Particle accelerators are machines that propel charged particles at incredible speeds, generally to collide with other particles. It's highly advisable that the particles the high-speed particles collide with should not be part of your head, as one man learned the hard way.
On July 13, 1978, particle physicist Anatoli Bugorski was working his job at the U-70 synchrotron, the largest particle accelerator in the Soviet Union. The 36-year-old was inspecting a piece of equipment that had malfunctioned when the accident happened. Unbeknownst to him, several safety mechanisms had also failed, meaning that when he leaned over to get a good look at his task, a proton beam shot through the back of his head at close to the speed of light.
Or at least, closer to the speed of light than you'd like a proton beam to be traveling at when it shoots clean through your face.
At first, he felt no pain. He knew what had happened, as he had seen a light “brighter than a thousand Suns," as well as the gravity of the situation. At this point, he didn't tell a soul, and merely completed his day's work before heading home and waited for the inevitable to happen.
Absorbing 5 grays (500 rads) of radiation would usually lead to death. Though he didn't yet know it, he had been hit with between 2,000-3,000 grays (200,000-300,000 rads). In the night, his face began to swell beyond recognition, prompting him to visit the doctors the following morning. From there, he was taken to a clinic in Moscow, though largely so that his death could be observed rather than for any expectation that his life could be saved.
The next few days saw his skin peel off around the entry and exit wounds, showing a clean path burned right through his skin, skull, and brain.
Remarkably, he did not die. The brain tissue continued to burn away over the ensuing years, and his face became paralyzed on the left side, where his hearing was also lost. Weirder still, as he aged the right side of his head showed signs of aging, while the left side did not.
Over the next few decades, he experienced seizures but remained functional, continued his work as a physicist, and completed a PhD. As far as people who have put their heads into a particle accelerator go (and to be fair, that's a demographic of one) he was pretty lucky. The narrow focus of the beam, though it caused massive damage, likely kept the damage limited to an area of brain that he could live without.
For the decade after his accident, he was unable to tell anyone about it, given the notorious secrecy of the Soviet Union. He survived well beyond the end of the USSR, however. In fact, the man who put his head in a particle accelerator and lived to tell the tale remains alive to this day.