what is the simplest set up for detecting ions?
The simpliest schematics is probably amplifier from any ion smoke detector.
Connect base of n-p-n darlington (the one without internal resistors) to metal box and emitter to ground. Apply 50% of transistor rated c-e voltage (10..20v) through 1..10k resistor with LED. 



For fun, try 4-6 transistors in enourmous darlington chain with LED. It will work like "detector of everything". In my experiment it detected a lit match few feet away with "antenna" 1 inch long. (Sorry no proof of working in vacuum due to budget limitations)