This reply was stimulated by your post about TIER for one of these tubes.
Exercise TIER calculation for SNM-11 tube
Richard replied that he would expect to see more counts from the test set-up you have.
Earlier I made a reply to your BNC connector post.
Making a BNC adapter for SNM-11 boron neutron counters
In that, I mentioned the excellent paper by Bob Higgins about using these Corona Neutron Detector tubes. It is the best reference I have seen on the subject. You said you had read it in great detail, but then you said some things about voltages that make me wonder if you really follow how these tubes are intended to be used. You said,
"However the SNM-11 works best at 700V. In fact I've had one test where the tube worked correctly at 670V. I didn't try lower voltages.
In fact even other corona tubes such as the SNM-32 which are also said to work at 1500-1700V; my experience is that they are unusable at voltages above 1275V because they start oscillating at high frequency (and I've had independent confirmation from somewhere else that it's indeed their normal and expected behavior based on a sizeable sample of them). But they work well at 700V. "
The ~100 Mohm resistor in series with these tubes is critical. I was trying to point that out in the BNC thread.
Here's a table with specs for a good number of these Russian tubes.
This was google-translated from Russian. The places where it says "crown" should be "corona". It has listings for both the SNM-11 and SNM-32. The SNM-11 is a Boron tube while the SNM-32 is 3He but both are expected to be run in corona mode. The letters SNM are westernized from Russian, I've seen CHM, CNM, etc. all seem to be equivalent.
These corona tubes are intended to have an internal light corona discharge all the time they are operating. The normal configuration has a 100 Mohm resistor (or maybe a bit more) in series with the anode (center conductor) of the tube. The supply voltage goes through this resistor, not directly to the tube.
The pdf table above has 3 key columns of values: Nominal operating voltage, Starting voltage, and discharge current max. The nominal voltage is the HV that is applied across the series connected tube and 100 Mohm resistor. Per the table, for the SNM-11 or 32 this should be in the 1500 to 3000 V range.
As this voltage is first applied, the tube appears as an open connection, drawing no current, so the tube will see the whole voltage. This starts the corona discharge in the tube, causing it to draw current. The current through the 100 Mohm resistor causes voltage drop across it making the tube see a lower voltage. The system finds a stable operating point with 600 to 700 V across the tube and the corona discharge drawing about 10 to 20 uA current. In the table, the Starting Voltage column gives the voltage across the tube in corona mode and the discharge current max column is the highest current that the tube should be allowed to draw in its idle mode.
The 100 Mohm resistor in series with the HV supply effectively makes a constant current supply for the tube in the 10-20 uA range. In his paper Bob did some calculations. He assumed the corona voltage would settle to the midpoint of its start voltage range or 650 V. He picked a supply voltage of 1800 V, so the voltage across the 100 M resistor is 1800-650 = 1150V, so the current through the resistor (and tube) calculates to 11.5 uA which seems OK and well below the max current. This is a ballpark calculation. The tube will find its own operating point but it should be close to this.
The 100 M in series also helps to quench the tube after a neutron pulse. The pulse draws more current making the drop across the big resistor greater and lowering the voltage across the tube a bit.
I think you said you bought your solution from someone else. The circuit feeding the tube needs to be roughly constant current, either via the 100 Mohm or some other method. If I was you I would try to figure out the circuit and ensure you have a very large resistor (~ 100 Mohm) from the HV supply. The specified operating voltage of 1500 to 3000 V is into this series resistor, so the voltage across the tube should settle to around 600-700V. If you want to measure this directly bear in mind that the effective impedance of the tube is around 650/12u or about 55 Mohm, so your meter should have an input impedance of 500 Mohm or more to not shift the reading much from what just the tube sees.
Here's Bob's simple diagram of the detector.
He is coupling the neutron detection pulses from the anode through a DC-blocking 27 pF capacitor to the amplifier and comparator. He put the battery-operated detection and HV supply circuitry very close to the corona tube. Bob feeds the resulting pulses through a coax to some kind of counter.
If I understand your configuration, you may have the same general circuits but you are planning to run a coax from the tube to the circuits to carry both the high voltage and signal. It would look like this.
This may work OK but it puts the capacitance of the cable from the anode to ground. This may make it harder to get a clean pulse and might make the whole set-up more likely to oscillate. The connections to the cable here are high impedance so more susceptible to noise. I think you'll do better if you can put the front end circuit close to the tube and send the processed pulses from a low impedance driver through the cable as Bob did.
Bob Higgins' paper covers all this in more detail than I have provided, but your mention that 700V works better than >1200V makes me think you might not be seeing the distinction between the supply voltage and the corona voltage across the tube.