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Radiation Basics

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

Source Exposure
External Background Radiation 0.60 mSv y-1, U.S. Average
Natural K-40 and Other Radioactivity in Body 0.4 mSv y-1
Air Travel Round Trip (NY-LA) 0.05 mSv
Chest X-Ray Effective Dose 0.10 mSv per view
Radon in the Home 2.00 mSv y-1 (variable)
Man-Made (medical x rays, etc.) 0.60 mSv y-1 (average)

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.

  • Alpha Radiation
    Alpha radiation is a heavy, very short-range particle and is actually an ejected helium nucleus. Some characteristics of alpha radiation are:
    1. Most alpha radiation is not able to penetrate human skin.
    2. Alpha-emitting materials can be harmful to humans if the materials are inhaled, swallowed, or absorbed through open wounds.
    3. A variety of instruments has been designed to measure alpha radiation. Special training in the use of these instruments is essential for making accurate measurements.
    4. A thin-window Geiger-Mueller (GM) probe can detect the presence of alpha radiation.
    5. Instruments cannot detect alpha radiation through even a thin layer of water, dust, paper, or other material, because alpha radiation is not penetrating.
    6. Alpha radiation travels only a short distance (a few inches) in air, but is not an external hazard.
    7. Alpha radiation is not able to penetrate clothing.
    Examples of some alpha emitters: radium, radon, uranium, thorium.
  • Beta Radiation
    Beta radiation is a light, short-range particle and is actually an ejected electron. Some characteristics of beta radiation are:
    1. Beta radiation may travel several feet in air and is moderately penetrating.
    2. Beta radiation can penetrate human skin to the "germinal layer," where new skin cells are produced. If high levels of beta-emitting contaminants are allowed to remain on the skin for a prolonged period of time, they may cause skin injury.
    3. Beta-emitting contaminants may be harmful if deposited internally.
    4. Most beta emitters can be detected with a survey instrument and a thin-window G-M probe (e.g., "pancake" type). Some beta emitters, however, produce very low-energy, poorly penetrating radiation that may be difficult or impossible to detect. Examples of these difficult-to-detect beta emitters are hydrogen-3 (tritium), carbon-14, and sulfur-35.
    5. Clothing provides some protection against beta radiation.
    Examples of some pure beta emitters: strontium-90, carbon-14, tritium, and sulfur-35.
  • Gamma and X  Radiation
    Gamma radiation and x rays are highly penetrating electromagnetic radiation. Some characteristics of these radiations are:
    1. Gamma radiation or x rays are able to travel many feet in air and many inches in human tissue. They readily penetrate most materials and are sometimes called "penetrating" radiation.  
    2. X rays are like gamma rays. X rays, too, are penetrating radiation. Sealed radioactive sources and machines that emit gamma radiation and x rays, respectively, constitute mainly an external hazard to humans.
    3. Gamma radiation and x rays are electromagnetic radiation like visible light, radiowaves, and ultraviolet light. These electromagnetic radiations differ only in the amount of energy they have. Gamma rays and x rays are the most energetic of these.
    4. Dense materials are needed for shielding from gamma radiation. Clothing provides little shielding from penetrating radiation, but will prevent contamination of the skin by gamma-emitting materials.
    5. Gamma radiation is easily detected by survey meters with a sodium iodide detector probe.
    6. Gamma radiation and/or characteristic x rays frequently accompany the emission of alpha and beta radiation during radioactive decay.
    Examples of some gamma emitters: iodine-131, cesium-137, cobalt-60, radium-226, and technetium-99m.

How Is Radiation Measured?

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.

In the United States, radiation absorbed dose, dose equivalent, and exposure used to be measured and stated in traditional 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. 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 Sv = 1,000 mSv. Micro (μ) means 1/1,000,000. So, 1 Sv = 1,000,000 μSv.

Conversions are as follows:

  • 1 Gy = 100 rad
  • 1 mGy = 100 mrad
  • 1 Sv = 100 rem
  • 1 mSv = 100 mrem

With radiation counting systems, radioactive transformation events can be measured in units of "disintegrations per minute" (dpm) and, because instruments are not 100 percent efficient, "counts per minute" (cpm). Background radiation levels are typically less than 0.10 μSv 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 5.5 MBq of activity per pound, while cobalt-60, with a 5.3-year half-life, has nearly 19,000 TBq 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.

