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:
Geiger Counter, with Geiger-Mueller (GM) Tube or Probe A GM 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). GM probes (e.g., "pancake" type) are most often used with handheld radiation survey instruments for contamination measurements. However, energy-compensated GM tubes may be employed for exposure measurements. Further, often the meters used with a GM 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.
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.
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.
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.)
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 GM 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.
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 CR-39 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:
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.
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.
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.