So i purchased all of the Western Electric 427A tubes. A quick search brought up that these tubes had 4uCi of the Kr-85 in them (manufactured August/1966).
Kr-85 is a rather "uncommon" isotope along hobby spectroscopers ;)
Kr-85 is a radioactive gas with a half-life of 10.7year, leaving me with around 0.1uCi of Kr-85 left.
Kr-85 emits 251kev Betas with 99.5% intensity and also emits some gammas, the only engery thats notable is 513kev@0.4%
So out of the 3.7k disintegrations/s only around 15 are the gammas @ 513keV
The tube boxes warn about radiation of Kr-85 but don't indicate a activity
Three pieces on a Ludlum 44-9 pancake tube give around 200cpm
I used three pieces to make a 60min spectrum run. (Theoretically 45 gammas/s @513kev)
Below you can find the spectrum with subtracted background and the spectrum in relation to background (grey). The FWHM with subtracted background is around 12%
The high peaks in the 0-200kev range are most likely emission artifacts of the beta radiation
If someone wants to grab its own sample of Kr-85 search ebay for 427A tube, but be sure to ask the seller if the package has the warning sign. I'm not sure if later date code models used the isotope too, because there are alot of these tubes around that don't have the warning on the box!
