Answer to Question #6104 Submitted to "Ask the Experts"

Q

I work for the hazardous materials team of my local fire department. We are now required to have a detector that measures alpha, beta, and gamma, but it must also identify the radionuclide. We are looking for advice on acquiring a radionuclide reader. We have been given talks on CST and CdTe radionuclide identifiers. The CdTe is a small portable detector and would seem to fit in our needs category, but we do not have the expertise to differentiate between. I am looking for someone to help in this area.

A

You state that you are required to have a detector that measures alpha, beta, and gamma radiations and that the detector must also be able to identify the radionuclide. If you are looking to get a single instrument to perform adequately all of these tasks, I fear you will have a difficult time. You can get a reliable instrument that will detect alpha, beta, and gamma radiation and provide at least semiquantitative results in terms of count rates for alpha and beta radiations and possibly exposure or dose rate for the gamma radiation. The most commonly used portable detectors for such measurements are Geiger-Mueller (GM) detectors that are available from a number of manufacturers and/or suppliers. Here are some links to a few, including Ludlum, Technical Associates, and Gamma Scout; you can find numerous others through the Internet.

The handheld instruments available that are intended for radionuclide identification are, to my knowledge, virtually all gamma-sensing instruments that identify respective radionuclides by the energies of the gamma rays emitted and detected. Such units commonly use solid detectors (as opposed to the gas used in GM detectors) to yield reasonable efficiency for the gamma radiation; among the more common detector materials are sodium iodide detectors (NaI) and cadmium telluride (CdTe). I am not familiar with the CST detector that you refer to (there is a cadmium zinc telluride [CZT] detector that is a modification of the CdTe that is in use).

There is a radiation performance standard that has been published by ANSI, the American National Standards Institute, that specifies performance requirements for handheld instruments for radionuclide identification; it is ANSI N42.34 "Performance Criteria for Hand-Held Instruments for the Detection and Identification of Radionuclides" and was published in 2003. This standard incorporates some rather demanding requirements for instruments to be used for radionuclide identification and, to my knowledge, no available instruments are able to meet all of the test criteria. I am not able to recommend a specific instrument to you, but I would recommend that you contact the suppliers that you have been dealing with (and/or others if appropriate) and ask them to provide you with information as to how their systems stack up against the standard. You can get an idea of what kind of testing has been done, relative to the standard, by searching the Internet. Here are two links—one to a study that was conducted by the National Institute of Standards and Technology and another that was reported on by Canberra Industries. Here are a couple of other links, one to a Canberra site that offers a NaI detector based, handheld identifier system and another to a site where XRF offers a CdTe-based system. I have included these because they are two relatively well-known systems; their inclusion should not be viewed as a specific recommendation on my part. Based on what I have read and my past experience, I would judge that some NaI-based systems may offer somewhat more flexibility than do CdTe systems, especially when a relatively large-volume NaI detector (such as a two-inch by two-inch cylinder) is used; it likely will exhibit a notably higher detection efficiency for higher-energy gamma rays than will a CdTe detector, which will be much smaller in volume. If you can afford longer counting times (for example, two or three minutes instead of one minute), the CdTe has the advantage that the small detector size makes for a smaller and more convenient package. Additionally, I would expect the CdTe to be considerably more rugged than the NaI and to provide somewhat better energy resolution (ability to distinguish between closely spaced gamma energies).

I'm sorry we cannot make a single, straight-out recommendation to you, but I hope the above information is helpful to you in making a decision. Good luck.

George Chabot, PhD, CHP