Answer to Question #9614 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 have a 137Cs reference standard for daily checks. Its activity is about 3.7 MBq. I want to perform a leak test on it. What is the method of performing the leak test?

A

The methods for leak testing vary somewhat depending on the amount of radioactivity in the source, the physical characteristics of the source, and the nature of any enclosure that holds the source and obstructs direct access to the source.

We should first note that, at least in the United States, the regulating agencies, including the U.S. Nuclear Regulatory Commission and individual states that control the possession and use of radioactive materials, often specify a maximum activity content for sealed sources at or below which there is no legal requirement for performing wipe tests of the source(s). For beta/gamma emitting sealed sources the typical such limit in the United States is 3.7 MBq. Sources having this much or less radioactivity require no leak testing. Naturally, an individual or facility may still elect to perform sealed source leak tests even for sources that are exempt by regulation. In the discussion that follows I am assuming that we are dealing with truly sealed sources with all surfaces of sufficient construction so that wiping of the source surfaces will not damage the surface. This is almost always the case for sealed sources intended to emit gamma radiation but may not be the case for sources intended to emit alpha or beta radiations since the latter sources usually require a thin covering over the source.

For cases involving beta/gamma activity where the radioactivity content is sufficiently low so that external radiation exposure to the hands or any other body part is not a significant concern, a direct wipe of the source surfaces is often the desirable method to be used. In such cases a dry filter paper wipe is often used. The paper is held in a gloved hand and quickly rubbed over the accessible surfaces of the source. The source that you describe could be wiped using this direct procedure with minimal exposure to the person carrying out the test as long as the procedure is done quickly and efficiently so that exposure time to the hands is low (10 seconds or less is all that should be required for this test). The paper wipe may then be counted using an appropriate instrument. A gas flow proportional counter is often used for beta-emitters, such as 137Cs, although other instruments may also be appropriate, depending on availability. Liquid scintillation counters are a common choice, especially if the source emits low-energy beta radiation. For sources that require leak testing, it is necessary to know the counting efficiency of the system used since the licensee must demonstrate that the activity on the wipe is less than a specified limit, most commonly 185 Bq in the United States for generally used sealed sources.

For sources containing amounts of radioactivity that present too high an external radiation field to use direct wiping with handheld wipes, or if the individual performing the tests prefers not to handle the source directly, other techniques may be used. For some accessible sources, the source surface may be wiped using an extension tool to hold the wipe. In other cases the source may not be accessible or of too high an activity to allow access to the source surface, and indirect wipe testing is appropriate. For such cases the source surfaces are not wiped but rather “the wipe sample must be taken from the nearest accessible point to the sealed source where contamination might accumulate" (U.S. NRC Regulation 10 CFR 39.35). In some such instances, for example where the radiation beam emerges through a small port when the source is in the irradiate position, filter paper wipes may not be the best choice because of difficulty in accessing the inner port area. Some such sources are better tested using devices such as long Q-tips (cotton-tipped wood or plastic rods) to access the area close to the source. Typically, the source would be in the shielded position when the wipe test is being done.

Some other source types may require special procedures to evaluate leakage. For example, for some sealed gas sources wipe testing would not be a valid means to assess leakage, and other methods for appropriately collecting leaking gas and then measuring it would have to be implemented.

I hope this answers your question.

George Chabot, PhD, CHP

Ask the Experts is posting answers using only SI (the International System of Units) in accordance with international practice. To convert these to traditional units we have prepared a conversion table. You can also view a diagram to help put the radiation information presented in this question and answer in perspective. Explanations of radiation terms can be found here.
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