We have no way to detect radiation with our senses – unlike so many hazards we’re familiar with, it’s invisible, tasteless, odorless; neither can we feel it nor hear it. The only way that we have to tell whether or not we’re in the presence of a radiological hazard – or even a radiological inconvenience – is by using instruments. But even here we’re not off the hook because there are a plethora of radiation instruments and if you use the wrong one then you might still miss detecting what you’re looking for.
One example comes to mind – a researcher at a university I used to work for. He was working with an isotope that emits low-energy gamma radiation and he was surveying with a Geiger counter (specifically, a pancake-type GM detector like the one shown here). Unfortunately, this type of detector simply can’t detect low-energy gammas and even though he had flawless survey technique he failed to detect the contamination he’d spread around his lab. And as a result, a number of researchers became contaminated. There are all sorts of similar stories but, rather than regaling you with them, let me give you a little background information and then explain how to use this information to select an appropriate radiation instrument.
Types of radiation:
Alpha particles are high-energy particles that can only penetrate a few microns into tissue and only a mm or so in air. Once they contact living cells they can cause massive damage, but they can be shielded with tissue paper.
Beta particles are high-energy electrons that can penetrate up to 1 cm in tissue and up to 5-6 meters in air. They cause only minor amounts of damage to cells that they hit.
Gamma rays are high-energy photons that can penetrate through the whole body. Like beta particles, they cause only minor damage to cells that they hit.
Neutrons are sub-atomic particles that can have a wide range of energies. They can penetrate the whole body, they can cause massive cell damage, and they can cause material to become radioactive.
Types of surveys:
Radiation surveys are performed to measure radiation dose rates, which are a measure of energy deposited in an object. These are used to identify possible health risks, to demonstrate regulatory compliance, and to look for the presence of unexpected radioactive materials. Radiation surveys can also be used to check for leaks in radiation shielding. Radiation dose rates are measured in units of gray (Gy) and Sievert (Sv) internationally and in units of rad (R), roentgen (r), or rem in the US.
Contamination surveys are used to survey for the presence of radioactive materials in a place that they’re not expected to be (i.e. on a laboratory workbench following an experiment). Contamination surveys can be used to identify leaking radioactive sources, sloppy work practices, to check for contamination on persons exiting contaminated areas, and so forth. Radioactive contamination is measured in terms of count rates – counts per second or counts per minute.
Radionuclide identification is performed in order to identify specific radionuclides present in a source or as contamination. Every radionuclide emits radiation with a particular “fingerprint” of radiation energy. For example, radioactive cesium-137 always emits a gamma ray with an energy of 662,000 electron volts (abbreviated 662 keV).
Nuclide identification can be performed with alpha, beta, and gamma radiation although it can only be done easily for gamma radiation. An example of a gamma spectrum is shown here with a number of gamma peaks identified – in this example, the Ba-133 and Cs-137 peaks are artificial radionuclides and the others are naturally occurring.
Types of radiation detectors:
Geiger-Mueller tubes (also called Geiger counters or GM tubes) are gas-filled detectors that are sensitive to medium- to high-energy alpha, beta, and gamma radiation. They have a high sensitivity to radiation, but they cannot distinguish between radiation energies. This means that they have limited utility for measuring radiation dose rate unless they are a type called “energy-compensated” GM detectors.
Sodium iodide detectors are crystals used to detect gamma radiation. They have a moderate sensitivity to gamma radiation and cannot detect alpha or beta radiation at all. They can also distinguish the different radiation energies so they can be used to identify gamma-emitting radionuclides by characterizing the unique “fingerprint” of gamma rays emitted by a particular radionuclide. Sodium iodide detectors can be designed for specific purposes – for example, there are detectors that can only detect low-energy gamma radiation, that can be used to measure radiation dose rate, or that can be used for nuclide identification.
Ionization chambers (and pressurized ionization chambers) are gas-filled detectors used to measure radiation dose rates. They are sensitive to beta and gamma radiation, but will usually have a beta shield that can be slid aside to make beta measurements. They are most accurate for measuring radiation dose rate and are poor at measuring contamination.
Zinc sulfide crystals are used to detect alpha radiation; they cannot detect beta or gamma radiation.
Neutron detectors are used to detect neutron radiation. There are neutrons found in nature, emitted from some types of radioactive sources, or emitted from nuclear reactors. Some linear accelerators also produce neutrons, as do nuclear weapons. There are a number of materials that can be used to detect neutrons, including helium-3 (He-3), boron tri-fluoride (BF3), uranium-235 (U-235), and some forms of lithium. Most neutron detectors cannot be used to measure any other form of radiation.
Selecting a radiation detector
You need to match the type of instrument and detector you’re using to the type of radiation you’re looking for and the type of survey you’re preforming. So, for example, the scientist I mentioned earlier should have known that he was unable to detect low-energy gamma radiation with a GM “pancake” probe and he should have used a sodium iodide detector designed to pick up low-energy gamma radiation. Here’s a table that should help you to sort this all out.
|Type of survey||Type of radiation||Type of detector or instrument||Comments|
|Contamination||Alpha||Zinc sulfide||We normally don’t worry about neutron contamination – neutron-emitting substances tend to also emit alpha, beta, or gamma radiation as well|
|Radiation||Beta||Ion chamber||Alpha radiation has such a short range in air that we don’t consider alpha radiation to have a dose rate and this is not measured|
|Neutron||Specialized neutron detector|
|Nuclide ID||Gamma||Sodium iodide
|Nuclide ID can only be performed in the field for gamma-emitting radionuclides. Other types of radiation must be identified in the laboratory|