Algodoo

Interesting. I looked for spinthariscope on YouTube and your simulation looks believable. I don't know much about radiation, but a spinthariscope doesn't seem like a very safe toy.

They actually are quite safe. Just don't eat the radioactive material! LOL
All kidding asside, you can buy one of these from United Nuclear (the company owned by the famous Area-51 scientist, Bob Lazar). You can also find them on Ebay and other sites. The radioactivity in a spinthariscope is << the radiation from a smoke detector (which is typically <1uC)

I suspect that is much less than the radiation from the vacuum tube diode in the high voltage section of an old television set.

That's a different kind of radiation. The hv diode in old TV's often emitted "soft" X-rays, but the spinthariscope emits mostly Alpha and Beta particles from the result of radioactive material (typically Americium-241). It's all ionizing (bad) radiation, just different flavors.

EDIT - There is usually ZERO detectable radiation outside the walls of the spinthariscope because alpha and beta particles cannot penetrate the plastic housing. They truely are very safe. That's why they can be handled by children under adult supervision.