Detecting and Measuring Radiation
Radiation has no odor or property that allows it to be seen. However, there are many types of devices used to measure it. Some devices are qualitative – they simply tell you that radiation is present but don’t tell you anything about the rate at which you are being exposed to the radiation or the amount of radiation present. Some are quantitative – they might indicate the rate of radiation exposure, the quantity, or both. Although there are many types of devices, the basic requirement for measurement is that the radiation must be able to interact with the detector. If the radiation cannot get in to the detection chamber because it doesn’t have enough energy or the chamber wall is too thick, then it won’t get measured. This section will discuss only those devices commonly used to measure beta and gamma emitting radionuclides.
Local emergency responders will have some of these devices when they respond to incidents that might involve radioactive material. Your local department of health will use one or more of these devices to analyze samples taken from around nuclear power plants to determine if any radioactive materials are being released.
Geiger-Mueller (GM) Meter
The GM survey meter is the most common meter used for the detection of radioactive contamination. You might hear this meter referred to as a “pancake” or “GM pancake” because of the look of the device. A GM meter is often portable, and the efficiency of a GM for detecting radiation depends on four factors:
- The type of radiation.
- The energy of the radiation.
- The amount of activity in the source.
- The direction of the source relative to the instrument detector active area.
If a beta particle or gamma ray can enter the active portion of a GM meter and interact, it can be detected. Because the detection efficiency of the GM meter is dependent upon the energy of the particle or ray, the efficiency of a GM meter is much higher for detecting beta particles than it is for detecting gamma rays. Beta particles do not travel far, so if they are able to enter the active portion of the meter, they will be absorbed. Gamma rays, on the other hand, have enough energy to enter the active portion, but still may exit without interacting.
Scintillation Detector (routinely called a “sodium iodide”)
A scintillation detector is often portable. The scintillation detector is not as versatile as the GM meter although it can be used to look for contamination from some radioactive materials. The scintillation detector's active portion for detecting radioactivity is a solid crystal (that is the scintillator) with which the radioactive emission must interact. This essentially limits use of the detector to gamma rays since beta particles cannot penetrate the crystal and, therefore, cannot interact.
Scintillators can be made in different sizes and the size of the scintillator determines its ability to absorb and detect certain gamma-ray energies. A thin scintillator is an excellent choice for low energy gamma rays. The gamma ray will be absorbed within the thin scintillator and the light produced by this interaction will be able to pass through the remaining thickness to allow the gamma ray to be detected. A high-energy gamma ray is likely to pass right through the thin scintillator without interacting.
A thick scintillator is the choice for radionuclides emitting high-energy gamma rays. This scintillator is thick enough to absorb the gamma ray but not too thick to prevent the light that is produced from being detected. A thick scintillator is not very good for low-energy gamma rays; they will interact but the scintillator is too thick and will absorb the light that is produced before it can be detected.
Ionization Chambers
An ionization chamber (or ion chamber) is often portable. An ion chamber is used to measure the rate of radiation exposure (how much radiation exposure is being received in a specified period of time; mrem per hour). The ability of the ion chamber to measure the dose rate from a radionuclide is based upon the ability of the emission to reach the active portion of the meter and the energy of the emission. Ion chambers are used where there is exposure to or potential for exposure to x and gamma rays.
Liquid Scintillation Counters
A liquid scintillation counter generally is not portable. Liquid scintillation counting is the most widely used technique for the detection and quantification of radioactivity. This measurement technique is applicable to all types of emissions though it is most often used for beta particles. Liquid scintillation counting is an analytical technique that measures activity of radionuclides from the rate of light photons emitted by a sample.
Samples to be counted are prepared by adding a scintillation fluid (cocktail) to the beta emitter. As beta particles are released and interact with the fluid, photons of light are produced and measured. The intensity of the light is proportional to the energy of the beta particle; therefore, the spectra for different-energy beta emitters are somewhat unique.
Gamma Counters
A gamma counter often is not portable. Gamma counters do just as the name implies - quantify the activity of a gamma-emitting sample. In principle, the gamma counter is like a scintillation detector with the exception that with a gamma counter the scintillation crystal surrounds the sample. The gamma rays interact with the crystal, are absorbed, and produce light. As with the scintillation detector survey meter, if the energy of the interacting photons is too low, the photons of light that are produced may be absorbed in the scintillation crystal and never be detected. If the energy is too high, the incoming photons may just pass right through without interacting. The thickness of the crystal is critical and the efficiency of the instrument for detecting certain gamma-ray energies is dependent upon the thickness of the crystal.
Personal Radiation Monitors (“Badges)
Personal radiation monitors are often referred to as “badges” or “film badges.” Some types of these monitors still use film while others use a small, thin crystal or even some special powder. All of these devices are used to measure the amount of radiation to which someone has been exposed, usually in an occupational setting (e.g., an x-ray technologist in a hospital).
The degree of darkening on a film badge tells us how much radiation exposure there was, the amount of light given off when the crystal is heated tells us how much radiation exposure there was, and the amount of light given off when the powder is excited by a certain wavelength laser tells us how much exposure there was.
These monitors generally are issued to people exposed to penetrating radiation such as x-rays and medium to high-energy gamma rays. They are not routinely issued to people working with radionuclides in academic research laboratories for two reasons: the activity of the radioactive materials used is too small to measure external exposure and the most commonly used radionuclides are not external radiation exposure hazards (many are low-energy beta emitters).
Urine Bioassays
We can monitor individuals who work with certain beta emitters with a urine bioassay if there is reason to believe we need to look for internal contamination. A urine sample measured on a liquid scintillation counter can indicate whether the worker has taken some radioactive material internally. Nearly 100 percent of some beta emitters are excreted in the urine, for others it may be just a few percent. The excretion of the radionuclide into the urine is dependent upon the chemical form of the radionuclide, how it is metabolized, and the half-life of the radionuclide. Internal contamination with some gamma emitters can also be measured in this manner; however, the next two paragraphs discuss other techniques more commonly used.
Thyroid Assay
Thyroid counting is performed for determination of internal radioactive iodine uptake. This is a noninvasive test. Radioactive iodine when taken into the body seeks out the thyroid gland and deposits there. A thyroid count generally consists of the placement of a scintillation detector in front of the thyroid gland for one or more minutes with the resultant count indicating the presence or absence of radioactive iodine.
Whole-Body Assay
Whole-body counting is performed to detect the presence of medium- to high-energy gamma-emitting radionuclides within the body. This is noninvasive. Whole-body counting can also be used to determine the distribution of a gamma emitter throughout the body. A whole-body counter may be a chair, table, or standing type. In nearly all cases, individuals being counted have a series of detectors near them (usually scintillation detectors) that will detect the gamma photons being emitted from the radionuclide source in their body.