As a graduate student I did a lot of work using radioactive materials, so I have some idea what a billion Bq means!
“With a 1/2 life of 138 days, there would be nothing left after 8 years.”
Depends on how much you started with. If you start with a billion Becqurel, after nine years you’d have 60 or so Bq left, that’s 60 disintegrations per second, which is measurable with a good counting system. It’s a few handfuls of atoms. The problem is the statistics, to distinguish that from the natural background.
As a graduate student I did a lot of work using radioactive materials, so I have some idea what a billion Bq means!