Answer to Question #10004 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:


I have a liquid scintillation counter (LSC) and a gamma counter (GC). Which counter is most appropriate to use to detect the leakage of 137Cs from a sealed source, given the facts that the GC has a lower counting efficiency than the LSC, but the GC will count only gammas and not betas?


Since you are counting only a single radionuclide, there is no need for energy discrimination, and the LSC will provide much higher detection efficiency than will the GC. On this basis, I would opt for the LSC to detect possible leakage on an appropriate wipe of the source.

If you have been using the LSC for measuring low-energy beta emitters, you naturally will have to adjust the counting channel to accommodate the higher-energy beta particles from 137Cs. The higher sensitivity of the LSC might afford the opportunity to detect low levels of leakage, much below the usual limiting value of 185 Bq, and alert you to potential problems with the source. You will require a counting efficiency to evaluate the activity of the wipe using the LSC; depending on the capabilities of your LSC, this may not be problematic, but you may have to do some work to establish the proper value.

As you have noted, the GC will see only the gamma radiation from the 137Cs (and possibly some low-energy characteristic x rays and bremsstrahlung). The gamma counting efficiency depends on the detector size and source-detector geometry, but a 137Cs counting efficiency on the order of 3 percent would be reasonable. The background on the GC may be higher than that of the LSC and this, combined with the lower efficiency, makes for lower sensitivity.

With a 3 percent photopeak detection efficiency, the GC would yield an expected net gamma count rate of about 330 cpm if the wipe contained 185 Bq of activity. This is probably sufficient to identify the leakage at this level with a relatively short counting time, but it is much below what you might expect from the same amount of activity on the LSC, which would likely be 5,000 to 10,000 cpm. Clearly, your ability to see much lower levels is enhanced by using the LSC.

I hope you are satisfied with this response.

George Chabot, PhD
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