| T O P I C R E V I E W |
| Alchemy2 |
Posted - 01/11/2015 : 21:14:01
Happy New year to all!
Well, I recently retrofitted my first 320+ unit with a SBT-11A as well. This one has about a 450mm extension of RG-174/U coax to a probe box housing the tube. This permits me to have a more "remote" sensing of radioactivity and still have the GQ unit data logging but without having it as proximal to the GM tube sensor.
I really like these Russian SBT-11A tubes: they are relatively cheap, they are sensitive, small, robust, easy to run on any GM kits out there, and respond to alpha, hard and soft beta, soft and hard X-rays, and gamma. Overall, they are superior to the 4011 and SBM-20 tubes (though I like the 20s too!), given that they have greater sensitivity and alpha particle response. IMO, a SBT-11A equipped GMC-320+ is almost as good as one needs for almost everything.
However, I do plan to try connecting a SBT-10 to my unit once my anode resistors arrive. Having a sensitive tube is one thing, but a BG of some 80CPM or more is just silly. I like the ~30CPM range that the SBT-11A gives.
The SBT-11A runs in the 270-320 CPM/uSv/h regime. My current one is running at 318, the integrated unit I setup before is running near 270 CPM/uSv/h. dead time measurements put it in the 50uS regime, which is just fine. I can max out the CPM reading and it's still counting!! So, no issues with the cable length and parasitic capacitance of the cable. Nice.
I'll post dead time measurement info on another message for those interested. It's very easy to do, the math can be done on a standard calculator. Doing those measurements allows one to evaluate their system to see how well it responds and whether there is anything limiting performance.
Anyway, I'm totally sold on the 11As - they are a nearly perfect match to this unit - no additional resistors required!!! Also, for those doing X-ray experiments, I think that this tube is a good choice. I can get 140CPM off a beta light keychain. This is NOT beta (since the tritium tube is embedded in acrylic which absorbs the 12/6kEv max beta particles); it's bremsstrahlung from the betas, which are at or below 12keV. This puts the response in the soft X-ray regime.
Alchemy2 |
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