Chapter 7
Principles of Radiation Safety in the Laboratory
The subject of radiation safety is an immense one and covers a wide variety of environments, from nuclear power plants to educational settings. In this brief discussion we will focus on the laboratory use of radioactive sources only. Should your application involve other radiation environments (such as engineering nuclear reactors), it is your and your supervisor's duty to ensure that you are fully trained in proper operational and safety procedures before you enter the environment. Remember: without the required safeguards and/or monitoring equipment, you could receive a significant dose of radiation without realizing it, with years passing before physical effects become apparent.
In addition to the material of this chapter, documents listed below and prepared by the University's Radiological Health Department, which you should consult when appropriate, are available in the Physics Department Stockroom (grey filing cabinet). These documents give a thorough discussion of the basics of radioactivity and radiological health principles, and describe the regulations that must be met when using radioactive materials. In addition, they contain copies of all the forms that one would need when disposing of radioactive waste. The documents are Fundamentals of Radiation and Radioactivity, Biological Effects of Radiation, Radiation Safety Information, the University of Utah Radiation Safety Policy Manual, and Radiation Safety Procedures and Records for Radioisotope Users.
If you are ever in doubt about procedure or policy in the acquisition, handling, or disposal of radiation sources, or in the proper safety precautions, don't take chances: either contact a member of the Physics Department Safety Committee, or directly contact the University's Radiologic Health Office (1-6141). Prior to working with radiation sources, all University employees are required to attend a class on radiation health and safety given periodically by the Radiological Health Department. For the date and location of the next class, call Radiological Health at 1-6141.
7.1 Radiation Safety for Non-technical Staff
For staff members of the Physics Department whose duties are non-technical, it is not necessary to read past this section in this chapter. The remainder of this chapter has been written for staff and students whose work in the Department may require use of radioactive materials, and so beyond this section the discussion is quite technical. 
The basic principle of radiation safety for non-technical staff is simple: do not handle any radioactive materials, and avoid areas which display the required and universally recognized radiation-alert sign, shown at left.
Every lab that contains radiation source materials is supposed to have a sign bearing this symbol on its door, informing you of what types of radioactive materials are present in the lab. Most radioactive materials sources within the department emit relatively little radiation and so are quite benign if handled appropriately. However, in the absence of detailed knowledge about any given radioactive material, you should treat the radioactive material as potentially hazardous, and under no circumstances should you touch it. If you must be in an area with radioactive sources, be sure that you know where they are so that you can avoid them.
The other two primary sources of ionizing (harmful) radiation in the Department are the X-ray machine and Van de Graaff generator, both located in B-39, near the loading dock. The direct beam of the X-ray machine presents a potentially serious health hazard. Exposure of body parts (fingers) to the direct beam will cause permanent, irreparable damage in a few seconds. The background radiation in the remainder of the room is sufficiently weak as to present no health hazard, particularly to infrequent visitors. (Note that the X-ray machine is used infrequently.) The Van de Graaff generator in the same room presents a serious radiation hazard at all times when it is running. Under no circumstances is anyone allowed in the room when it is running (which is also infrequently). A loud siren is activated whenever the machine is operated, so there is no chance of accidental exposure.
If you believe that there are radiation sources or radiation hazards in an area which is not properly labeled as such, you should immediately contact your supervisor for corrective action. If you have a continuing concern about possible radiation hazards (or any other safety issue within the Department), directly contact the Chair of the Physics Department Safety Committee.
7.2 Terminology and Units
Before discussing safe laboratory practice, we need to define a number of radiological terms and units (see References 1 and 2 for additional elementary discussion):1. Radiation: a form of energy that can be transmitted through vacuum. Ionizing radiation has sufficient energy that its interaction with atoms or molecules can separate those atoms or molecules into charged components. Ionizing radiation can cause irreparable damage to tissue, including chromosomes. Examples of ionizing radiation include neutrons, electrons, ions and electromagnetic waves from the ultraviolet, x-ray and gamma regions of the spectrum.
2. The Curie (Ci): the unit of radioactivity, defined as 3.7·1010 disintegrations/sec. For example, a calibrated mCi 210Po source will emit 3.7·104 alphas per second. Typical laboratory sources are in the micro Curie to milli Curie range. Although the radioactivity of a source alone does not give you the complete story on its biological effects, which depend on the type and energy of the emitted particles as well; if you are dealing with sources in the mCi range and above, extra caution is advisable.
