What is Radiation?
Radiation is energy that comes from a source and travels through space and may be able to penetrate various materials. Light, radio, and microwaves are types of radiation that are called nonionizing. The kind of radiation discussed in this document is called ionizing radiation because it can produce charged particles (ions) in matter.
Ionizing radiation is produced by unstable atoms. Unstable atoms differ from stable atoms because unstable atoms have an excess of energy or mass or both. Radiation can also be produced by high-voltage devices (e.g., x-ray machines).
Unstable atoms are said to be radioactive. In order to reach stability, these atoms give off, or emit, the excess energy or mass. These emissions are called radiation. The kinds of radiation are electromagnetic (like light) and particulate (i.e., mass given off with the energy of motion). Gamma radiation and x rays are examples of electromagnetic radiation. Gamma radiation originates in the nucleus while x rays come from the electronic part of the atom. Beta and alpha radiation are examples of particulate radiation.
Interestingly, there is a "background" of natural radiation everywhere in our environment. It comes from space (i.e., cosmic rays) and from naturally occurring radioactive materials contained in the earth and in living things.
Radiation Exposure from Various Sources
What Types of Radiation are There?
The radiation one typically encounters is one of four types: alpha radiation, beta radiation, gamma radiation, and x radiation. Neutron radiation is also encountered in nuclear power plants and high-altitude flight and is emitted from some industrial radioactive sources.
How is Radiation Measured?
In the United States, radiation absorbed dose, dose equivalent, and exposure are often measured and stated in the older units called rad, rem, or roentgen (R), respectively. For practical purposes with gamma and x rays, these units of measure for exposure or dose are considered equal. This exposure can be from an external source irradiating the whole body, an extremity, or other organ or tissue resulting in an external radiation dose. Alternately, internally deposited radioactive material may cause an internal radiation dose to the whole body or other organ or tissue.
Smaller fractions of these measured quantities often have a prefix, e.g., milli (m) means 1/1,000. For example, 1 rad = 1,000 mrad. Micro (μ) means 1/1,000,000. So, 1,000,000 μrad = 1 rad, or 10 μR = 0.000010 R.
The International System of Units (SI) for radiation measurement is now the official system of measurement and uses the "gray" (Gy) and "sievert" (Sv) for absorbed dose and equivalent dose respectively. Conversions are as follows:
With radiation counting systems, radioactive transformation events can be measured in units of "disintegrations per minute" (dpm) and, because instruments are not 100% efficient, "counts per minute" (cpm). Background radiation levels are typically less than 10 μR per hour, but due to differences in detector size and efficiency, the cpm reading on fixed monitors and various handheld survey meters will vary considerably.
How Much Radioactive Material Is Present?
The size or weight of a quantity of material does not indicate how much radioactivity is present. A large quantity of material can contain a very small amount of radioactivity, or a very small amount of material can have a lot of radioactivity.
For example, uranium-238, with a 4.5-billion-year half-life, has only 0.00015 curies of activity per pound, while cobalt-60, with a 5.3-year half-life, has nearly 513,000 curies of activity per pound. This "specific activity," or curies per unit mass, of a radioisotope depends on the unique radioactive half-life and dictates the time it takes for half the radioactive atoms to decay.
In the United States, the amount of radioactivity present is traditionally determined by estimating the number of curies (Ci) present. The more curies present, the greater amount of radioactivity and emitted radiation.
Common fractions of the curie are the millicurie (1 mCi = 1/1,000 Ci) and the microcurie (1 μCi = 1/1,000,000 Ci). In terms of transformations per unit time, 1 μCi = 2,220,000 dpm.
The SI system uses the unit of becquerel (Bq) as its unit of radioactivity. One curie is 37 billion Bq. Since the Bq represents such a small amount, one is likely to see a prefix noting a large multiplier used with the Bq as follows:
How Can You Detect Radiation?
Radiation cannot be detected by human senses. A variety of handheld and laboratory instruments is available for detecting and measuring radiation. The most common handheld or portable instruments are:
The most common laboratory instruments are:
How Can You Keep Radiation Exposure Low and Measure It?
Although some radiation exposure is natural in our environment, it is desirable to keep radiation exposure as low as reasonably achievable (ALARA) in an occupational setting. This is accomplished by the techniques of time, distance, and shielding.
Time: The shorter the time in a radiation field, the less the radiation exposure you will receive. Work quickly and efficiently. Plan your work before entering the radiation field.
Distance: The farther a person is from a source of radiation, the lower the radiation dose. Levels decrease by a factor of the square of the distance. Do not touch radioactive materials. Use remote handling devices, etc., to move materials to avoid physical contact.
Shielding: Placing a radioactive source behind a massive object provides a barrier that can reduce radiation exposure.
Administrative and Engineering Controls: The use of administrative and engineering controls is essential for keeping radiation exposure ALARA.
Monitoring occupational radiation exposure is a fundamental aspect of radiation protection. This can be done by measuring radiation fields with handheld instruments described above and, if exposure conditions are predictable and relatively low (i.e., less than 10% of the regulatory limit), expected exposures can be calculated and documented. Alternately, regular radiation field survey measurements can be performed, and personnel dosimeters are issued to workers.
