How to determine the radioactivity of a nuclide source when required absorbed dose is known ?

How to determine the radioactivity of a nuclide source when required absorbed dose is known ?

DEARS

I need to choose a Beta source for radiobiological experiment.
A coin-shape source will be attached to animal skin for
irradiation,now the required absorbed dose is know, how can I
calculate or estimate the radioactivity of a source for the purpose ?

It's a little bit urgent, could anybody help ?
Thanks