Quote Originally Posted by budrichard View Post
If a radioactive isotope. it has a half life i.e. after a certain amount of time only one half will be left to disintegrate and provide readiation to check your GM tube. Therefore you need to know the isotope and half life to determine if the source is still effective in checking your GM. In short, just having a GM counter without the proper documentation and/or training, you don't really know if your detecting anything.-Dick
Yes, this is all pretty common knowledge for anyone with a little interest. No, you do not need to know the isotope and half life for a check source. I think you have an inverted view of how to use a check source. If as you imply, a radioactive isotope with a very short half life was used as standard check source , and disentragated long before my test, the result would be that the check source test would not register. This opens up the possibility of a false-negative, i.e. a dead detector that's not actually dead. However, there is no room for a false-positive, as you are concerned, i.e. a working detector that is actually dead. What can not be done with an unknown check source, is to callibrate the actual reading.

I'm not really clear on what's driving this bad detector discussion. As I mentioned earlier, regarding the 1960's quip, I don't believe it to be the case. My detector was last callibrated in 1996, and it's a model that can be bought on-line today. Do you have one of the lenses that I have listed and believe it to be radioactive?