Overview
In this experiment we will try to prove that the gamma rays created from the annihilation of positrons and electrons resulting from the decay of Na22 into Ne22 conserve both energy and momentum. We will do this by checking that annihilation events produce gamma rays of 0.511 MeV with trajectories 180 degrees from each other such that they maintain zero momentum relative to their rest frame.
Procedure
It will be necessary to place your NaI detector inside of a lead tube, or some other protective cavity, with its opening toward your source so that noise can be reduce from any nearby or distant emitter that could be producing radiation. Each detector should be placed a reasonable distance from the source as compared to its activity so that a fair amount of counts will be maintained. Need for safety due to proximity of a radio-active source should always be taken into account. For our source a distance of approximately 5 inches was close enough to get around 3000 single counts per minute. However, the detector on our goniometer arm was capable of being maneuvered from a few feat away.
Each detector works by advantage of the photo-electric effect. When a photon is incident with the detector an electron is often ejected in the same direction as the motion of the photon. This electron then heads towards a photo-multiplier tube which uses successive dynodes held at a potential relative to each other to further eject electrons. The end result will be a cascade of electrons large enough that a voltage reading can be taken from the capacitor at the end of the photo-multiplier tube. Given our detector this effect happens with an efficiency of around 30 percent. A pre-amplifier will read this change in voltage as a pulse which it will then send to and amplifier that scales the pulse to a level which can be read by a single channel analyser. The single channel analyser should be set so that it will only allow readings for a pulse associated to a 0.511 gamma ray detection. Each single channel analyser should then send a square pulse to your coincidence counter. It is likely that a delay time will need to be set in one arm so that pulses coming from an event correlated in time will arrive as well at the same time. This can be done by adjusting the delay until it seems that a maximum number of counts are being received over time and should be done while the detectors mirror each other about the source. A coincidence counter capable of taking singles counts from both arms \(A\) and \(B\) separately as well as count total number of coincidences from these arms given a specified interval will be useful .
Angular correlation can now be checked by maneuvering detector \(B\) about some fixed central position while keeping track of coincidence counts. It may also be useful to check for randomly correlated events by adjusting the time delay so that any coincidence counts would only be coming from such an event.
Set-up
The Block diagram shown below should resemble the set up for this lab.