I have been testing a number of different design variations and have identified the following issues.
1) Power supply requires good filtering and regulation - Completed High Voltage Regulated Power Supply
2) Tubes vary in voltage requirements from one tube to another even between the same make, model and age
3) Oscillation is a problem as the supply voltage and/or coupling plate surface area increase
4) Internal filament electrodes must be insulated, even loose coupling increases oscillation and spurious pulses
5) Coupling plates should be positioned back 1cm from the tip of the internal filament electrodes
6) Oscillation occurs as the circuit forms a basic relaxation oscillator
Although oscillation is an unwanted artifact, it would also seem there is a point before oscillation begins where the tube increases sensitivity to radiation as the voltage increase and approaches a point where oscillation begins. However, radiation (cosmic or terrestrial) is also what first triggers the tube to jump into an unstable state before free oscillation begins.
Nevertheless, I will investigate this further to see how oscillation could be regulated through some form of negative feedback or quenching circuit as this may yield useful results.
Compton Scattering
Tests using a Geiger–Müller array detector have revealed a problem which will equally effect fluorescent tubes detector called Compton Scattering, this is where an interaction between charged electrons within the detector and high energy photons result in the electron being given part of the energy, causing a recoil effect of a high energy photon into the adjacent detector causing a false coincidence detection. In other words this causes cross-talk interference between Detector Tubes