Record the highest measurement for each area and the final measurement after decontamination. If the levels of beta and gamma contamination are high enough, they may cause the metre to go off scale. This would indicate that a dose rate measurement may be required. The factors effecting the efficiency are show in the diagram below. Counting a standard source of known activity with your detector.
Some radiation goes directly from the radioactive material P, into the detector.
Some radiation will backscatter off the surface, into the detector. Some radiation is absorbed by the detector covering. Most radiation doesn't even get detected. If the detector was closer, this radiation would be detected. Wipe each of the locations shown on the plan of the working area with a filter paper.