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Registered Member #96
Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4059
Hi all.
Making progress with this project. See
I did notice that even at 290V the typical el cheapo (ie low sensitivity) GM tubes do generate detectable EMI with a sufficiently sensitive receiver next to the tube.
It is interesting to note (sorry for the confusion Stella!!) that many of the problems associated with Ebay GM tubes can be resolved simply by shielding the counter properly and a good test is to use a plasma globe. If the shielding is done right there should be no change in count near the globe. The sensitivity problem also exists with opening microwaves too soon and 3/4G, this has been the cause of many an annoying problem which only seemed to happen at odd times.
Never underestimate the effectiveness of an HF bypass to Gnd from tube HV side, I found a 1000pF+10M+10M did the trick on some particularly stubborn miniature tubes.
Registered Member #122
Joined: Fri Feb 10 2006, 12:55PM
Location: Milano Italy
Posts: 148
Some end-window and pancake GM tube can be sensitive to ultraviolet light also!
There are many ways to shield a GM tube, but all of them depends on the radiations also: for a gamma-only tube, a simple allumium box will suffice. for a gamma-beta tube, you need a metallic box with an entrance window covered with tinfoi. For Alpa-beta-gamma tube, the EMI shield may be a problem!!! you can use a metallic grid to cover the window and/or a metallized mylar foil (none of these solutions provide a complete shield against EMI and all of them cause a substantial reduction in alpha sensitivity).
to avoid false counts with ultraviolet light (and sun also!), the window can be covered with a thin substrate of graphite (graphite spray is the easiest option!)
Registered Member #96
Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4059
Yeah, for that matter some (cough SBM /cough) are sensitive to broadband blue light as well. I noticed that this might have occured with my recent lightning research as counts went up exactly as the bolts hit yet the shielding was EMI proof.
Re. graphite, guess who "discovered" that PG makes a very nice though lossy alpha window..
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GM tube hacking
