How to get signal out of Geiger-Muller tube?
You're feeding a 1nF capacitor with a 50pF capacitor.  That's a 200:1 ratio, which means that the very first thing you're doing is effectively shorting out your signal.
If you're measuring 150mV on the oscilloscope with that 1nF cap in-circuit, then if you take it out your signal should be HUGE.  If you're not, then you need a preamp circuit that'll present the smallest-possible capacitance (and impedance) to the cold end of that 50pF cap.
In my heart of hearts I think I'm a hot-shot analog circuit designer -- and I'd start by looking at other people's Geiger counter circuits.  Only if I couldn't find something suitable would I roll my own circuit.  If I did roll my own, I'd probably start with a small-signal MOSFET in source-follower configuration, driving a boring old op-amp buffer circuit, thence into a comparator.