3) Efficiency Scan

GOAL : determining how often a particular detector "sees" a particle event, if that event "definitely" took place. In a 4-detector array, it's helpful for data analysis if all 4 detectors are the same efficiency. And if the efficiency of a detector drops off, it may indicate a problem with the PMT, scintillator, or optical glue bond between them. You'll characterize the efficieny of each detector over a range of voltages, to find the optimum operating voltage for each PMT.

a) Set up a "telescope". This is 3 detectors in a vertical array. The top and bottom detectors serve as your backup. If both of them send a pulse within a 100 nanosecond window, the logic gate interprets that as a signal : a real particle event. Then, if the array is vertical, it's reasonable to assume that the particle that created the coincidence between top & bottom MUST have passed through the middle detector, and MUST have created an event. The question is "If there was an event in the middle detector, did the middle detector actually record it ?"

The way we'll actually think about it is in terms of per cent efficiency : For definite events, what per cent of the time did a particular detector actually record a signal ?

This can be calculated as

(# of counts of 3 detectors in coincidence) / (# of counts for top & bottom detectors in coincidence) x 100 = % efficiency

b) use the logic gate to set up the discriminator outputs from the top & bottom detectors into a 2-way coincidence. Run this output into one scaler (counter).

c) use the second channel on the logic gate to set up a 3-way coincidence between all 3 detectors. Run this output into a second scaler (counter).

d) For each detector, collect counts and compare efficiencies at different high-voltage settings for THE MIDDLE DETECTOR. Test a range of voltage settings above and below "nominal optimum" for your type of PMT. Graph efficiency on the y-axis, and voltage setting on the x-axis.

NOTE :The top & bottom detectors should run off a DIFFERENT high-voltage power supply, which should be set at "nominal optimum" for your type of PMT.

e) The best high-voltage power setting for each detector is the high-voltage where the efficiency first reaches a maximum. For most detectors, this will be at an efficiency of approximately 90 %.

f) Repeat this process, placing a different detector in the middle.