P.M.T. Characteristics
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Data is probably wrong (see here)
Dark Testing
Equipment:
- Photo Multiplier Tube
- Hamamatsu R7081-10, 10 inch diameter
- Ortec 584 Constant-Fraction Discriminator
- Ortec 974 Quad Counter/Timer
- Ortec CO4020 Quad 4-input Logic Unit
Dark Event Counting
The last few weeks were spent studying the above mentioned PMT. The first tests were done with the PMT in a dark box without any stimulation from a scintillater. The PMT signal went to a discriminator with its trigger set at a certain level. The discriminator would then send a logic pulse to the counter. By varying the trigger level of the discriminator and observing the count rate, data was obtained as seen in the graph to the right. Very similar exponential looking plots were made from data for PMT input voltages from 1100V to 1800V (click here).
Decaying Count Rates
However, there are two time dependent variables unaccounted for in this experiment. The first is how long the PMT has been in darkness. The longer in the dark the less noise there should be. The second variable is how long the input voltage has been at its level. The problem was discovered when the input was changed from 1500V to 1600V. The count rate began at near 300 kHz, but dropped very fast to near 40 kHz in 2 minutes and 10 kHz in 10 minutes. This decay of the count rate is shown on the graph to the right. Similar measurements were done from 1500V to 1800V and plotted (click here).
The first time this was observed the PMT had been in darkness for several hours, and on the later tests it had been for a day or more. As similar decay was observed at all times. It was concluded that the decay is due to the change in voltage and not exposure to light. It also appears that the decay is more pronounced at higher voltages.
Signal Strengths
Using the logic unit along with two discriminators a histogram of signal strengths was generated. The discriminator's triggers were set two millivolts apart, and the logic unit was set up to pass a signal to the counter when the lower discriminator triggered, but not the higher. In this way signals in a two millivolt range could be counted. The data on the histogram to the right was then collected by taking ten second counts and moving that range up the scale.
Note that this is still a dark run without any scintillating device in the dark box with the P.M.T. There was strong double peak expected. A definite wide second peak would have been expected if there had ben a scintillater in the dark box.
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