Module 10: Radiation Safety for X-Ray Units

10.1 Nature of Analytical X-rays

Analytical X-ray instruments produce intense beams of ionizing radiation that is used for diffraction and fluorescent studies. At U of T, there are also numerous X-ray producing machines for medical and/or dental applications.

The part of the beam that is corresponding to the shell (K, L, M, etc.) emission of the target material is called characteristic x-ray radiation. In addition to the characteristic radiation, a continuous radiation spectrum, ranging from a very low energy to the maximum setting, is produced. Undesirable parts of the x-ray may be filtered out using cones, diaphragms, and collimators.

The primary beam is not the only source of ionizing radiation. Any high voltage discharge is a potential source of X-rays. Faulty high-voltage vacuum-tube rectifiers may emit X-rays of twice the voltage applied to the X-ray tube. Other sources of ionizing radiation are:

  1. Secondary emissions and scattering from the sample, shielding material, and fluorescent screens
  2. Leakage of primary or scattered X-rays through gaps and cracks in shielding
  3. Penetration of the primary beam through or scattering from faulty shutters, beam traps, or collimator couplings
  4. X-rays emitted from an open, un-collimated port form a cone of about 30 degrees. A collimator reduces the beam size to about a 1 millimeter diameter

10.2 X-ray Hazards and Biological Effects

X-rays produced by diffraction machines are readily absorbed in the first millimeters of tissue, and therefore do not contribute any dose to the internal organs of the body. However, the lens of the eye can receive a significant dose from X-rays of this energy. Overexposure of lens tissue can lead to the development of lens opacities and cataracts.

An absorbed dose of a few Gray may produce a reddening of the skin (erythema) which is transitory in nature. Higher doses to the skin – 100 Gy and greater – may produce significant cellular damage resulting in pigment changes and chronic radiation dermatitis. Exposure to erythema doses may not result in immediate skin reddening. The latent period may be from several hours to several days.

X-rays used for medical purposes are about one order of magnitude shorter in wavelength. Diagnostic rays are designated for tissue penetration and are carefully filtered to avoid X-ray damage to the skin caused by the longer, more readily absorbed wavelengths.

10.3 Safety Precautions, Shielding

The normal shielding, safety equipment, and safety procedures are applicable for up to 75 kV-peak X-rays. Additional precautions are necessary for instruments operating at higher voltages.

Prior to the removal of shielding or before beginning work in the sample area, the operator must check both the warning lights and current meter on the console. The best way to avoid accidental exposure is to turn the machine off before working in the sample area.

Never put any part of the body in the primary beam. Exposure of any part of the body to the collimated beam for even a fraction of a second may result in damage to the exposed tissue.

If an instrument malfunction is suspected, the machine should be turned off and unplugged. A notice should be placed on the control panel until the instrument is repaired. A qualified person must perform all repairs. Alignment procedures also require special training and knowledge. Such service to X-ray emitting devices must be reported to the Radiation Protection Service.

10.4 Eye Protection

The use of safety glasses is encouraged when working with analytical X-rays. While glasses cannot provide complete protection to the eyes, they can reduce X-ray exposure. Glass provides about 10 times the protection of plastic. Neither one will adequately protect the eye from direct exposure to the primary beam.

10.5 Tube Status Indicators

There must be a visual indication located on or near the tube head to indicate when X-rays are being produced. This is usually an assembly consisting of two red bulbs, wired in parallel and labelled X-RAYS ON. If one of the bulbs is burned out, the operator should either replace it before leaving the room or leave a note on the light assembly indicating that the bulb is burned out. An unlit warning bulb does not necessarily mean that X-rays are not produced. Never trust a bulb, unless it is illuminated ON! Always check the control panel when the bulb is off.

10.6 Safety Devices – Interlocks

Interlock switches are used to prevent inadvertent access to the beam. They should not be bypassed. Interlocks should be checked periodically to ensure that they are functioning properly.

Interlocks and other safety devices, including warning systems, are not foolproof or fail-safe. A safety device should be used as a back up to minimize the risk of radiation exposure – never as a substitute for proper procedures and good judgment.

10.7 Registration of X-ray Instruments

Users of X-ray producing devices on U of T campuses (or in areas that are controlled by U of T) must register their instrument with the University RPS. Registration is necessary for the reporting requirements of the Ontario Ministry of Labour (MOL) and the Ministry of Health.

The following information is required for registration:

  1. Type of device (dental, crystallography, fluorescent, medical, etc.)
  2. Name of Manufacturer
  3. Model of the device
  4. Serial number
  5. Maximum voltage
  6. Maximum current
  7. Building and room number where the instrument is located
  8. Department to which instrument belongs
  9. Name and telephone number of the person in charge of the instrument

More information about the U of T X-ray safety program can be found at: