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Hello,

Looking for an alpha capable tube, I found an SBT-9 on eBay It is due to arrive next week. It will come with matching clip and anode resistor (I'll give you the value on this forum as it is hard to find!)

I have two questions about wiring the SBT-9 to the Geiger kit.

- do you think it is safe to have both SBM-20 and SBT-9 connected in parallel at the same time, considering they operate at almost the same voltage (respectively 380 and 400v)? I'd rather avoid to connect/disconnect the SBM-20 too often by fear of breaking it and I don't want to permanently replace the SBM-20 as the SBT-9 seems to be less sensitive in the gamma-beta range. My plan is to add an easy connector (like header pins) to connect the SBT-9 to read alpha rays and disconnect it to read beta and gamma with the SBM-20.
- apparently the anode resitor for the SBT-9 is 10 MOhms. Can I simply put another 4,7 MOhm resistor in serial with the 4,7 MOhm resistor of the kit or is it better to have a single 10 MOhm resistor as close to the anode as possible? And what power should the resistance be able to withstand (e.g 1/4w, 1/2w, 1w...)?

As a bonus, here is a graph of some measurements made with the kit :



From 0 to 300 secondes the background radiation oscillates between 10 and 35 CPM, then a big granite cylinder puts the count between 25 and 50 CPM up to 850 seconds when I swhitch back to background and then again with the granite cylinder from 1100 to 1400 seconds.

Best regards,
Vincent Becker

It should be ok to put both tubes in parallel. You can feed each tube with it's own anode resistor and tie both of the cathodes (negative ends) of the tubes together. Some commercial geiger counters do this.

Try connecting a 10 Meg resistor to the SBT-9, you can tie the other side of the resistor to the HV+ test point (TP2). If the resistor is too big, the geiger counter may not register all counts from the SBT-9. Some experimentation might be necessary.

Good luck.

Thank you! I'll give you some news as soon as I get the tube.

Hello,

As promised here are some details of the mounting of my SBT-9 alpha tube on the Geiger kit. The tube I found on eBay went with two matching metal clips, 10 Mohm resistor and HV cable.

As stated above, I didn't want to attach it permanently. I wanted to keep the SBM-20 tube for beta-gamma measurements as it is more sensitive in that range (more below) and I didn't want to switch tubes as they are both very fragile.

I first added an external connection port to the kit. I used some of the HV cable to connect a female pin header to the HV+ and cathode connectors. The header pin is attached to the board with crazy glue.



The tube itself is attached to a piece of prototyping board with two metal clips. I added a 10 Mohm resistor right behind the anode clip. The whole anode part was then isolated with heat-shrinking tube and electrical tape, leaving only the cathode parts visible. The tube is thus safe to handle even when conneted. With the leftover HV cable, I added a small extension and a male header pin.



The tube can now be very quickly attached or removed from the kit at the back of the protoboard.



As the operating voltage of the SBT-9 is 380V and the SBM-20 is 400V, I tuned the voltage of the kit to 390V. This should be well within tolerable parameters for both tubes. The SBM-20 seems to perform similarly at 400V or 340V, which was the voltage of my kit before I bought a 1 GOhm resistor to measure it accurately.

Before putting the SBM-20 back in place, I took some comparative measurements with each tube. Both measurements where taken in similar conditions : 10 min of background, 5 min on top of a big granite cylinder (which was my most radioactive source at the moment of the measurement, but I just found old radioactive lamp mantles!) and 8 additional minutes of background. I don't have any alpha source to test the SBT-9 in that range yet. As americium-based smoke detectors are banned in Europe I'll have to find something elese...



The blue curve is the SBT-9, the red curve the SBM-20. The horizontal lines are the average of each measurements.

As you can see, the SBT-9 is about 30% less sensitive in gamma-beta than the SBM-20. It is also about 30% shorter, so it makes sense.

I didn't measure it yet but I guess that with both tubes connected at the same time their sensitivities would be added.

Best regards,
--
Vincent Becker

Vincent,

Excellent, thank you very much for sharing this. I'll have to buy one of the SBT-9 tubes, that is one tube that I don't currently have in my collection.

Hello friend, I would be helpful for you to share the schematic for the tube SBT 9 .., Thank you!