Horace,
It is not possible to get an exposure in a vacuum from degassing species using the same exposure time as with atmospheric pressure gas. This is not even a close call. The exposure times are way too long. The radioactive species gets immediately evacuated.
Do you have a reference for this high initial degassing rate, followed by a subsequent almost complete degassing turn-off ?
I would have thought that following a high initial rate (few seconds) which takes the loading down 10% or so, from the starting level, that the subsequent rate of degassing would be slow and steady.
And besides - what kind of beta decay or radioactivity produces ~300 eV electons?
Jones
