Physics307L:People/Mondragon/Notebook/071003
Poisson distribution
This is an exercise in dealing with data that tends to fall into a Poisson distribution
Lab equipment and setup
The scintillator and PMT are one unit. It looks like a large flashlight. The scintillator is a large Thallium doped Sodium Iodide crystal in the wider end of the detector device. What the scintilator does is emit a pulse of ultraviolet light every time it absorbs high energy electromagnetic or particle radiation. Read more about scintillators and scintillation. The burst of UV light has an energy roughly proportional to the energy of whatever radiation the scintilator absorbed, but this isn't important in this experiment.
The PMT is on the other end of the detector. Read about Photomultiplier tubes. The PMT will produce a current roughly proportional to the energy of the ultraviolet light falling on its photocathode. How proportional and by what proportion aren't of much importance here either, since we are using the device to count events, not to measure their energy.
The PMT needs a high voltage power source to turn the pulses of light hitting its photocathode into currents, usually in the range of 1000V to 2000V. Increased voltage usually corresponds to increased sensitivity. The point of this exercise is to find what the probability of distribution function of an unlikely event looks like, so throughout the experiment, use a voltage of 1000V. To further decrease the number of events detected, keep the detector in a lead brick "cave."
The output of the PMT can either come from the anode or dynode connectors at the back of the PMT. output after the PMT has detected a light burst will be a positive voltage spike coming out of the dynode and a negative spike coming out of the anode. This is important because the event counter needs pulses of an amplitude larger than the PMT produces and the amplifier has settings for amplification of either positive or negative pulses. Try to look at the output of the PMT with an oscilloscope to make sure whether the pulses are positive or negative. The pulses are sharp, meaning they are short time wise and may be difficult to see, and come at random intervals, meaning triggering on the oscilloscope may not act nice. For this part you may want turn the high voltage up to 2000V and take the detector apparatus out of the cave so that the detector detect more events per second. This will make the pulses easier to see on the oscilloscope.
The counter itself will be a MCA (MultiChannel Analyzer) card inside the computer next to the rest of the equipment. The card has an input suited for connecting to a coax cable connector, so that you can run a coax from the amplifier to the back of the computer without any special connectors. A MCA normally counts and bins incoming voltage pulses according to pulse height (Pulse Height mode, PHA), but for this exercise you will want to use the card in MCS mode (Multi Channel Scaling), where it will count and bin according to when the pulses arrive.
final graph
Shows the Probability distribution function as is drifts from a Poisson distribution of a rarely occuring event to the more familiar Gaussian
