are beta emitters. Beta particles are usually stopped by the skin but can cause serious damage to skin and eyes.

  • Gamma rays and x-rays, extremely energetic photons, have no mass or charge. Gamma rays are generally emitted from the nucleus during nuclear decay, and x-rays are emitted from the electron shells. Gamma rays are also produced by particle accelerators and nuclear reactors. Extremely dense materials such as lead or depleted uranium are required to shield against these very energetic, penetrating forms of radiation.

  • Neutrons, uncharged particles, are emitted from the nucleus during decay. Shielding materials for neutrons include water, paraffin, boron, and concrete.

Radioactive decay rates are reported in curies (1 curie (Ci) = 3.7 x 1010 disintegrations per second) or in the International System of Units (SI) in becquerels (Bq) (1 becquerel = 1 disintegration per second). The decay rate provides a characterization of a given source, but provides no absolute guide as to the hazard of the material. The hazard depends on the nature of, as well as the rate of production of, the ionizing radiation. In characterizing human exposure to ionizing radiation, it is assumed that the damage is proportional to the energy absorbed. The radiation absorbed dose (rad) is defined in terms of energy absorbed per unit mass: 1 rad = 100 ergs/g (SI: 1 gray (Gy) = 1 joule/kg = 100 rads). For electromagnetic energy, the roentgen (R) produces 1.61 x 1012 ion pairs per gram of air (SI: 1 coulomb/kg = 3.876 R).

For evaluation of the risk of exposure to ionizing radiation in humans, the dose equivalent in rem (roentgen equivalent man) is defined as

where the absorbed dose is given in rads, Q is the quality factor, and N is the tissue factor. Q is 1 for x-rays and gamma radiation of any energy, and for beta radiation. For alpha radiation, Q is 20. For neutrons, Q is 2 to 10, depending on their energy. In the United States, the applicable Standards for Protection Against Radiation from Sealed Gamma Sources (U.S. National Committee on Radiation Protection and Measurements, 1960), defines dose equivalents as follows: for x-ray, gamma ray, and electron radiations, Q x N = 1 and so 1 rad = 1 rem; for neutrons or high-energy protons, Q x N = 10 and 1 rem = 0.1 rad.

Damage may occur directly as a result of the radiation interacting with a part of the cell or indirectly by the formation of toxic substances within the cell. The extent of damage incurred depends on many factors, including the dose rate, the size of the dose, and the site of exposure. Effects may be short-term or long-term. The acute short-term effects associated with large doses and high dose rates, for example, 100,000 mrads (100 rads) in less than 1 week, may include nausea, diarrhea, fatigue, hair loss, sterility, and easy bruising. In appropriately managed workplaces, such exposures are impossible unless various barriers, alarms, and other safety systems are deliberately destroyed or bypassed. Above 600 rads, all exposures are probably fatal. Long-term effects, which develop years after the exposure, are primarily observed as cancer. Exposure of the fetus in utero to radiation is of concern, and the risk of damage to the fetus increases significantly when doses exceed 15,000 mrems. The U.S. Nuclear Regulatory Commission has set limits for whole-body occupational exposure at 500 mrems per quarter and 2,000 mrems/year and recommends that student exposures not exceed 500 mrems/year. Exposure limits are lower in facilities operated by the Department of Energy and other agencies. No completely safe limit of exposure is known.

As with all laboratory work, protection of the worker against the hazard consists of good facility design, operation, and monitoring, as well as good work practices on the part of the worker. The ALARA (as low as reasonably achievable) exposure principle is central to both levels of protection. The amount of radiation or radioactive material used should be minimized. Exposures should be minimized by shielding radiation sources and workers and visitors and by use of emergency alarm and evacuation procedures. Physical distance between personnel and radiation sources should be maximized, and whenever possible, robotic or other remote operations should be used to reduce exposure of personnel.

(Also see Chapter 5, section 5.E.)



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