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Chemistry LibreTexts

5.6: Measuring Radiation

  • Page ID
  • Skills to Develop

    • Define units for measuring radiation exposure
    • Explain the operation of common tools for detecting radioactivity
    • List common sources of radiation exposure in the US

    Several different devices are used to detect and measure radiation, including Geiger counters, scintillation counters (scintillators), and radiation dosimeters (Figure \(\PageIndex{1}\)). Probably the best-known radiation instrument, the Geiger counter (also called the Geiger-Müller counter) detects and measures radiation. Radiation causes the ionization of the gas in a Geiger-Müller tube. The rate of ionization is proportional to the amount of radiation. These type of devices measure alpha and beta radiation quite well. If a Geiger counter is altered, it can detect gamma and x-rays. A scintillation counter contains a scintillator—a material that emits light (luminesces) when excited by ionizing radiation—and a sensor that converts the light into an electric signal. Radiation dosimeters also measure ionizing radiation and are often used to determine personal radiation exposure. Commonly used types are electronic, film badge, thermoluminescent, and quartz fiber dosimeters.

    Three photographs are shown and labeled “a,” “b” and “c.” Photo a shows a Geiger counter sitting on a table. It is made up of a metal box with a read-out screen and a wire leading away from the box connected to a sensor wand. Photograph b shows a collection of tall and short vertical tubes arranged in a grouping while photograph c shows a person’s hand holding a small machine with a digital readout while standing on the edge of a roadway.

    Figure \(\PageIndex{1}\): Devices such as (a) Geiger counters, (b) scintillators, and (c) dosimeters can be used to measure radiation. (credit c: modification of work by “osaMu”/Wikimedia commons).

    A variety of units are used to measure various aspects of radiation (Table \(\PageIndex{1}\)). The becquerel (Bq), and the curie (Ci) are used to describe rate of radioactive decay. Named in honor of Henri Becquerel and the the Curies, these units do not describe the damaging effects of radiation. Early medical applications used the units of of rad or gray (Gy) to indicate energy emitted onto an absorbing material. Today, most industries rely on the sievert (Sv) or rem. These two units measure tissue damage caused by radiation. Looking at the chart below, please note the different units and what they describe. It is not necessary for you to perform conversions (with the exception of rem/sV) with units that involve activities or absorbed doses.

    Table \(\PageIndex{1}\): Units Used for Measuring Radiation
    Measurement Purpose Unit Quantity Measured Description
    activity of source becquerel (Bq) radioactive decays or emissions amount of sample that undergoes 1 decay/second
    curie (Ci)

    amount of sample that undergoes \(\mathrm{3.7 \times 10^{10}\; decays/second}\). 

    The body normally contains .1\(\mu\)Ci of C-14 radiation. 1 Ci is a huge amount of radiation.

    absorbed dose gray (Gy) energy absorbed per kg of tissue 1 Gy = 1 J/kg tissue
    radiation absorbed dose (rad)

    1 rad = 0.01 J/kg tissue

    Acute radiation sickness occurs before 100 rad.

    biologically effective dose sievert (Sv) tissue damage Sv = RBE × Gy
    roentgen equivalent for man (rem)

    Rem = RBE × rad

    An average American is exposed to around 360 mrem of radiation/yr. Acute radiation poisoning can occur at levels as low as 25 rems. Chernobyl liquidators were exposed to between 7.5x105 and 1.3x106 mrems of radiation.

    The roentgen equivalent for man (rem) is the unit for radiation damage that is used most frequently in medicine (100 rem = 1 Sv). For testing or therapy, radiation values will typically be given in mrem (millirem) or mSv (millisievert). Knowing how to convert these units will enable you to compare them to given standards. In addition, understanding the exposure limits in mrem, mSv,Sv, and uSv can give you an idea of the level of toxicity involved in nuclear reactor accidents or nuclear weapons.

    Effects of Long-term Radiation Exposure on the Human Body

    It is impossible to avoid some exposure to ionizing radiation. We are constantly exposed to background radiation from a variety of natural sources, including cosmic radiation, rocks, medical procedures, consumer products, and even our own atoms. We can minimize our exposure by blocking or shielding the radiation, moving farther from the source, and limiting the time of exposure.

    Two images are shown. The first, labeled “Rate of radioactive decay measured in becquerels or curies,” shows a red sphere with ten red squiggly arrows facing away from it in a 360 degree circle. The second image shows the head and torso of a woman wearing medical scrubs with a badge on her chest. The caption to the badge reads “Film badge or dosimeter measures tissue damage exposure in rems or sieverts” while a phrase under this image states “Absorbed dose measured in grays or rads.”

    Figure \(\PageIndex{2}\): Different units are used to measure the rate of emission from a radioactive source, the energy that is absorbed from the source, and the amount of damage the absorbed radiation does. Image used with permission (CC BY-SA 3.0; OpenStax).

