GM tube with two electrodes?

[AllenWallace]

I have a couple of pancake GM tubes salvaged from an air monitor which have an unusual electrode arrangement. I'd like to understand what going on, so I can devise a circuit which will adapt the 2 BNC system to a single BNC system.

I figure the HV BNC supplied the HV and the standard BNC is the output. The bare tube has a standard mica window and four electrodes. One wire goes to electrode A. The other wire goes to another electrode B, which is connected via a RC to the third C and fourth D electrodes. I'm pretty sure that the metal shell of the tube is the cathode. I think that electrode C and D support an internal grid.

Any ideas on what's happening inside this tube? any advice on how to adapt to a survey meter which combines HV and signal into the same coax connector?

[John Futter]

Wallace

you have only given half the info ----where do the two external connectors go to?
ie which pins on the detector head?

[Frank Sanns]

SHV on the left for HV and the BNC on the right is for signal out. Typical configuration for scintillator probes.

Using just the SHV connection will work. Search the site for the details with scintillaition preamps.

Frank Sanns

[Richard Hull]

I see the limiter resistor is in the probe. That would demand two connectors. one for High voltage and one for the signal out. I don't understand why they did this. They must have a brother-in-law in the hv bnc biz.

Richard Hull

[DaveC]

On Richard's observation, the only reason I could imagine would be noise reduction

The easiest approach, assuming the rest of it works, is to invest in some plastic cabling wrap, to make one cord, or heat shrink...To actually eliminate one cable, requires putting the HV supply in the probe and feeding with a LV power/signal return cable. or use Triax cable and relocate the high value resistor (2.2 Meg) to the main box. The HV return would need to have a current sense resistor in it,

If you could put the HV outgoing wiire through a small current transformer located in the main instrument housing, you might be able to read the current pulses that way. This would eliminate the need for a Triax line.

Dave Cooper



D

[Richard Hull]

GM tubes are gas amplified at the device. Noise is virtually never an issue. typical GM pulses at the tube can be on the order of 70 volts, depending on biasing.

I personally float the GM shell against ground with a 5k resistor to get postive going pulses on the shell. Works great. You still need the 2 meg limiter from the central lead to the HV however. This setup makes for a smaller positive going output pulse that can DC drive logic and amps. No capacitor needed and lowers the impedance of the output line. I then typically drive cmos logic or an NPN darlington to pump the signal up a bit.

This setup definitely requires two lines.

Richard Hull