2) Setting Thresholds

GOAL : finding an optimum voltage setting for the PMT which maximizes the ratio of real particle signal counts to the random thermal noise produced in the detecctor system. You will produce a graph of counts per 5 minutes on the y-axis vs discriminator threshold on the x-axis.

a) connect the signal cable from light tight detectors to discriminators. With the right discriminator equipment, and 4 scaler (counter) channels, you might be able to to connect 4 detectors to 4 different discriminator channels, and do all 4 thresholds at once.

b) Set your high voltage supply to the nominal optimum operating voltage for your PMTs. This varies according to PMT type, and is something you should already know.

c) Set your discriminator thresholds at 30 mV and record the counts per 5 minutes for each detector

d) Reset discriminator thresholds at 50 mV. Recount.

e) Repeat this process in 20 mV steps up to 170 mV

f) Graph your data. You should see (roughly) 2 distinct regions in the graph. The steeper region on the left corresponds to a region where thermal noise is dominating your count rate. The less steep region on the right corresponds to a region where counts are dominated by actual particle events of interest.

g) Your ideal discriminator threshold setting is the vertex on the graph between these 2 regions. Roughly estimate where that point is. These points may be quite different for different detectors. You can eyeball this, or calculate slopes for before and after your "eyeball vertex", and find the point where they cross.

h) Go back and collect more threshold data at 5 mV intervals near each vertex, until you are reasonably satisfied you have located an appropriate operating threshold for each detector.