Plateau
From MariachiWiki
For each counter, the voltage supply to the PMT and the voltage threshold for discriminating signal pulses from the PMT both need to be individually calibrated. This insures the counters operate at the highest possible detection efficiency.
Voltage
The only input to the gun-case is a low voltage power supply in the range of 5-7 V at ~.05 Amps. Each set of 5 counters is powered off a single 7.0 V supply. The voltage to each counter is dropped to the desired voltage with a resistor inside the circuit board housing box. Inside the guncase is a DC-to-DC voltage converter that steps the low voltage supply up to the ~1000 V that the PMT requires. The voltage conversion is roughly linear.
Running the PMT's at the proper voltage is critically important. The detector's efficiency for detecting secondary cosmic rays depends on the voltage on the PMT. We hope that the efficiency for detecting cosmic rays is close to 100%. In order to determine the proper voltage for operation, the efficiency is measured as a function of voltage. This measurement is made by stacking 3 counters vertically(shown below).
This figure at left shows the efficiencies for 4 different counters. The efficiency "plateaus" at roughly 95%. The PMT's should be operated near the middle of this plateau so that small changes in voltage do no alter the efficiency and thus the rate of cosmic rays we will measure.
The counter 1D plateaus at a significantly different voltage than the other counters. Most likely, this is caused by differences in the gain of individual photomultiplier tubes even though they are all the same model. It is important that all counters are operating on their efficiency plateaus. You can see that the counters will stop counting if the voltage exceeds a certain value. The highest voltage points on 1C and 1B show the efficiency falling drastically due to this. These 4 counters can't be simply run at the same voltage because at voltages high enough to reach 1D's plateau the other PMT's will shut down. We solve this problem by using resistors of different values in series to drop the voltage to each counter to a value on the middle of its plateau.
Resistors
This calibration is completed when the counters are built at Stony Brook. We begin by setting thresholds to 15.0 mV. A switch is then used to adjust the resistor in the power supply path for the counter being calibrated. Three stacked counters are again implemented to make efficiency measurements for the middle counter as a function of resistance. A resistor near the middle of the plateau is then selected. Resistors will be installed at Stony Brook.
Fine tuning of the efficiency should then be done at the schools by measuring an efficiency plateau as a function of discriminator threshold voltage. Making these measurements requires a screw driver to adjust the threshold, a voltmeter to measure the threshold, and a PC running the Labview data collection software to measure the efficiency.
Threshold
The discriminator selects pulses from the PMT above a certain height(in voltage) and turns them into square waves with a standard height and duration. The threshold to trigger the discriminator is set to filter out noise. The average pulse height of signals from the PMT depends on the voltage on the PMT.
In the figure above, counter 1D has a low efficiency at 6V because most of the pulses from cosmic rays are not large enough to trigger the discriminator. The threshold on the discriminator can be lowered for this counter so that the efficiency increases at 6V. We can adjust the threshold and make a plateau curve for the efficiency as a function of threshold voltage.
At left is an example of such data. All the data in the first plot was taken using a threshold of -50.0 mV. The circuit board has a manually adjustable discriminator threshold for each individual input.
