This story is almost certainly true. I earned an M.S. in physics and have worked on several experiments at the Thomas Jefferson National Accelerator Facility in Newport News, VA. (It is the largest "continuous" beam electron accelerator in the world.)
Many people have said that sticking your head into the beam path would involve breaking the necessary vacuum. This is not true. Particle accelerators use magnets to accelerate charged particles to near light speed, and a vacuum is required for this. But in many cases the experiment does not take place inside the beam pipe. Instead, the beam pipe runs right up to the face of the experiment's target. Then the already-accelerated particles exit the sealed end of the pipe, fly through a small amount of "outside" air and into the target chamber (type of target chamber depends on the experiment). If I remeber the story correctly, Bugorski was checking on something in his experiment's target chamber, not on the particle accelerator when this accident occured.
Many people have said that sticking your head into the beam path would involve breaking the necessary vacuum. This is not true. Particle accelerators use magnets to accelerate charged particles to near light speed, and a vacuum is required for this. But in many cases the experiment does not take place inside the beam pipe. Instead, the beam pipe runs right up to the face of the experiment's target. Then the already-accelerated particles exit the sealed end of the pipe, fly through a small amount of "outside" air and into the target chamber (type of target chamber depends on the experiment). If I remeber the story correctly, Bugorski was checking on something in his experiment's target chamber, not on the particle accelerator when this accident occured.