Okay, that would have been the (IIRC) "isolinic reaction" that allowed the particulates in orbit to be collected. This is also the source of the heat that require the Baku to be removed.
The reason for the collection was not just to gather some elixir of youth for the Son'a. An equally important reason for the operation was to ruin the fountain of youth for good, so that the Ba'ku would die of old age just like any other members of their species. I'm sure the Son'a would have made sure that the rings (or whatever the fundamental source of the health effects) would be completely destroyed, hopefully also with some painful side effects to those down on the planet.
It was a mission of vengeance and destruction, after all, and the Son'a were the only ones who knew how the technology worked. Either they would have slipped in a destructive element or eleven without the technologically less advanced Starfleet noticing - or then they would have slipped in those elements come hell or high water, since destruction of Ba'ku happiness was a non-negotiable mission goal.
It was explicitly stated in the film that harvesting the radiation using the Son'a collector would render the planet uninhabitable. It wouldn't just make the Ba'ku mortal, it would kill them.
That's the whole reason they had to bother about trying to surreptitiously move them.