As always, I'll keep the style of my answer and decompose the question into several subquestions in order to explain in better:
- what is the source of radiation in those lenses
- what type of radiation is present and what are the consequences
- as always, should you buy one, just as a cool item
Why Thoriated glass?
The advantage of thorium glass is due to its high refractive index and low dispersion, two most important parameters for "practical optics".
Here is the example of such a graph.
Refractive index is simple to understand, keeping Snell's law in mind. High refractive index = more efficient "curving" of the light.
Abbe number (V-number) is a bit more complex but intuitive to understand. Different wavelengths will behave in a different manner and will take different angles. If you play with the eyeglasses, you will be able to see rainbow-like structures.
For camera lenses, it's useful to describe the refractive indexes at several specific wavelengths, so called Fraunhofer D-, F- and C- spectral lines (589.3 nm, 486.1 nm and 656.3 nm respectively). In simple words, green, violet and red.
What about radioactivity?
Good scientific literature is very scarce, but this paper is wonderful.
The full sequence could be found on Page 7, and the most important for you is to notice that there are two types of radiation: α and β-.
Both types have an incredibly short reach and can be stopped by basically anything. Including the - air.
Practical guide
As always, I'll give you some ideas for shopping. Here is the full list and those are my favourites:
SMC Takumar 50mm f/1.4
This is one of the most popular f/1.4 lenses and among the cheapest with the 100 $ price tag. The mount is M42, so you will have no problems to properly adapt it.
Canon FL 50/1.4, eary versions
Dating back from the 60's with a similar price tag. Include the adapter as well (probably). It's not EF, not FD, it's FL!
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