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- If you want to examine the picture of a collision very carefully – to find small angle kinks, for example – it is a good idea to print the picture and look at it from a very low angle.
Click here for two examples of low angle kinks!
- All charged particles visible in a bubble chamber have charges of +/- that of the electron - this makes it easy to use charge conservation.
(see lesson plans.)
- It is rare to be able to identify every particle in an event just by looking at the pictures – for example, many pions are produced in high energy collisions and they usually leave before they decay. Since the number of bubbles per centimetre (`ionisation density', ID) depends on speed , all charged particles moving close to the speed of light have the same ID and are indistinguishable. (The greater the speed, the lower the ionisation density : the impulse imparted to the electron depends on the time spent by the ionising particle in the vicinity of the electron; a relativistic particle is thus `minimum ionising'; click here for full detailed discussion).
So, for energies greater than, say, 5 GeV tracks that just leave the chamber can not be identified. If we are studying the strong interaction, such tracks are often referred to as `unknown hadrons'; they may be pions, kaons or protons, but they are usually pions because these are produced more copiously. Click here for example
Proton targets often only receive a gentle glancing blow from the beam, and therefore move with speeds well below that of light; these will have dark tracks – high ID – and will often stop, making protons easy to identify. Click here for example.
- For connoisseurs: very occasionally, a negative track will look like a stopping proton. This is an example of `pion capture' in the reaction
     . For example click here.
- If you only have one view of a picture, it is sometimes difficult to tell what is going on because, for example, one track might overlap another. To be able to sort out such Problems (and, more importantly, to be able to reconstruct the event in 3 dimension) Three or for picture of each event are taken Click here for an example of the same event on two views.
- To be able to reconstruct events, it is essential to have a few reference points whose positions in space are accurately known. These so-called `fiducial marks' are crosses on the windows or walls of the bubble chamber. Click here for example.
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