Single electron non-perturbing detector I am designing an experiment where I need to trigger the release of an electron by a radioactive source (Sr-90).
The easy way to do it is to use a thin scintillator right after the source collimator. The problem is that it would interfere with the electron particle causing multiple scattering. 
Do you know a better way to detect the presence of a single relativistic (E=200 KeV gamma=0.38) electron without interfering with it?
We are building a small chamber to measure the drift properties of electrons in various gas mixes. The chamber is similar to that described in (http://www.sciencedirect.com/science/article/pii/S0168900298013886). I would like use the setup to measure the primary ionization yield of an electron in a mixture. The idea is to place a collimated Sr90 source in the gas volume, near the first plate. The beta electron travel at c toward the drift tube ionizing the gas. The electric field drift the ionized electrons toward the drift tube were they are collected and counted. The trigger is provided by a scintillator near the exit point of the electron from the camera (after the drift tube) in coincidence with the tube. 
Tube drift tube is somewhat noisy so I would like to have another trigger signal, preferably near the source collimator.
 A: Have you built the instrument yet? If not I've had a brain storm which might solve the problem.
Considerations
Your biggest difficulty here is that at those energies electrons will range out in less than $10 \text{ g/cm}^2$ (see the PDB), which means that putting a massive front side detector is a bad thing. 


*

*Cerenkov is out, your electrons are too slow. 

*Likewise for the usual transition radiation detectors (I'm not terribly familiar with these and can't say if there are low energy variants which would work).

*Scintillation counters are much better than they used to be and you don't need a cm of plastic scintillator anymore: a couple of mm will do. But you have to make them light-tight so you're getting into a significant amount of straggling even so.

*Solid state detectors might work, but tend to be expensive if you don't want a lot of mass.


So I started thinking about gas detectors. After all you will already have a container and you could afford a slightly longer gas volume. 
A possibility
You could build a few simple sense wires at the front (entrance side) of the device and trigger on those. Of course, you must screen the test detector from the field associated with the trigger wires; a plane of grounded wire will do that. As you are only using these to trigger you can make them small and run in them in full saturation like a Geiger counter if you want. Use a small spacing to keep the delay down. Use two planes for fewer false triggers at the cost of some efficiency.
What you can't fix
Unless you have access to ta deep underground lab, you are going to have a problem with cosmic background. A top-and-sides array of hodoscopes as a veto is the usual hardware solution. Or you can take cosmics data in the absence of the source and subtract at the cost of longer running.
