I bought a couple of 18-1 tubes some time back. This was earlier last year - before these 'corona tubes' got much of an outing in the forums, and I wasted a considerable amount of time trying to get the *** things working - and failed!
All this talk recently encouraged me to look closer. Well, with a new oscilloscope I bought in the intervening time period, it picks out the pulses! Cool!
Both 18-1 tubes work, giving around 4 to 10 pulses per minute background (little groups of bursts distort the minute-by-minute statistics, it seems). I also have two SNM-17 tubes, I tested one that seems to give only one pulse every several minutes, but bear in mind these have no moderation around them at all, so we're looking at direct cosmics, here.
I ran the tube with 100Mohm serial (seems to work fine - is there any benefit of the higher serial resistance?) and 100k to ground (this gives bigger pulses but doesn't actually improve the S/N that much). Whatever resistors I choose, the pulse seems to be around the 10pC worth of charge. I guess in previous attempts, my circuits were simply suppressing so little charge and I didn't notice the pulses just didn't become 'visible', and got swallowed up in noise.
Now I turn to detector circuits... oh boy!!.. I went around this loop for ages - dozens of 'man hours' before already to no success. Similarly today and yesterday I've spent around 12hrs experimenting with damned circuits. Yes, Doug, I tried yours early on, at least with the transistors I have available. Maybe that is critical. But I found out something else, which makes me hesitate on your circuit. In one test, I manually adjusted the bias on a sensitive signal transistor to give a response. The circuit was set up to be 220kOhm input impedance. I did get signals, but it was significantly missing many out, it was only picking up the very biggest. So even with that high impedance, the signals still got pulled down.
I tried further and further, but the more I tried to push up the impedance of the detector, the further I seemed to get from detecting those pesky 10 pC's!
I don't know if the output on these tubes is a bit different to the ones you are running, but I am also sure I'm not skilled to make your circuit 'sing'. So I am planning on going to buy some JFET opamps and see if upping the input impedance gives me a more suitable circuit. I've spent too long now with the parts I have to try carrying on with them, so I'll try some new components, see if they give me the 'answer'. The cheaper JFET opamps can be found with 10^12 input impedance and 3MHz response, so I'll go for that level of spec first.
Here's all I have to show for a day's worth of 'effort'...