Detection Devices

Choosing a Portable Meter

Survey meters are typically sold in two parts — the meter itself, and the detector. Meters can usually be calibrated to work with most styles of detector, such that one meter could be used with a GM detector for beta use and then re-calibrated with a NaI crystal detector for low-energy gamma use.

The most popular meter is Ludlum Model 3. For beta work, the Ludlum 44-9 detector is usually fitted. For gamma work, the 44-3 crystal detector is used; for gamma detection, consider the 44-21 NaI crystal and plastic scintillator or 44-98 with a BGO scintillator. The Model 3 is preferred for its reliability, durability, and ease of use on the bench and in hand for contamination surveys.

Another meter commonly used on campus is the Ludlum 2401-P, which is a combination meter and detector in one unit. Its lower cost and higher portability are useful, but it may not be as convenient for bench work since it is less readily used hands-free for area monitoring or to check for glove contamination.

Johnson Nuclear also makes a fine meter for lab use, the GSM-115. Johnson meters are popular in the nuclear industry and are made to high standards. Most Johnson products are geared towards much more rigorous measurement than most campus users require.

Finally, RPI Corporation makes two meters, the beta-use GM-1 and the gamma use SD10. These meters have an easily readable display that is geared towards bench-top use. The drawback to this design is less portability.

Image Technology, Inc. is a supplier of instruments and may offer a discount to University of Wisconsin employees and students. They may be reached at 1-800-599-2643,
or at 319-373-0944.

Not Recommended

We do not recommend Eberline or Bicron meters. They have merged with several other nuclear-related companies, their meters are being consolidated and the meter you buy may no longer be supported by the vendor.

We also do not recommend meters with digital readouts. It is better to have a dial with numbers and a needle that moves from 0 upward. This provides an instantaneous indication of the field strength and reduces confusion. Radiation emission is a random process. Meter efficiency is directly related to a variety of factors including geometry, energy, detector speed, etc. Digital meters suggest an accuracy that is not real.

Portable Devices

All authorized users of radioactive material must have a survey meter which is sensitive to (i.e. is able to detect) the type and energy of radiation emitted by the radioactive material being used. Thus, researchers working with beta-gamma emitting nuclides are required to have a meter with a thin-window GM detector. Users working with gamma emitters are required to have a scintillation detector.

Geiger counters are used for radiation surveys at the University because of their high sensitivity for beta particles. Nearly every beta particle that penetrates the detector will cause a discharge and produce a count. A thin-window GM (Geiger-Muller Meter) has a conducting shell with one surface covered by a thin (e.g. 1.5 – 4mg/cm2) mica or mylar cover. This “window” allows particles to enter the chamber. The shell of the detector is usually made of steel or coated glass approximately 200 mg/cm2 that does not let beta particles penetrate.

Low energy gamma (LEG) probes are highly efficient for low energy gamma rays in the 20 to 70 keV range. They normally use NaI crystals approximately 0.04″ to 0.08″ thick. Because the higher energy gamma rays are more penetrating, scintillation detectors designed to detect and measure photons with energies between 100 keV and 2 MeV are thicker, often more than 1″ thick. NaI crystals produce a portable system sensitive to both low energy gamma rays and beta particles. This type of detector would be an excellent choice for a lab group that uses both beta emitting nuclides and 125I or 51Cr.

Many labs use both gamma-emitters as well as beta emitters in their research. Several detectors are on the market which combine detector elements to be capable of detecting both beta emitters and low-energy gamma emitters. The two most common type of detectors are the sandwich and the single crystal. The sandwich probe (e.g. Ludlum Model 44-21) is a NaI crystal fronted by a thing plastic scintillator (similar in function to liquid scintillation cocktail). The single crystal (e.g. Ludlum Model 44-98) uses a single BGO (bismith-germanium-oxide) that produces light when either beta or low energy gammas are absorbed in the crystal. One caution about these, and all scintillation detectors, is that the detector is fragile and shock sensitive, and is easily broken if mishandled. For that reason, we recommend labs use thin-window GM detectors unless a specialty detector is needed and lab personnel understand how to properly handle the detector to prevent damage.

Meters must be calibrated annually (at least once per year). The UW–Madison Department of Environment, Health & Safety is able to calibrate most types of meters at no charge. All meters listed on the web page are able to be calibrated by EH&S. Meters that cannot be calibrated byt EH&S must be sent to an approved calibration service; the authorized user is then responsible for calibration fees and other charges.