The SI system uses the unit of becquerel (Bq) as its unit of radioactivity. The older, traditional unit previously used in the United States is the curie (Ci).

Common multiples of the becquerel are the megabecquerel (1 MBq = 1,000,000 Bq) and the gigabecquerel (1 GBq = 1,000,000,000 Bq).

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:

  • 37 GBq = 37 billion Bq = 1 curie
  • 1 MBq = 1 million Bq = ~ 27 microcuries
  • 1 GBq = 1 billion Bq = ~ 27 millicuries
  • 1 TBq = 1 trillion Bq = ~ 27 curies

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:

  1. Geiger Counter, with Geiger-Mueller (G-M) Tube or Probe. A G-M tube is a gas-filled device that, when a high voltage is applied, creates an electrical pulse when radiation interacts with the wall or gas in the tube. These pulses are converted to a reading on the instrument meter. If the instrument has a speaker, the pulses also give an audible click. Common readout units are roentgens per hour (R/hr), milliroentgens per hour (mR/hr), rem per hour (rem/hr), millirem per hour (mrem/hr), and counts per minute (cpm). G-M probes (e.g., "pancake" type) are most often used with handheld radiation survey instruments for contamination measurements. However, energy-compensated G-M tubes may be employed for exposure measurements. Further, often the meters used with a G-M probe will also accommodate other radiation-detection probes. For example, a zinc sulfide (ZnS) scintillator probe, which is sensitive to just alpha radiation, is often used for field measurements where alpha-emitting radioactive materials need to be measured.
  2. MicroR Meter, with Sodium Iodide Detector. A solid crystal of sodium iodide creates a pulse of light when radiation interacts with it. This pulse of light is converted to an electrical signal by a photomultiplier tube (PMT), which gives a reading on the instrument meter. The pulse of light is proportional to the amount of light and the energy deposited in the crystal. These instruments most often have upper and lower energy discriminator circuits and, when used correctly as single-channel analyzers, can provide information on the gamma energy and identify the radioactive material. If the instrument has a speaker, the pulses also give an audible click, a useful feature when looking for a lost source. Common readout units are microroentgens per hour (μR/hr) and/or counts per minute (cpm). Sodium iodide detectors can be used with handheld instruments or large stationary radiation monitors. Special plastic or other inert crystal "scintillator" materials are also used in place of sodium iodide.
  3. Portable Multichannel Analyzer. A sodium iodide crystal and PMT described above, coupled with a small multichannel analyzer (MCA) electronics package, are becoming much more affordable and common. When gamma-ray data libraries and automatic gamma-ray energy identification procedures are employed, these handheld instruments can automatically identify and display the type of radioactive materials present. When dealing with unknown sources of radiation, this is a very useful feature.
  4. Ionization (Ion) Chamber. This is an air-filled chamber with an electrically conductive inner wall and central anode and a relatively low applied voltage. When primary ion pairs are formed in the air volume, from x-ray or gamma radiation interactions in the chamber wall, the central anode collects the electrons and a small current is generated. This in turn is measured by an electrometer circuit and displayed digitally or on an analog meter. These instruments must be calibrated properly to a traceable radiation source and are designed to provide an accurate measure of absorbed dose to air which, through appropriate conversion factors, can be related to dose to tissue. In that most ion chambers are "open air," they must be corrected for change in temperature and pressure. Common readout units are milliroentgens and roentgen per hour (mR/hr or R/hr). (Note: For practical purposes, consider the roentgen, rad, and the rem to be equal with gamma or x rays. So, 1 mR/hr is equivalent to 1 mrem/hr.)
  5. Neutron REM Meter, with Proportional Counter. A boron trifluoride or helium-3 proportional counter tube is a gas-filled device that, when a high voltage is applied, creates an electrical pulse when a neutron radiation interacts with the gas in the tube. The absorption of a neutron in the nucleus of boron-10 or helium-3 causes the prompt emission of a helium-4 nucleus or proton respectively. These charged particles can then cause ionization in the gas, which is collected as an electrical pulse, similar to the G-M tube. These neutron-measuring proportional counters require large amounts of hydrogenous material around them to slow the neutron to thermal energies. Other surrounding filters allow an appropriate number of neutrons to be detected and thus provide a flat-energy response with respect to dose equivalent. The design and characteristics of these devices are such that the amount of secondary charge collected is proportional to the degree of primary ions produced by the radiation. Thus, through the use of electronic discriminator circuits, the different types of radiation can be measured separately. For example, gamma radiation up to rather high levels is easily rejected in neutron counters.
  6. Radon Detectors. A number of different techniques are used for radon measurements in home or occupational settings (e.g., uranium mines). These range from collection of radon decay products on an air filter and counting, exposing a charcoal canister for several days and performing gamma spectroscopy for absorbed decay products, exposure of an electret ion chamber and read-out, and long-term exposure of CR39 plastic with subsequent chemical etching and alpha track counting. All these approaches have different advantages and disadvantages which should be evaluated prior to use.

