Likely zero, as it was encapsulated with resin. They would have had to grind the ring to dust and then eat the whole damned thing.
It was also commonly in record and camera cleaning brushes, as it could be used to induce a static charge, which would attract dust. Likewise, encapsulated, so the risk with normal use was minimal, but again, if you ground the brush to dust and ate it, all bets are off.
> Likely zero <…> They would have had to grind the ring to dust and then eat the whole damned thing.
Never had kids, huh?
But it was a spinthariscope. If the resin blocked all the alpha particles, then they wouldn't produce the visible flashes in the screen. It seems more likely that the radioactive substance would be embedded in the surface of the resin. And therefore eating it would still allow it to damage your tissues.