Co, 137Cs, 90Sr and 226Ra calibration sources.
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4.3. DEVICES AND CATEGORY 3 SOURCES