Film Badge A film badge is one of the earliest devices used to measure worker exposure to gamma radiation from radium and x rays. Initially packets of dental x-ray film were worn and developed periodically to view the degree of darkening. Later special metal filters were used in a x-ray film holder, with an open window to provide unattenuated film area for high-energy beta measurement. With appropriate calibration of exposure versus optical density, these devices provide an accurate measure of worker external exposure and a permanent record.
Thermoluminescent Dosimeter (TLD) Badge The TLD badge is a personnel monitoring device with special chemical compounds (e.g., lithium fluoride) in powder of solid form that retain deposited energy from radiation exposure. These TL materials emit light when subsequently heated in a reader. The light is detected by a PMT, and through calibration the electrical current provides a proportional measure of radiation exposure. However once read out, the signal from these devices is erased for the most part. Thus, quality control on measurements must be to the strictest standards. The National Institute of Standards and Technology (NIST) has developed a national voluntary laboratory accreditation program (NAVLAP) for all external dosimetry (e.g., film, TLD) processors. Cross-checks, reviewed procedures, on-site inspections, etc., all provide assurance that dosimeter results are of the highest quality.
Optically Stimulated Luminescence (OSL) Badge The OSL dosimeter/reader technology is relatively new and uses a laser to stimulate an aluminum oxide material that was in the badge for personnel radiation monitoring of occupationally exposed workers. With optically stimulated luminescence, a tiny crystal traps and stores energy from exposure to ionizing radiation fields. The amount of exposure can be determined by shining a green light on the crystal and measuring the intensity of the blue light emitted. OSL systems allow instantaneous readings that can be repeated, as opposed to TLDs which take 20 or 30 seconds for a one-time-only reading.
Pocket Ionization (Ion) Chamber This is a sealed cylindrical air-filled chamber, sometimes called a direct reading dosimeter (DRD) or quartz fiber dosimeter (QFD), with a charged quartz fiber that can be directly viewed through a built-in microscope. This filament can be seen against a scale from typically 0 to 200 milliroentgen or 0 to 5 R. Ionizing gamma radiation passing through the chamber causes a discharge of the device and a deflection of the fiber upscale. When properly manufactured, maintained, and calibrated, these devices provide a fairly accurate direct measure of external exposure. In the late 1980s a thin-walled type was introduced that was more sensitive to diagnostic-energy x rays. The advantage of these DRDs or QFDs is instantaneous indication of radiation exposure. However, they are fragile devices and subject to leakage. Frequent calibration and leakage checks are recommended, as well as the use of two dosimeters side by side. Readings that do not reasonably match should be suspect.
Electronic Dosimeters Electronic dosimeters have been available since the early 1980s. These devices use energy-compensated Geiger-Mueller tubes or solid-state detectors with supporting electronics in a package typically the size of a deck of playing cards. Features vary with respect to size, ruggedness, user control, display of accumulated dose and/or dose rate, alarm set point, battery life, computer interface, etc.
What is Radioactive Contamination?
If radioactive material is not in a sealed source container, it might be spread onto other objects. Contamination occurs when material that contains radioactive atoms is deposited on materials, skin, clothing, or any place where it is not desired. It is important to remember that radiation does not spread or get "on" or "in" people; rather, it is radioactive contamination that can be spread. A person contaminated with radioactive material will receive radiation exposure until the source of radiation (the radioactive material) is removed.
How Can You Work Safely Around Radiation or Contamination?
You can work safely around radiation and/or contamination by following a few simple precautions:
Is it Safe to Be Around Sources of Radiation?
A single high-level radiation exposure (i.e., greater than 10,000 mrem) delivered to the whole body over a very short period of time may have potential health risks. From follow-up of the atomic bomb survivors, we know acutely delivered very high radiation doses can increase the occurrence of certain kinds of disease (e.g., cancer) and possibly negative genetic effects. To protect the public and radiation workers (and environment) from the potential effects of chronic low-level exposure (i.e., less than 10,000 mrem), the current radiation safety practice is to prudently assume similar adverse effects are possible with low-level protracted exposure to radiation. Thus, the risks associated with low-level medical, occupational, and environmental radiation exposure are conservatively calculated to be proportional to those observed with high-level exposure. These calculated risks are compared to other known occupational and environmental hazards, and appropriate safety standards and policies have been established by international and national radiation protection organizations (e.g., International Commission on Radiological Protection and National Council on Radiation Protection and Measurements) to control and limit potential harmful radiation effects.
Both public and occupational regulatory dose limits are set by federal agencies (i.e., Environmental Protection Agency, Nuclear Regulatory Commission, and Department of Energy) and state agencies (e.g., agreement states) to limit cancer risk. Other radiation dose limits are applied to limit other potential biological effects with workers' skin and lens of the eye.
The information and material posted on this website is intended as general reference information only. Specific facts and circumstances may alter the concepts and applications of materials and information described herein. The information provided is not a substitute for professional advice and should not be relied upon in the absence of such professional advice specific to whatever facts and circumstances are presented in any given situation. Answers are correct at the time they are posted on the Website. Be advised that over time, some requirements could change, new data could be made available, or Internet links could change. For answers that have been posted for several months or longer, please check the current status of the posted information prior to using the responses for specific applications.
|This page last updated 27 August 2011. Site Map | Privacy Statement | Disclaimer | Webmaster|