Answer to Question #13023 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:
What is the correct method to determine an average surface activity using a GM pancake detector? Using a detector with an active surface area of 15 cm2, and an efficiency of 15%, how do you determine the average surface activity over 100 cm2, from a single point reading? I feel like a simple multiplier of 6.66, that is 100 cm2/15 cm2, would give you the total activity of the area, but not the average. Would the average be assumed to be the single point reading (all other things being equal), or is there something I'm missing?
Your simple approach of ratioing areas to obtain the expected 100 cm2 activity may be legitimate, but there are a number of considerations that may apply.
There is commonly uncertainty regarding the interpretation of surface contamination surveys because of a number of influencing factors that affect detector response as well as the validity of the approach used in specifying the number and locations of measurements. These include things such as whether the activity is relatively uniformly distributed over a surface or present as random spots or individual small areas of contamination and how large an area is contaminated, whether the radiation emitted from the contamination is attenuated by foreign material on the surface or because of penetration into the surface, whether the type and energy distribution of radiation and the spatial distribution of the radioactivity used for calibration is very similar to that being measured in the field, and whether the distance of the probe from the surface is maintained constant and the same as that used in calibration.
If the measurement conditions, including area distribution of radioactivity, radiation characteristics, and survey technique are substantially similar to conditions used for calibration, many uncertainties are removed, although it is not always possible to validate this similarity. Usually measurements of surface contamination when beta or alpha radiations are being detected are made with the active surface of the probe held about 1 cm from the surface. In some instances we may know the characteristics of the spatial distribution of activity—e.g., a laboratory drops a single drop of radioactive solution from a pipette to the bench or floor—but more often the distribution is not known and we assume, for purposes of interpretation of a given reading, that the activity is more or less uniformly spread over an area as large as or larger than the active facial area of the detector.
In such cases we should apply efficiencies based on uniform areal distribution of activity—e.g., cps per Bq-cm-2. I assume you are dealing with beta- or possibly alpha-emitting radioactivity. Often the manufacturer may not provide efficiencies for direct interpretation of areal concentrations. The efficiency may be given as a simple percentage value, such as the 15% that you cite, which is usually a 4π efficiency, meaning that if a source of given emitted beta or alpha energy characteristics is positioned below the open window at a close distance from the window, you can expect to record a count rate that is 15% of the beta particle emission rate from the source. Often such sources may be smaller in area than the detector window, and in such cases the detection efficiency may be greater than would be observed if the same source activity were spread out uniformly over an area equal to the active facial area of the window. It may be necessary to check with the instrument manufacturer to find out what the calibration conditions were. The distance of the probe window from the surface is also important since it affects geometric efficiency.
If we assume that you have a reasonable number for the detection efficiency and that it is a 4π efficiency, then your conversion to areal contamination, expressed as activity per 100 cm2, by assuming the activity is proportional to the contaminated area and taking the ratio 100/15 as a multiplier to calculate activity over the 100 cm2 area based on the determined activity over the 15 cm2 area is an approach commonly used. This calculation is based on the assumption that areal contamination is uniform over at least 100 cm2 of which the measured 15 cm2 result is representative. Generally, moving the detector over a suspected contaminated area to measure at a number of detector locations will provide evidence of whether such an assumption is valid. In fact, if the contaminated area is appreciably larger than the detector facial area, you must make enough measurements over the contaminated area to be able to characterize the area using an average of the results obtained or, if there is a lot of variability, possibly breaking the large area up into smaller areas, each characterized by possibly different average contamination levels. One measurement is usually not sufficient to characterize the surface contamination level unless the contaminated area is very small.
While there is more we could say about this topic, such discussion is beyond what I believe is necessary to address your question. I hope this is helpful to you.
George Chabot, PhD, CHP