Subtracting background from a radioactive measurement To measure the activity of a certain sample, we need to also measure and subtract the background. An experiment I attended had explicit instructions that the background should be measured before AND after the measurement of the sample and then to use an average of these backgrounds. I didn't really understand why it was important to measure the background before and after. It's clear that an average provides a "safer" value, but why couldn't I just measure the background both times before the experiment and use their average, or two times afterwards? It was encouraged that we consider the importance of measuring background before and after a specific experiment.
Why is it important to measure background before and after specifically in that order?
A follow up question to this is:
Is the background unique for one single measurement locally speaking? If I e.g measure the background before the experiment, and then measure the activity of my sample, and afterwards the background again. But then I immediately take another measurement of another sample (of the same material), can I use the same background for both these measurements? or will I get "bad" values for the second experiment.
(The Chi squared test actually gave a better fit for the second one)
I don't think you could use the background from a completely different occasion though, but since these two experiments are very close in space and time. Is it valid to use the same background for both?
 A: If the background was constant in time, then measuring it twice before or twice after or once before and once after would all provide equivalent information.  The problem is that you cannot be sure that the background is constant, and measuring before and after is better.
For a radioactivity measurement, the background could change with time for several reasons.  Most obviously, a large part of the background might be due to natural radon in the air, and the amount of radon in a room can change dramatically, e.g. if someone opens a window or ventilation fans turn on or off. Depending on the detection system, the background could also include non-radioactive contributions such as electromagnetic noise which can also change dramatically with time, e.g. if a nearby electric motor turns on or off.  The background for some radiation detectors can also change with the temperature of the detector, which can also change with time.
It is best to monitor the background for extended periods before and after the measurement, but if (as is often the case) you only have time for two background measurements, before and after is better.  Since the actual (non-background) measurement is done in between the two background measurements, the two background measurements will be farther apart in time than two measurements done before or after.  This means you are more sensitive to slow changes in the background than with two measurements done back-to-back either before or after. The average of before and after measurements is  (at least roughly) also effectively the interpolated value for the background at the time of the actual measurement.  If you make two measurements before or after, their average is not the extrapolated value at the time of the actual measurement.  Even if you did extrapolate, this is rarely as good as an interpolation.
When taking multiple measurements, it is still best to measure the background before and after each measurement to monitor the background. As noted above, some backgrounds could change quickly.  Of course, if you take multiple background measurements and they are unchanging, then one may decide it is not too risky to take fewer background measurements.  Experimental science is always resource constrained and one has to make choices about where to spend time, but one always needs to be clear about any experience or assumptions that affect those choices.
