Answer to Question #6973 Submitted to "Ask the Experts"

I infer from what you describe that the instrument you are calibrating is to be used for measuring surface contamination. If this is the case then the calibration should be carried out at approximately the same distance from the source as the probe is held from the surfaces to be assessed in actual use. If you are trying to measure alpha-emitting radioactivity, it is important to keep the distance small so as to minimize air attenuation effects, and distances of 1 cm or less are generally desirable. In general, the closer the detector is to the source, the greater will be the effects of small changes in the source-to-detector distance on detection efficiency. If you are assessing beta-emitting surface contamination, somewhat greater distances are permissible but preferably should be less than 5 cm. For calibration purposes you can maintain the detector at the desired distance by using a simple spacer device to hold the detector face at a fixed height above the source; alternatively, you may be able to invert the detector and hold the source at a fixed distance above the detector face.

Such spacing devices need not be very sophisticated. For example, if you decide that 1 cm is the desired distance, you can fabricate three plastic posts, each 1 cm in height and thick enough to stand easily on its own. Most pancake-type G-M probes have a metal ring around the active window area, and the detector can be held centered above the source with the posts positioned under and around the metal ring so as to maintain the correct distance (but be careful not to damage the window with the posts). If you are doing calibrations frequently you would probably be better served to fabricate a spacing device with a rigid body that requires minimal manipulation to hold it in place. If you are measuring beta emitters, the detector or source holding devices should be low in mass and fabricated of low atomic number material, such as plastic, to minimize beta particle scattering effects. If you are measuring beta emitters, the type of surface (e.g., wood, metal, etc.) can affect the degree of beta particle backscatter that occurs, and this impacts detector efficiency. It is then desirable to have the calibration source backing similar in backscatter characteristics to what is present in the field.

I believe you will be able to come up with a simple and acceptable means to accomplish your objective. Good luck.

George Chabot, PhD, CHP