Checking Detector Efficiencies and Measuring the flux of Cosmic Ray Muons

Purpose

Once the high voltage has been correctly set, you are ready to check that your detectors are working efficiently and to measure the rate of cosmic ray muons.

Procedure

We can't use a single detector to accurately measure the rate of incoming cosmic rays. As we saw, if the high voltage is set high enough to have good efficiency, there will be a large number of noise "hits". In order to measure the actual cosmic ray rate, we need to stack at least two detectors vertically and measure the rate at which they have simultaneous hits. A cosmic ray muon passing through the top detector will also pass through the lower detector firing the two "simultaneously". Of course, there may be a slight time difference between the hits in the upper and lower detector so we need to set a time window within which we consider the two hits to be "simultaneous". In this experiment, we will set this coincidence window to be 100 nano seconds (100 ns). This will allow for any time jitter due to light collection in the scintillators, electron multiplication in the photomultilpler tubers, differences in cable lengths, etc. Actually, this is a fairly conservative window. It would probably be alright to make it narrower.

1. Stack the four detectors vertically. Try to make sure the areas overlap as much as possible
2. Run the "2fold" experiment.
3. Set the following configuration.

Delay Time:	0.1 μs
Gate Width:	0.2 μs
Channel Enable:	1, 2, 3, 4
Coincidence Level:	2

4. Count for one minute and note the number of counts for each of the six detector coincidence pairs.

Questions

• Why do the rates differ?
• If you count longer do they agree better? Why?
• Do you see a pattern to the differences? Why might there be these systematic differences?