Are there moments in particle collision experiments where the particle beam is in open air? I keep encountering the story of Anatoli Bugorski, who apparently got hit in the head by a proton beam at the U-70 PS in Protvino, Russia. 
I find it difficult to believe that this is actually possible, because I've always been told that particle accelerators (from CRT's to the LHC) require a vacuum to prevent beam loss and defocussing and whatnot. 
How do you get your head in the path of a particle beam if it travels in a vacuum tube? Or, is a vacuum always needed if you want to accelerate particles and do scattering experiments?
More concrete, is this incident possible? Was the U-70 operating without vacuum?
 A: I was working at CERN bubble chamber experiments back in the early seventies. The accelerators required a vacuum so as to be able to sustain the beam which makes many turns in the circle ( practically velocity of light), so the best possible vacuum is and was a necessity. 
After the generation of the beams , the beam lines did not need a vacuum because the probability of scattering in air is very small, and mainly a bit of ionization can happen. Even in the dense  liquid of the bubble chamber maybe one in ten incoming particles interact; the intensity was low, controlled to be ten or so  particles at a time,  for clear pictures :

This is an antiproton beam entering on the left.
When we got a glitch in data taking we would complain that the cat had entered the beam line ! Yes, a person could get in the beam. There were physicists in the early years who would center the beam by the Cerenkov light it made passing trough the eye. I know of one who  died of cancer of the retina.
In those early times a proton beam could be a primary beam steered towards an electronic detector. In this case the beam would have many more particles and would present a greater danger. Still it could go through the air, depending on the experimental setup, may be  for special checks with low intensity, but one should be careful of the radiation induced and certainly not to enter the beam line.
A: It's primarily the Earth's magnetic field that shields us from the Sun's protons, not the atmosphere. It's just that the losses are quite high for particle accelerators not in vacuum, and, they generate all sort of noise that would drown out whatever the physicists are looking at. Hence operating in vacuum.
Also, research particle accelerators are extraordinarily complex instruments. Perhaps he was working on the part of the device that injects the beam. Just because the beam might not be able to travel well several hundred meters in air doesn't mean it can't go through your head. Also, there seem to be very little details about the incident.
Finally, an X-ray machine is a particle accelerator that doesn't operate exclusively in vacuum.
So to answer your first question, no, a vacuum isn't required. How the hell his head got into the path of the beam is something better answered by experimentalists.
