Photomultiplier Plateaus

An efficient photomultiplier (PMT) is one that yields a high count number with a minimum amount of noise. By applying a range of voltages to a PMT and plotting them against the count efficiency at each level, one will see a plateau. At the start of this plateau, the efficiency of the PMT is greatest because the count rate is high and applying more voltage will only contribute more noise.

Setup

In the setup of the scintillation detector, there are two PMT's and a trigger. The PMT's are placed on either end of the 100" detector and the trigger is placed in one of three positions listed below. While one PMT is being tested, the other is kept at a voltage of -2200 V. The trigger is always kept at -1300 V. There are three channels being read in from the detector: (1) left PMT x right PMT x trigger, (2) left PMT x trigger, and (3) right PMT x trigger. So, dividing channel 1 by channel 3 will yield the count efficiency of the left PMT and, likewise, dividing channel 1 by channel 2 will yield the count efficiency of the right PMT.

Trigger positions:

First - 50" from left PMT (50" from right PMT)

Second - 5" from left PMT (95" from right PMT)

Third - 95" from left PMT (5" from right PMT)

Left PMT

Right PMT