Overview
In this experiment we will try to prove that the gamma rays created from the annihilation of positrons with electrons resulting from the decay of Na22 into Ne22 conserve both energy and momentum.
Questions
1) Explain what the SCA does for you in this experiment.
In this experiment the single channel analyser will determine if the pulse coming from the amplifier is in the specified range necessary to have come from an electron-positron annihilation event. This is necessary as the decay products of our sample also include a higher energy gamma ray that is produced when the an excited electron on the Ne22 product falls back to its rest energy.
2) Explain how the NaI detector works.
The NaI detector works by advantage of the photo-electric effect. When a photon goes into the crystal an electron is often ejected in the same direction as the motion of the photon. This electron then heads toward our photo-multiplier tube which uses successive dynodes held at a potential relative to each other to further eject electrons. The end result should be a cascade of electrons large enough that we can take a reading of voltage from the capacitor at the end of photo-multiplier tube.
3) Explain why the coincidence detector is so important.
The coincidence detector is important to maintain that any two pulse coming from detector A and B are coming from the same event and not from two random events occurring at the same time.
4) Explain how you will count your pulses
For this experiment we were able to count pulses by two methods. First, by a slightly more difficult method, we were able to count gamma rays collected at either detector with a labview program set to count a pulse when the single channel analyser (SCA) detected a pulse within a specified amplitude. Square wave pulses generated by both SCA's were then sent to the coincidence detector in our NIM bin where counts were only made when two pulses were within \(50nm\) of each other. I consider this slightly more difficult than the next method used as it was necessary to use a stop watch and the counter switch to make sure that counts were only made in the 60 second time frame that we specified for the collection of coincidence data. The start button on the labview program as well needed to be pushed when data collection began, which of course required another person and some caution in the timing of all the necessary buttons. In the second method we were able to collect all necessary data through labview with the addition of a coincidence detector and program courtesy of Dr. Professor Chair Extraordinaire Ayars. With this addition we were able to collect single counts from both detectors and coincidences all with a specified time interval and without the need of coordination by multiple people thereby reducing random error.
Set-up
This block diagram was taken directly from Positron-Electron lab handout.