How do you sample a gas Bremsstrahlung spectrum?
In short, the technique is the following: make the electron beam cross a volume of air (or appropriate gas) at atmospheric pressure, or, if the straight line is longer than about 10 m, at 1/10 atm. The results must be normalised to the actual pressure (dividing by a factor which is generally of the order of 1.E11~1.E12). Very important:
- multiple scattering in the gas must be suppressed (there is no appreciable scattering in the residual gas of very low density, but at atmospheric pressure scattering will introduce a non-physical angular spread of the photons)
- secondary electron and positron production thresholds (Moller and Bhabha thresholds) must be set very large, close to the incoming energy, in order to avoid angular spread coming from those processes as per multiple scattering
- kill the electrons at the end of their trajectory in gas (in real life they would be bent out of the way by some magnet). One way to do this is to make a very thin region of gas with electron cutoff higher than beam energy.