One of the major potential threats facing water and wastewater facilities is contamination by radioactive substances. Radioactive substances brought on-site at a facility could be used to contaminate the facility, thereby preventing workers from safely entering the facility to perform necessary water treatment tasks. In addition, radioactive substances brought on-site at a water treatment plant could be discharged into the water source or the distribution system, contaminating the downstream water supply. Therefore, detection of radioactive substances being brought on-site can be an important security enhancement.

The basic principles of radiation and radiation detection are described in the Radiation Detection Equipment Product Guide. As described in that document, different radionuclides have unique properties, and different equipment is required to detect different types of radiation. However, as is also discussed in that document, it is impractical and potentially unnecessary to monitor for specific radionuclides being brought on-site Instead, for security purposes, it may be more useful to monitor for gross radiation as an indicator of unsafe substances. An expanded discussion of the pluses and minuses of monitoring for gross radiation vs. specific radionuclides can be found in the document cited above.

In order to protect against these radioactive materials being brought on-site, a facility may set up monitoring sites outfitted with radiation detection instrumentation at entrances to the facility. Depending on the specific types of equipment chosen, this equipment would detect radiation emitted from people, packages, or other objects being brought through an entrance. Specific discussions regarding the differences in implementation/detection and effectiveness of the different types of monitoring equipment are provided under the Attributes and Features section below.

One of the primary differences between the different types of detection equipment is the means by which the equipment reads the radiation. Radiation may either be detected by direct measurement or through sampling.

Direct radiation measurement involves measuring radiation through an external probe on the detection instrumentation. Some direct measurement equipment detects radiation emitted into the air around the monitored object. Because this equipment detects radiation in the air, it does not require that the monitoring equipment make physical contact with the monitored object. Direct means for detecting radiation include using a walk-through portal-type monitor that would detect elevated radiation levels on a person or in a package, or by using a hand-held detector, which would be moved or swept over individual objects to locate a radioactive source.

As described above, some types of radiation, such as alpha or low energy beta radiation, have a short range and are easily shielded by various materials. These types of radiation cannot be measured through direct measurement. Instead, they must be measured through sampling. Sampling involves wiping the surface to be tested with a special filter cloth, and then reading the cloth in a special counter. For example, specialized smear counters measure alpha and low energy beta radiation.

Examples of both direct measurement and sampling equipment are described in more detail below.

Portal monitors can be used at facility entrances, or at entrances to locations within facilities that require extra security (for example, pump houses, etc.). Portal monitors are designed to monitor for gamma radiation only or for high-energy beta and gamma radiation. Because of their limited range in air and other materials, low-energy beta and alpha radiation are typically not detected by these monitors.

Portal monitors may be stationary or portable. Stationary portal monitors (See Figure) are heavy and more expensive than are portable portal monitors, but their increased shielding relative to portable portals lowers the amount of background radiation detected by the portal, and therefore increases the instrument's sensitivity. Portable portal monitors are generally less expensive than the stationary models, which allows for greater flexibility in their use, but they are less sensitive than stationary models.

An additional option for scanning personnel or packages entering a facility is to monitor them using hand-held monitors. For example, survey instruments such as a Geiger-Mueller (GM) detector (See Figure) can be used to frisk personnel or equipment entering a facility for alpha, beta, and/or gamma radiation. GM detectors and meters and similar survey instruments are manufactured by several companies, are generally easy to use, and are relativity inexpensive. Using this type of smaller, hand-held equipment may allow for more flexibility in frisking personnel coming through an entrance and in pinpointing the location of a radioactive source than does a portal monitor. However, the smaller probe size of a handheld monitor vs. a portal would also result in an increased monitoring time.

Specially designed hand and shoe monitors are available to detect alpha, beta, and/or gamma radiation on a person's hands or feet. To use this equipment, personnel to be scanned are required to stand on a platform and simultaneously place their hands in another part of the detector. While this type of detector may give highly accurate readings, it can also be time-consuming to screen all personnel coming into a facility. The adjacent figure shows a typical hand/foot monitor.

As described in the Radiation Detection Equipment Product Guide, alpha and low energy beta radiation does not travel very far in air, and can be shielded or blocked by many types of materials. Therefore, equipment that is more sensitive to alpha and low energy beta radiation, such as a smear counter (See Figure), may be required to detect these types of radiation. Smear counters require that a sample (or a "smear") be taken from the object or person being monitored. The smear sample is taken by wiping a small cloth filter over a certain area on a surface. The smear filter is then placed in a specially designed smear sample counter, and is read over a specific period of time (typically 1-30 minutes, depending on the required sensitivity). Alpha/beta smear sample counters are typically portable, so the analysis does not necessarily need to take place at the location where the sample was taken

A summary of the appropriate devices for detecting various types of radiation is summarized in Table 1 below.

Although the instruments listed above may commonly be used for evaluating alpha, beta, and gamma radiation, other methods or devices can be used. Local environmental conditions (e.g., temperature, humidity) or the properties of the specific radionuclides being detected may make other types of instruments or other detection methods better suited to achieve the measurement goals than the instruments noted above. Experts or individual vendors may be consulted to determine the appropriate measurement device for any specific application.

While certain radiation detectors are "maintenance free" in design, specialized expertise is usually needed for installation, setup, and routine calibration of radiation monitoring equipment.

Cost estimates of the instrumentation listed above are presented in Table 2.