a tube with kr85 has also found its way to me. Jonathan has explained that the tubes has had in 1966 approximately 4µCi Kr85. This are 148 kBq. The halflife is 10,7 years and from 1966 to 2018 its 52 years. So we have only 3.4% of the activity, ~ 5000 Bq. The gamma rays are only 0.4% of all and so near 20 Bq. It is a very low activity and so i measured about 9 1/2 hours. With all corrections to the geometry of the arrangement i could calculate 20 - 40 Bq.
In order to reduce the strong activity of beta I have used a copper disc 0.6mm in front of the tube. Here is my result.