The most common laboratory instruments are:

  1. Liquid Scintillation Counters. A liquid scintillation counter (LSC) is a traditional laboratory instrument with two opposing PMTs that view a vial that contains a sample and liquid scintillator fluid, or cocktail. When the sample emits a radiation (often a low-energy beta) the cocktail itself, being the detector, causes a pulse of light. If both PMTs detect the light in coincidence, the count is tallied. With the use of shielding, cooling of PMTs, energy discrimination, and this coincidence counting approach, very low background counts can be achieved, and thus low minimum detectable activities (MDA). Most modern LSC units have multiple sample capability and automatic data acquisition, reduction, and storage.
  2. Proportional Counter.— A common laboratory instrument is the standard proportional counter with sample counting tray and chamber and argon/methane flow through counting gas. Most units employ a very thin (microgram/cm2) window, while some are windowless. Shielding and identical guard chambers are used to reduce background and, in conjunction with electronic discrimination, these instruments can distinguish between alpha and beta radiation and achieve low MDAs. Similar to the LSC units noted above, these proportional counters have multiple sample capability and automatic data acquisition, reduction, and storage. Such counters are often used to count smear/wipe or air filter samples. Additionally, large-area gas flow proportional counters with thin (milligram/cm2) mylar windows are used for counting the whole body and extremities of workers for external contamination when exiting a radiological control area.
  3. Multichannel Analyzer System.— A laboratory MCA with a sodium iodide crystal and PMT (described above), a solid-state germanium detector, or a silicon-type detector can provide a powerful and useful capability for counting liquid or solid matrix samples or other prepared extracted radioactive samples. Most systems are used for gamma counting, while some silicon detectors are used for alpha radiation. These MCA systems can also be utilized with well-shielded detectors to count internally deposited radioactive material in organs or tissue for bioassay measurements. In all cases, the MCA provides the capability to bin and tally counts by energy and thus identify the emitter. Again, most systems have automatic data acquisition, reduction, and storage capability.

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.

  • A person is externally contaminated if radioactive material is on the skin or clothing.
  • A person is internally contaminated if radioactive material is breathed in, swallowed, or absorbed through wounds.
  • The environment is contaminated if radioactive material is spread about or is unconfined.

How Can You Work Safely Around Radiation or Contamination?

You can work safely around radiation and/or contamination by following a few simple precautions:

  1. Use time, distance, shielding, and containment to reduce exposure.
  2. Wear dosimeters (e.g., film or TLD badges) if issued.
  3. Avoid contact with the contamination.
  4. Wear protective clothing that, if contaminated, can be removed.
  5. Wash with nonabrasive soap and water any part of the body that may have come in contact with the contamination.
  6. Assume that all materials, equipment, and personnel that came in contact with the contamination are contaminated. Radiological monitoring is recommended before leaving the scene.

Is It Safe to Be Around Sources of Radiation?

A single high-level radiation exposure (i.e., greater than 100 mSv) 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 100 mSv), 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.

Annual Radiation Dose Limits Agency
Radiation Worker - 50 mSv (NRC, "occupationally" exposed)
General Public - 1 mSv (NRC, member of the public)
General Public - 0.25 mSv (NRC, D&D all pathways)
General Public - 0.10 mSv (EPA, air pathway)
General Public - 0.04 mSv (EPA, drinking-water pathway)

Updated by Kelly Classic, Medical Health Physicist
October 2013

The information posted on this web page is intended as general reference information only. Specific facts and circumstances may affect the applicability of concepts, 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. To the best of our knowledge, answers are correct at the time they are posted. Be advised that over time, requirements could change, new data could be made available, and Internet links could change, affecting the correctness of the answers. Answers are the professional opinions of the expert responding to each question; they do not necessarily represent the position of the Health Physics Society.
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