Answer to Question #10748 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements — Surveys and Measurements (SM)

The following question was answered by an expert in the appropriate field:

Q

I wish to buy a system that measures the amount of uranium, cesium, plutonium, and other harmful radioactive poisons. Geiger counters don’t measure specific radionuclides so they are not useful for me. Please let me know what my options are for systems that accurately measure these kinds of substances.

A

You do not specify what your particular requirements are, but I will attempt to provide some guidance. It appears that you may be interested in a handheld instrument that can identify the radionuclides of concern. There are several such instruments available, but it may be difficult to find a single one that will handle all of the radionuclides you mention.

These handheld instruments fall into the category often described as "isotope identifiers." The most common of such handheld devices use scintillation detectors, most often sodium iodide (NaI), to detect gamma radiation emitted by the radionuclides. They would easily identify common gamma-emitting cesium isotopes such as 134Cs and 137Cs present in significant amounts. They would have a more difficult time with natural uranium because 238U, the dominant species in natural uranium, does not emit any significant gamma radiation; however, if the uranium is associated with its radioactive decay progeny such as 214Pb and 214Bi, these species might serve as surrogate radionuclides to identify the 238U. If the uranium is enriched in the isotope 235U, as is typically the case for uranium in reactor fuel or in nuclear weapons, the isotope identifier may well be able to see the characteristic 185 keV gamma ray from the 235U. None of these handheld instruments capable of radionuclide identification are inexpensive, typically ranging in price from roughly $5,000 to $15,000.

Plutonium, with 239Pu being the isotope of most common concern in many situations, is difficult to detect because it emits no significant gamma radiation; some low-energy x rays are emitted, but cannot be measured effectively with many of the typical handheld systems. There are some thin-window detector systems that have been successful. Some have used combinations of scintillation detectors, in particular NaI and CsI, in a specially designed detector that allows discrimination against many normal background radiation-induced pulses in order to identify the low-energy pulses from the plutonium x rays. There has also been use made of cadmium zinc telluride (a semiconducting material) as a detector for the low-energy x rays from plutonium. Again, the costs of these detecting systems are relatively high, typically exceeding $5,000.

There are also full-blown laboratory systems that are more suitable if your concerns are with radionuclides contained in various media—e.g., soil, vegetation, water, food products, etc. Such systems use various detector types for specific applications. High-purity germanium detectors are the detectors of choice for measuring gamma radiation, although somewhat less expensive scintillation detectors are also useful if very high gamma resolution is not required. Other detectors, such as solid-state silicon detectors, are useful for measuring alpha radiation energies from isotopes of uranium and plutonium. These detection systems are also typically quite expensive, many being in excess of $10,000.

I don't know whether any of this is useful to you since I'm not sure what financial or other restrictions apply. I have not mentioned any specific suppliers/manufacturers, but if you have a continuing interest in any of these instruments, you may contact me for some suggestions.

George Chabot, PhD

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