    As shown in Table \(\PageIndex{2}\), the average person is exposed to background radiation, including cosmic rays from the sun and radon from uranium in the ground; radiation from medical exposure, including CAT scans, radioisotope tests, X-rays, and so on; and small amounts of radiation from other human activities, such as airplane flights (which are bombarded by increased numbers of cosmic rays in the upper atmosphere), radioactivity from consumer products, and a variety of radionuclides that enter our bodies when we breathe (for example, carbon-14) or through the food chain (for example, potassium-40, strontium-90, and iodine-131).

    Table \(\PageIndex{2}\): Average Annual Radiation Exposure (Approximate)
    Source Amount (mrem)
    radon gas 200
    medical sources 53
    radioactive atoms in the body naturally 39
    terrestrial sources 28
    cosmic sources 28
    consumer products 10
    nuclear energy 0.05
    Total 358

    The actual effects of radioactivity and radiation exposure on a person’s health depend on the type of radioactivity, the length of exposure, and the tissues exposed. Table \(\PageIndex{3}\) lists the potential threats to health at various amounts of exposure over short periods of time (hours or days). An average person receives around 400 mrems of radiation each year. This amount is equivalent to 0.400 rems and 4 mSV. Table \(\PageIndex{3}\) shows the acute toxicity level of radiation in the unit rem. Comparing yearly radiation to this table, one can get an idea of how low daily exposure levels are. Additionally, one can understand the radioactive exposure of the victims of Hiroshima and Nagasaki. Studies involving these groups of people show once exposure limits exceed 100 rem, it is most likely a person will develop some form of cancer.

    Table \(\PageIndex{3}\): Effects of Short-Term Exposure to Radioactivity and Radiation
    Exposure (rem) Effect
    1 (over a full year) no detectable effect
    ∼20 increased risk of some cancers
    ∼100 damage to bone marrow and other tissues; possible internal bleeding; decrease in white blood cell count
    200–300 visible “burns” on skin, nausea, vomiting, and fatigue
    >300 loss of white blood cells; hair loss
    ∼600 death

    Flying from New York City to San Francisco adds 5 mrem (or 0.005 rem) to your overall radiation exposure because the plane flies above much of the atmosphere, which protects us from most cosmic radiation.

    Radiation Exposure

    This 11 minute video shows how radiation can be measured. In addition, the scientists report their findings with various radiation units. While watching the video, answer the questions below to gain a better understanding of radiation around the world.

    1. Does a Geiger counter measure nonionizing radiation?
    2. At an acute level, how many Sieverts (Sv) of radiation will kill a person?
    3. Fill in the missing information of the chart below:


    Table \(\PageIndex{4}\): Who is the most radioactive?
    Substance or Location Amount of Radiation in μSv/hr
    Range of background radiation  
    Hiroshima today (event: ___________)  
    Uranium mine  
    Trinity Site (event: _____________)  
    Range of radiation while flying  
    Chernobyl (event: ___________)  
    Walking in Fukushima (event: ____________)  
    Pripyat, Ukraine hospital maximum reading  
    CT Scan  
    Fukushima resident (lifetime exposure)  
    1. What two places in Marie Curie's lab are still radioactive? What type of radiation is still lingering?
    2. Why did the residents of Chernobyl and Fukushima have black bags on the sides of their highways?
    3. In this video, what individuals are exposed to the most radiation in μSv/hr?
    1. 1 Source: US Environmental Protection Agency


    becquerel (Bq)
    SI unit for rate of radioactive decay; 1 Bq = 1 disintegration/s
    curie (Ci)
    larger unit for rate of radioactive decay frequently used in medicine; 1 Ci = 3.7 × 1010 disintegrations/s
    Geiger counter
    instrument that detects and measures radiation via the ionization produced in a Geiger-Müller tube
    gray (Gy)
    SI unit for measuring radiation dose; 1 Gy = 1 J absorbed/kg tissue
    nonionizing radiation
    radiation that speeds up the movement of atoms and molecules; it is equivalent to heating a sample, but is not energetic enough to cause the ionization of molecules
    radiation absorbed dose (rad)
    SI unit for measuring radiation dose, frequently used in medical applications; 1 rad = 0.01 Gy
    radiation dosimeter
    device that measures ionizing radiation and is used to determine personal radiation exposure
    relative biological effectiveness (RBE)
    measure of the relative damage done by radiation
    roentgen equivalent man (rem)
    unit for radiation damage, frequently used in medicine; 1 rem = 1 Sv
    scintillation counter
    instrument that uses a scintillator—a material that emits light when excited by ionizing radiation—to detect and measure radiation
    sievert (Sv)
    SI unit measuring tissue damage caused by radiation; takes into account energy and biological effects of radiation