3. The Rad (rad): defined as the amount of radiation that loses 100 erg/g in material that it traverses. The actual flux of particles required to achieve this degree of energy deposition is of course dependent on the type particle and its energy. For example, consider gamma-rays (at 1.172 MeV) from the decay of 60Co; the attenuation length for gamma rays at this energy in human muscle tissue is approximately 15 cm (muscle density being about 1 g/cm3). Thus, one gamma-ray will deposit on the average 1.172(1-exp(-1/15)) MeV = 0.076 MeV in the first one cm past the skin. By contrast, a 5.3 MeV alpha from 210Po decay will deposit its full 5.3 MeV within the first 0.04 cm past the skin surface.
4. The Roentgen-Equivalent-Man (rem): This is the biologically relevant unit, and is used in the measure of radiation exposure in humans. The actual density of energy deposited in tissue is given in units of rads, and it is clear that the higher the energy deposition by radiation, the greater will be the biologic effects (increased likelihood of DNA damage, for example). However, the amount of energy deposited per gram does not alone tell us the true biological "effectiveness" of the radiation; different types of particles will be more damaging than others, even though the total energy deposition is the same. Thus neutrons are approximately 10 times more damaging than gamma-rays for every erg/g deposited in human tissue. To account for these differing biological effects, the unit "rem" was established, and is defined as the amount of radiation that will produce the same biological effects as the absorption of 1 Roentgen of gamma-rays. The term "relative biological effectiveness", or "RBE", is defined for different particles and energies; it is defined by the expression (no. of rem) =(no. of rad)*RBE. The RBE for x-rays, gamma-rays and electrons is 1, while the RBE for protons and neutrons is 10, and for alphas is 20. As mentioned above, neutrons are particularly pernicious; since they are uncharged, they can penetrate quite deeply into body tissue even at low energies.
7.3 Radiation Dose Limits and Effects of Radiation
The National Committee on Radiation Protection has recommended the following radiation
dose limits for those personnel exposed to radiation sources in the course of their professional activities (see References 3 and 4 for details):
| Portion of Body | Annual Limit | Quarterly Limit |
|---|---|---|
| Whole body, gonads, Eyes, red bone marrow | 5 rem | 1.25 rem |
| Skin of whole body | 30 rem | 7.5 rem |
| Hands and forearms, feet | 75 rem | 18.75 rem |
| Other organs, tissues | 60 rem | 15 rem |
| Embryo or fetus | 0.5 rem | 0.15 rem |
The recommended annual dose limit for the general public is only 0.1 rem. A comparable amount of natural exposure occurs from background radioactivity and from cosmic rays, so this particular limit derives from the philosophy that persons should not be exposed to much more radiation than they would unavoidably get from living on the surface of the planet. This philosophy is buttressed by data that suggests that the probability of getting cancer at some point from radiation exposure is roughly proportional to one's accumulated radiation dose. (However, this is still a contentious issue.) As an example, the risk of mortality (i.e. death) from a single, 1 rad (~10 rem) neutron whole-body dose is about 0.25%.
To quantify your risk, you need to know what your exposure is. You may want to ask your dentist, for example, how much dose you get from each dental x-ray film; the usual answer, "Oh, it's a very small amount," does not allow you to quantify your risk. (In the case of dental x-rays, doses at the skin where the beam enters may be as high as 1 rem; because this is a localized dose, however, it is not nearly as significant as a 1 rem whole-body dose). It is good practice when handling radioactive materials to use a portable radiation detector to get an idea of what your potential dose rate is. Commercial radiation monitors exist for all types of particles, and are usually calibrated to give dose rates in rems. Different particles usually require different detectors; for example, a Geiger-Müller tube works for electrons and protons but not alpha particles ar neutrons. One suggested vendor for a variety of radiation monitors is Victoreen, Inc. at 216-248-9300.
Here are some typical radiation exposure rates for standard source energies and radiation types. The table assumes a 1 cm radius source is applied directly to the skin. Note that for radioactivity levels as small as 1 mCi direct exposure to the skin for as little as a few seconds (a source) is sufficient to give the maximum quarterly exposure for some body parts. Extreme care must be taken even with low level radiation sources, particularly a sources.
| Particle type | Particle energy (MeV) | Radioactivity (10-6 Curies) | Time for 1 rem exp direct skin: |
|---|---|---|---|
| g,e | 1 | 1 | 5.5 hr |
| p,n | 1 | 1 | 83 sec |
| a | 4 | 1 | 0.65 sec |
| a | 1 | 1 | 2.6 sec |
To give you an idea of what is truly dangerous, the following table gives the effects of a given whole-body radiation dose obtained within a few hours (from Ref. 2):
7.4 Laboratory Practice of Radiation Safety
The safe handling of radioactive materials involves a few clear principles:
- Minimize your exposure to all radiation! Under no circumstances should you inhale or ingest radioactive material; this means that absolutely no eating, drinking, or smoking may take place in any area in which there are radioactive sources. Wear gloves and/or other protective clothing if even a remote possibility of contamination of the hands or body exists. Subsequent ingestion of radioactivity may occur from hand-to-mouth transfer or from inhalation if these precautions are not taken. Ingested or inhaled radioactive material is far more hazardous than it is just on the skin.
