This certainly sounds like an interesting project, with the low noise signal circuitry being especially challenging. My only concern is the count rate forms such a small detection area. Do you have an estimated count rate? Many years ago, whilst at Bristol University, I was involved in the design of instrumentation for a large balloon-borne cosmic ray detector that was flown from Palestine Texas. It used a gas scintillater that was about four metres in diameter and used 25 photomultipliers!