Answer to Question #10132 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements — Instrument Calibration (IC)

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

Q

When constancy checks are performed for pancake probes, for instance, it was always recommended to use the printed side of a 137Cs button source. I continue to follow this recommendation using all button sources in order to stay consistent, but I would like to know the reason. Is there a significant difference in attenuation between the two sides? Would it differ between manufacturers?

A

You raise a legitimate question regarding consistency in positioning of check sources used to check for proper operability of portable instruments. For the 137Cs instance that you cite, I do not think a difference in gamma attenuation between the two facial orientations is very significant. There may be an added difference in readings from the change in geometry factor associated with different thicknesses. The latter is most significant for a small volume detector probe than it is for a relatively large probe such as the pancake probe that you mention.

For example, most of the common 1” diameter check sources have a nominal thickness of close to 1/8” or about 3 mm. If the perpendicular distances from the source material to the flat plastic surfaces of a given source differed by as much as 1 mm the expected difference in attenuation between the two orientations would be only about 1 percent, a value which is not a concern in establishing reading consistency (this is based on simple exponential attenuation using a linear gamma attenuation coefficient of 0.10 cm-1). When the detector probe being tested is placed in contact with the source surface the impact of the 1 mm difference in distances to the respective surfaces could be more notable. For example, if the distance from the source material to the detector center was 1 cm for one facial orientation and 1.1 cm for the other orientation, the difference in photon dose rates for these two distances, independent of differences in attenuation, would be about 20 percent, based on the simple inverse square law and an assumption that both the detector and the source material may be treated as points. Because many detector probes are relatively large, these distance effects are often less significant. For the typical pancake probe the dose rate is averaged over the volume of the detector, and the small distance differences associated with the two orientations become less important. The smaller the probe dimensions are, the greater influence the distance difference makes.

In any case, your habit of always orienting the source in the same fashion is appropriate to minimize effects of changes in geometry and attenuation (especially if a lower energy gamma ray or x-ray source were to be used). It is also desirable that the same source always be used to check a given instrument. If a different source is used, whether it is from a different manufacturer or not, you should evaluate the detector performance by comparing the readings when both the old source and the new source are used. Sources made by different manufacturers may differ somewhat in the way they are fabricated, and such differences may result in somewhat different results.

I hope this is helpful. Happy surveying.

George Chabot, PhD

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