- Some radiation sources are quite safe. For example, calibrated, low-activity gamma-ray sources are usually provided in sealed (plastic) disks, so that unless you deliberately cut into the protective packaging there is no danger of contamination. Other sources (of charged particles such as alphas) cannot be similarly protected because unacceptable energy degradation of the emitted particle would occur. These sources are frequently electroplated onto a foil; with age or mishandling the source material can flake off the foil, causing a potentially significant radiation hazard (primarily due to possible ingestion or inhalation). You should never touch the foil of one of these sources. The danger of irradiation is usually not the worry here; the main concern is the deposit of salty sweat you may leave on the foil, which may chemically weaken or dissolve the protective layer. If the protective foil is damaged, subsequent users might contamination ("permanent exposure") from the unprotected radioactive material itself. All such sources must be regularly checked for source integrity by the University's Radiological Health Office (100 OSH, 1-6141).
- Always have the appropriate radiation monitoring equipment on hand when using sources. Not only will you need this to determine the level of dose you are receiving during your use of the source, but you will also need this to check for accidental contamination of you and/or your work area after use of the source(s). If there is any chance of acquiring a dose >100 mrem to a major portion of the body in any calendar quarter, you may be required to wear a film badge. The badges are provided at no charge by Radiological Health but you must take their safety class to qualify for one. These badges are processed at regular intervals and tell what your integrated whole-body exposure has been.
- Keep your area safe for others: the fact that radiation sources are present, and what type they are, should be clearly labelled at the entrance to your lab, and the container holding the sources should also be clearly labelled. All sources should be clearly labelled with nuclide and activity (e.g. 1 mCi 22Na). Return sources to their storage containers when done with them; do not leave them lying on table tops, etc.. Do not take radioactive sources out of the laboratory without making a written record of the new location of the source and its intended use; this will insure that sources do not become lost. (Losing a radioactive source is a very serious affair!)
- Discard radioactive waste only according to approved methods, using properly designated and labelled containers. Never throw potentially contaminated items in trash cans! This is a very serious offense! If you have any sources whose integrity or identity is in doubt, contact the University's Radiological Health Office (1-6141) for source checks and/or disposal instructions.
- In case of an emergency: Any accident, injury, or loss of control of a radiation source that could cause an excessive or uncontrolled radiation exposure to any individual is considered a radiation emergency. The first action to take in any emergency is to provide first aid (if you are trained) to injured persons and/or prevent (further) injury. Persons should immediately leave the affected area until the extent of the radiological hazard has been determined, but should remain in the vicinity until they have been personally scanned for contamination. In case of fire or injury, first call Public Safety at 1-7944 (24 hours a day). If for some reason you cannot reach Public Safety, call Salt Lake City Emergency Services at 9-911 (24 hours a day). In the event of personal contamination, call Radiological Health at 1-6141 during normal office hours (1-7944 otherwise). Clearly state the nature of the emergency and follow all instructions; do not hang up the phone until directed to do so!
7.5 Summary of Radiation Safety Points
1. Know the identity and activity of all sources that you use, and have the appropriate radiation monitors on hand.2. Never eat, drink, or smoke when using radioactive sources, or in an area in which radioactive sources have been used.
3. Wear gloves and other protective clothing when danger of contamination exists.
4. Check hands and body for accidental contamination after use of sources.
5. Wear a film badge if there is any possibility of receiving >100 mrem per calendar quarter.
6. Never tamper with or physically modify a commercially prepared radioactive source!
7. Do not remove sources from the designated lab area unless a written record is made of their relocation.
8. Report all accidents to your supervisor, and to Radiological Health (1-6141) in cases of emergency.
7.6 References
1. G.F. Knoll, Radiation Detection and Measurement (John Wiley, 1979).
2. A.C. Melissinos, Experiments in Modern Physics, Section 4.5, (Academic Press, 1966).
3. J. Shapiro, Radiation Protection (Harvard U. Press, 1981).
4. Radiation Safety Manual of the University of Utah (available from the University's Radiological Health Office, 100 OSH). (There is a copy in the grey file cabinet in the Stockroom.)
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