I have done a paper exercise for the design of a high frequency ion detector. It will detect ion signals of 1E-8 Amps p-p broadband (50 MHz) or around 1E-10 Amps p-p narrowband (1 KHz).
http://iecfusiontech.blogspot.com/2007/ ... fiers.html
Response is DC to 3 MHz (low band) and .3 MHz to 35 MHz high band.
Update: the main limitation on the bandwidth is the input circuit. I'm assuming probe and stray capacitance of 16pF. If that could be reduced to 6 pF, bandwidth would double to 70 MHz.