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rugged case

Posted: Mon Apr 02, 2018 1:33 pm
by greglarious
Looking advice on building a rugged case for the MightyOhm. Not very hard to try it and find out but I have some specific concerns I could use help with:

1) First idea: seal the whole thing in a rectangular plastic box:
- will this interfere with detection of particles?
- if the box is air/water tight will that present a problem with heat buildup?

2) Second idea: seal the GM tube into a detachable cylinder (PVC pipe?) and connect to main unit with a coiled cord
- same 2 questions from above about detection and heat buildup
- will a 3 foot extension cable between the main board and the tube interfere with voltage or current?
- any special concerns about making an extension cable for 600v?
- how rugged is the GM tube ... does it need to be cushioned with foam inside the cylinder?


Re: rugged case

Posted: Mon Apr 02, 2018 3:14 pm
by mightyohm

Plastic will shield some radiation from reaching the tube, particularly beta particles. Thicker plastic will have more shielding effects, so I'd use thin low density plastic where possible. If you're mainly interested in observing trends, this may not be an issue as it's the change in measured radiation and not the absolute value that is often most interesting. I believe that most environmental sensors primarily detect gamma rays, so the loss of beta sensitivity may not be a problem depending on your application.

The kit wasn't designed to support remote mounting of the tube, but I've seen it done successfully. Try to keep the cable as short as possible to minimize interference and capacitance. You may need to use coax or twisted pair wiring. It really depends on the environment that the circuit is operating in. BNC is most common. There's an example of this here: viewtopic.php?t=519

The SBM-20 tubes are somewhat fragile, but as far as I can tell the primary risk is crush damage. I haven't seen isolation mounts used on commercial detectors that use this tube, but it's not a bad idea if the tube will be handled roughly. They may not survive being dropped.

BNC cables and connectors should work fine at 400V. Most BNC components are rated to at least 500V. There are special high voltage BNC connectors available. Check the specific coax and connectors that you are using.