Radiation Safety for X-ray Diffraction and Spectroscopy

Analytical x-ray machines produce intense beams of ionizing radiation that are used for diffraction and fluorescence studies. The most intense part of a beam is that corresponding to the K emission of the target material and is called characteristic radiation. In addition to the characteristic radiation, a continuous radiation spectrum of low intensity is produced ranging from a very low energy to the maximum kV-peak setting. This is referred to as ``bremsstrahlung" or white radiation.

X-rays in the range of 15 to 40 keV produced by diffraction machines are readily absorbed in the first 5-10 millimeters of the skin, and do not contribute to a deep dose to the internal organs of the body. However, the eyes, because of the aqueous nature of the tissue, do receive deep dose. Overexposure of lens tissue can lead to the development of lens opacities and cataracts; therefore, safety glasses should be worn when operating x-ray producing equipment (for this application, glass lenses are preferred).

Absorbed doses on the order of 100 rads may produce a reddening of the skin (erythema) which is transitory in nature. Higher doses - 10,000 rads and greater - may produce significant cellular damage resulting in pigment changes and chronic radiation dermatitis. It should be remembered that exposure to erythema doses may not result in immediate skin reddening. The latent (waiting) period may be from several hours to several days. (Note: X-rays used for medical diagnosis are about one order of magnitude shorter in wavelength for tissue penetration and are carefully filtered to avoid x-ray damage to the skin caused by the longer wavelengths).

Avoiding the primary beam does not necessarily mean that one is not being exposed to ionizing radiation. Faulty high-voltage vacuum-tube rectifiers may emit x-rays of twice the kilovoltage applied to the x-ray tube. Other sources of potentially hazardous radiation are: 1. Secondary emission (scattering) from the sample, shielding material, and fluorescent screens. 2. Scattering from a faulty beam trap. 3. Leakage of primary x-rays through gaps and cracks in shielding. 4. Penetration of the primary beam through faulty shutters, or through insufficient thickness of shielding material.

The equipment operator is responsible for his own safety and the safety of others when using an analytical x-ray machine. The written procedures developed for individual instruments should be sequentially followed. 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 few seconds may result in damage to the exposed tissue. A person not knowledgeable about x-ray equipment should not attempt to make repairs or remedy malfunctions. Always consult the designated representative first. Remember, safety devices and warning systems are not fool-proof 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. The operator may use a radiation survey meter to detect the presence of unwanted radiation and to trace the origin of leaks. The recommended instrument is a Geiger-Mueller meter with a thin window "pancake" detector. It should be remembered that most meters do not respond accurately all the energies used for analytical x-rays. Correction factors of 3x to 10x may be required. Each user of APD x-ray equipment is recommended to wear a film badge. It must be recognized that the badge indicates only the level of whole body radiation dose intercepted by the badge, or the level of scattered radiation in the room. The office of radiation safety offers a quarterly course on radiation safety.

In the event of an accident or unusual incident involving an analytical x-ray machine, proceed as follows: 1.Turn off the machine. 2. Call the Office of Radiation Safety at 894-3600. 3. Call the principal investigator responsible for the machine. 4. Record all important parameters (e.g. kV-peak, mA, nature and duration of the possible exposure, and distance from the x-ray source).