9. Types & characteristics of radiation-emitting devices

9.1 BASIC PRINCIPLES

A radiation-emitting device is any device that is capable of emitting ionizing radiation when activated. This definition applies both to devices such as X-ray machines, whose sole function is the generation of ionizing radiation, and to devices such as electron microscopes and some types of cathode-ray tube, which generate ionizing radiation as a by-product of some other function.

An important distinction between a radiation-emitting device and a radioactive source is that the former can be switched off, whereas a radioactive source can only be shielded and continues to emit radiation even when it is not in use. It is important to emphasize, especially to cleaners and non-technical staff, that a radiation device is harmless when switched off.

*Accelerators may be an exception to this statement if they operate at energies high enough to induce radioactivity in surrounding materials. Special rules therefore apply in such cases.

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9.2 X-RAY MACHINES

An X-ray machine used for experimental purposes is essentially an irradiator in which the useful beam is confined to a specific direction, shape and size by means of slit, cone, diaphragm or other collimating device. The collimator may provide only a single fixed shape and size of the beam, or may allow for variation in these factors. Where possible, the useful beam should be permanently directed into an enclosed and shielded box into which the object to be irradiated can be inserted. The door or lid of the box should be interlocked with the on/off switch of the X-ray machine. The dose rate outside the box should be such that, taking into account the workload of the X-ray machine and the occupancy of the surrounding area, the annual dose limit for Radiation Users is not exceeded. Non-radiation workers should be excluded from the immediate vicinity of the X-ray machine during the whole of any period in which the machine is in use.

If the conditions set out above cannot be met, and it is impossible to restrict the X-ray beam to a single direction and/or to confine their radiated space to a shielded box, then the X-ray apparatus must be installed in a separate shielded room. The conditions set out in Section 7.4, for irradiators containing large radioactive sources, will apply.

Notwithstanding any shielding and interlocking arrangements, Radiation Users should take great care to avoid exposing any part of their bodies to a direct X-ray beam. This is particularly important in the case of X-ray diffraction units, where a narrow but very intense beam may need to be manipulated under manual control.

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9.3 IMAGING PROCEDURES

A diagnostic-type X-ray machine may be used for imaging in research, for example with experimental animals. Such images must be obtained either by radiography or by fluorographic techniques using an image amplifier. Direct-viewing fluorography is forbidden.

Normally, the operator of an X-ray machine should stand behind a protective screen or in another room during exposures. However, sometimes it is essential for the operator to stand near to an unshielded X-ray beam during an exposure, for example to carry out a procedure on an experimental animal. In such cases the worker must wear appropriate protective clothing (lead-rubber aprons and gloves), or be provided with a portable protective screen. In any case, it is important to avoid exposing parts of the body to a direct X-ray beam.

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9.4 RADIATION SURVEY OF X-RAY EQUIPMENT

Outside the useful beam, X-ray machines give rise to leakage radiation, i.e. stray radiation transmitted through the housing of the X-ray tube when the beam is "on", and also scattered radiation originating in objects placed in the direct beam. A radiation survey must therefore be made in the area surrounding the X-ray machine, if this space is accessible during exposures. Alternatively, if the machine is housed in a separate shielded room (Section 9.2 above) then the survey should be conducted in the area outside the room. Environmental Health & Safety will normally undertake such surveys. The results must be documented and filed.

X-ray generators that are many years old may include thermionic-tube rectifiers (more modern equipment use silicon rectifiers). Such thermionic rectifiers do not normally present a hazard, but if a circuit fault develops, it is possible for the rectifier to become, in effect, an X-ray tube and to emit ionizing radiation. This emission is associated with a sharp drop in the intensity of the emission by the X-ray tube proper. Owners of old X-ray equipment should be aware of this possibility and the rectifiers should be included in any radiation survey.

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9.5 OTHER RADIATION-EMITTING DEVICES

Electron microscopes are likely to generate measurable levels of ionizing radiation (soft X-rays). This is not usually a problem, because the exposure rate is normally barely distinguishable from background, but nevertheless a radiation survey should be carried out similar to that required for an X-ray machine (Section 9.4). The results must be documented and filed.

In principle, cathode-ray tubes (especially colour tubes) can generate soft X-rays, but surveys carried out by Health Canada and by several other organizations have shown that such emissions are negligible. The glass on the front screen effectively absorbs any radiation, which may be generated and the measured dose rate in the position of the operator is indistinguishable from background. No individual radiation survey is needed.

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9.6 INTERNAL PERMIT FOR RADIATION-EMITTING DEVICES

Radiation-emitting devices such as X-ray machines are licensable by the Canadian Nuclear Safety Commission only if they are capable of accelerating particles to a high energy or of inducing radioactivity. In practice this means that X-ray machines and some accelerators operating below 8 MeV are exempt from CNSC license requirements. Nevertheless, in the interest of the overall control of radiation hazards in McGill University, these machines require an Internal Permit similar to that issued for radioactive materials. A separate application form is used (see Appendix H-2).

Electron microscopes and other devices, mentioned in Section 9.5 above, do not require an Internal Permit. Nevertheless, it is recommended that a survey of stray radiation be carried out around any device capable of emitting significant radiation.