> He didn't know anything was happening at the time but ended up losing a couple limbs.

Surely that would have been reported by the university (PR, damage control) and in the news? Can you confirm?

He told me about it back in the early 90s, and it had happened years before that, though I don't know how many.

A quick google doesn't bring anything up, but a local newspaper would have had to put their old archives online without paywall.

I'm not sure I understand - how did the injured man actually place himself in the path of the particle beam? Given that particle accelerators are a sealed vacuum environment? Or could you be harmed by simply standing next to the accelerator while it's operating? If so, how is that energy reaching you?

For experiments, it's not unusual that the actual test is taking place in atmosphere. There is no issue as you can simply put a solid metal cap on the end of the accelerator and the beam will just blast through it with no issue and the atmosphere isn't going to slow it down significantly over the next few meters.

Thanks, I didn't know that. In a situation like that, is most of the danger to a human from bremsstrahlung, or the actual original beam of particles?

Both and more. The danger from the beam is limited to a specific energy intensity, otherwise the beam will pass through without depositing much energy (there is ways to calculate this). The bigger dangers are all the atoms the beam knocks out on it's way that are likely to transmute into radioactive isotopes via various modes of actions as well as the hard bremsstrahlung generated by the beams interactions as well as some of the stuff it knocks along (there is enough energy that if the beam knocks into an electron, the electron will dump some bremsstrahlung while it slows down again).

Interesting. I always assumed the radiation was from the target.