How can a person be hit by a high-energy proton beam? There is this somewhat famous story of a Russian particle physics Ph.D. student from the 70s, who stuck his head into a particle collider and got hit by a beam of high-energy protons. For more details see here, here or here.
I am confused about how exactly this is even possible. As far as I understand one needs to create an ultra-high vacuum in a particle collider to let different particle beams collide. If there is "air" in the collider, which there certainly would be some in the case of maintenance work, wouldn't the high-energy protons just interact with the surrounding air and ionize it? This would also happen in a relatively short distance (this is intuition, not sure if it is true and how one could check...), so I'm not really sure how a person can be "hit" by a proton beam...
 A: 
I am confused about how exactly this is even possible. As far as I understand one needs to create an ultra-high vacuum in a particle collider to let different particle beams collide.

The ultra high vacua in the present collider experiments are needed for two reasons

*

*To make sure that the detectors are getting the debris of pure proton-proton hits, and not of random molecules ( which still exists because no vacuum is complete,and have to be included in a background for the Monte Carlo simulations of the interaction).


*Keeping the beam intact , i.e not losing energy and direction through the scattering  you describe, and becoming useless for accurate measurements.
In the early times when one was studying interactions of particles in bubble chambers and detectors , the beams were crerated in vaccuum, but arrived at the detectors through air, at least in bubble chambers I am sure it is so, because I worked on Kaon experiments at CERN. That is because the incoming particles were seen one by one parallel, any interactions would take them out of the beam and they were not so many because the intensity of the beam was kept so that a small number of particles arrived at the detector.


Picture from CERN 2-metre hydrogen bubble chamber exposed to a beam of positive kaons  with energy 10GeV entering from the bottom of the picture.

If we lost the beam we used to joke that "a cat had entered the beam line".
For more intense beams used in electronic detectors the air beam would be dangerous, even if not  intense enough to be visible.
Edit after finding this reference, page 343:


The first fast ejected beam from the PS was
obtained on 12  May 1963.  This photograph shows
the beam lighting up blocks of  plastic scintillator
as  it travels in air on its way into the South Hall

The photo is in order to show how the beam interacted with the scintilators, no light is seen in the air .The hall was well lighted all times, the photo could not have been taken with the hall lightings on.
So the physicist must have been  checking a beam line, thinking the synchrotron was off, but it either was still on, or it came on without alarms going off.
A: Here is a slide that I found by Goggling "Proton beam in air"

