Plateuing Detectors

Purpose

Before starting any other experiments, you should adjust the sensitivity of the detectors so that they have a high efficiency for detecting cosmic ray muons while not producing a lot of accidental "hits" due to noise. This is done by setting the high voltage on the photomultiplier tubes to a suitable value.

Procedure

The rate of cosmic rays though your out stretched hand (100 cm2) is about one per second. The detector paddles are 30 cm x 30cm (900 cm2) so the rate of cosmic rays through them should be about 500 per minute. We are going to adjust the high voltage on the photomultiplier tubes so that we get about 1000 counts per minute. That will guarantee that we have very high efficiency for detecting cosmic rays. Since the count is twice what is due to cosmic rays, there is one count due to noise for every real hit. That's perfectly OK, because in most of the experiments we are going to use a coincidence of two or more of the detector paddles. The time window on the coincidence will usually be set to 100 nano (10-9) seconds or less. With a noise rate of 10 noise counts per second, the probability of getting a noise hit in two detector paddles within the 100 nano second window is very small.

(10 per second)   x  (10 per second)   x   (100 nanoseconds)   =   10-5 per second

This is six orders of magnitude smaller than the cosmic ray rate and so is negligible. Note that even for a noise rate one hundred times larger, 1000 per second (60,000 per minute), the contribution of noise to the two fold-coincidence rate would still be only 1%. This means that you don't have to set the rate very precisely. Anything around 1000 counts per minute will be fine.

  1. Run the "justcounting" experiment.
  2. Set the following configuration.
    3. Delay Time: 0.1 μs
    Gate Width: 0.2 μs
    Channel Enable: 1, 2, 3, 4
    Coincidence Level: 1
  3. Set the high voltage on all four photomultiplier tubes to be 750 V. This should put them in the right ball park. You will need to use a voltmeter for this. The monitor points on the black voltage supply box should all read 750 mV.
  4. Count for 30 seconds and note the number of counts for each of the four detectors.
  5. Adjust the high voltage on the individual detectors as needed so that you get about 500 in 30 seconds for each of the detectors. I recommend making these adjustments in steps of 25 V.

Questions

This page is maintained by Prof. Steve Schnetzer.