Answer to Question #7119 Submitted to "Ask the Experts"

I am looking for suggestions on the appropriate instrument to use out in the field to evaluate contamination swipes for beta/gamma emitters.

For example, a GM (Geiger-Müller) pancake probe lists approximate alpha and beta efficiencies and some specifics for certain radionuclides, but lists "inefficient for gamma." I'm looking for efficiencies so that I can convert cps to Bq.

Specifically, when counting a swipe from ¹³⁷Cs or ⁶⁰Co what instrument would you use?  

For ¹³⁷Cs, since the GM probe sees "events" would it see the beta and the gamma and count them as two events or is the probe so inefficient for gamma that it would be negligible and the activity could be determined from the beta events utilizing the beta efficiency?

A GM detector is often the choice for doing routine assessments of beta-gamma-emitting radionuclides in the field. It is generally cost-effective, and a thin-window probe with a decent active area, as is afforded by the commonly used pancake probe that you mention, has good sensitivity to beta emissions from a variety of radionuclides. The intrinsic gamma detection efficiency is typically low, often less than 1%, which makes the GM detector a not-so-good choice if you are concerned with low to moderate levels of surface contamination by pure gamma-emitting nuclides.

Both ¹³⁷Cs and ⁶⁰Co decay by beta decay 100% of the time. The maximum beta energies from ¹³⁷Cs are 0.51 MeV (94.6% of decays) and 1.12 MeV (5.4% of decays), and from ⁶⁰Co the maximum beta energy is 0.32 MeV (100% of decays). For a spot of contamination smaller in area dimensions than those of the active area of the detector and at a distance of 1 cm or less from the window, the detection efficiencies of a typical thin window (1.5 to 2 mg cm^-2) pancake probe for each of these radionuclides likely would be in the range of 20% to 30%. The gamma radiation from the decay of these radionuclides would contribute very little to the response of the detector at close distances from the source. The manufacturer of the thin-window detector can usually provide efficiency data for various beta emitters.

If there is a need or desire to measure the gamma radiation from surface contaminants, a detector such as a sodium iodide scintillation detector is a better choice since such a detector of modest size will have much greater gamma detection efficiency than will a typical GM detector. Such detectors commonly are encased in metal that is sufficiently thick to stop many beta particles from reaching the active volume.

If a GM detector is used to measure removable beta-emitting radionuclides on wipes, it is possible to use the detector connected to a ratemeter, although it is generally preferable, from an analytical standpoint, to use a digital scaler to record counts from the wipes; this allows for more direct statistical analysis and greater flexibility in adjusting counting times as necessary to obtain an adequate count. Gas proportional counters have also been used routinely for assessing removable beta and/or alpha activity on wipes. Proportional counters have the advantage of being able to handle higher count rates than similarly sized GM detectors; additionally, they allow for separation of alpha counts from beta counts when one is concerned with mixed contamination.

There are numerous manufacturers and suppliers of instrumentation for contamination assessment. You can find information about many providers by consulting the Buyer's Guide that includes Health Physics Society Affiliates. Good luck.

George Chabot, PhD, CHP