I do believe that this really did happen, however the 500-600 rads being enough to kill someone is not very accurate. That equals 5-6 gray and would not likely kill a healthy adult. Something more in the 1000 rads or 10 gray of TOTAL BODY irradiation would do the job via the hematopoetic syndrome. A syndrome in which the bone marrow is killed off because of the insult/damage from the radiation and no longer produces red or white blood cells. It is used as a common total body threshold for Bone Marrow Transplantation. However 5 to even 10 gray would not likely kill anyone in a localized exposure unless it were to go through a vital organ. That would certainly have been the end of him if it were to have hit something vital such as the medulla.

Another thing is like Semi said a proton beam would have to be very slow to only be the speed of sound. Speed of light would be more accurate.

As an answer to your question Sasha, the exact energy of the beam is determined by the amount of Voltage applied to create the beam in the first place, which is manually controlled and documented by who ever was running the machine.