I have been giving some thoughts to ionization gauges lately in terms of pressure control. What if we used one without a filament to detect ions? Put it in line with a beam path but out of the way of the beam so it doesn't become an added loss mechanism and see if you get a good signal. Bursts of high energy Heliums.
There will be problems. High energy Helium ions are going to be a heating problem. Secondary ionizations will be a problem. If the bias voltage needs to be in the MV range that will be a problem.
Still, I think something useful could be worked out. Low power test reactors can probably get by with some high temperature wire. For higher powers we may need to think of something else. By then of course we ought to know a lot more and the solution will be obvious.