This question can be answered by Anatoli Petrovich Bugorski, a particle physicist, who in July 1978 actually put his head in the proton beam trajectory of the synchrotron (a type of particle accelerator) at the Institute for High Energy Physics in Protvin, based in the Soviet Union. An unfortunate series of circumstances led Bugorski, at that time still a PhD student, to this unique experience in history. On that day, Bugorski was about to repair a fault in the accelerator he had been used to working with. He therefore told the control room to stop the proton beam in order to safely enter the laboratory where the beam, on exiting the accelerator, was going to impact a detector. The control room, however, did not carry out the instruction in time, leaving the beam active. On top of that, the door to the lab where Bugorski was supposed to go should have been closed if the beam was active. Instead, the door was left open that day by a colleague who had deactivated the security system. And, just like a final adverse fate, the light bulb inside the laboratory had just blown out, which would light up when the beam was active.
While Bugorski sensed that something was amiss, he did not realise the potential danger. Getting closer to the equipment he wanted to work on, he placed his head in the path of the beam. He saw a flash “brighter than a thousand suns” – and that is how he described the incident. Nevertheless, he felt no pain. A beam of protons with speeds of hundreds of thousands of kilometres per hour, at an energy of 76 billion electron volts, had hit him on the back of the head, pierced his entire brain and exited at nose level. What was his reaction? He finished the work he was doing, filled in the lab diary noting the day’s progress, and finally went home without telling anyone about what had happened.
Over the next few hours, though, Bugorski could not continue to pretend that nothing had happened. His face began to swell alarmingly and he was forced to go to the doctor. Over the next few days, his skin began to peel off, and his hair to fall out, revealing the precise entry and exit points that the protons had made through his head. Also, beyond the outward signs, the beam had burnt his bones and his brain tissue as well. The doctors calculated the radiation dose Bugorski had been exposed to. It was a hundred times higher than the fatal dose, so the doctors thought his end was sealed.
And yet, not only did he survive but, he even completed his PhD and continued working at the same institute where he had experienced that incredible accident.
He suffered some permanent damage, for example, he became deaf in his left ear and lost feeling in many facial nerves, which caused a paralysis of the left side of his face, but despite this, his intellectual abilities were virtually intact.
But how is it possible that Bugorski survived an accident of this magnitude? Part of the explanation lies in the nature of protons. Every particle, when it travels through a physical material (such as someone’s head), transfers its energy to whatever it crosses in its passage. If the particle has a high velocity and thus high energy, in passing through the material it will ionise atoms, destroy molecules, break structures of various kinds. As it collides and releases energy, the particle slows down to a stop. It is possible to calculate what is the average path that a particle with a certain initial energy has to travel inside a material before it stops. The peculiar characteristic of protons is that they give up most of their energy when they are getting close to a standstill. This is the principle underpinning proton therapy: a cutting-edge medical technique in which, basically, the Bugorski incident is repeated but in a controlled fashion, so as to destroy tumours that have grown in areas that are difficult for traditional surgery, for instance the brain. By appropriately choosing the speed of the protons, it is possible to direct these particles precisely to the tumour, causing them to release most of their energy into it, thereby eventually destroying it. In Bugorski’s case, the protons involved in the accident had such a high energy that the scientist’s head was not thick enough to effectively stop their course; so much so that the protons passed right through him without releasing most of the energy they possess. Nonetheless, the radiation dose received was considerable, enough to have caused his own death. So how did he survive? To answer this question, we have to bear in mind that the radiation absorbed, even though it was considerable, was concentrated in a very narrow space. Certainly, Bugorski suffered considerable damage, but apparently in a small enough area not to cause his death. In other words: would you prefer a hundred-kilogram anvil to fall on your chest, or a five-thousand-kilogram anvil falling on your little toe? Although the latter is much more dangerous in itself, the fact that it falls on a such a tiny part of your body would guarantee your survival. As previously mentioned, Bugorski survived and continued in his career as a scientist because, apparently, the proton beam disintegrated a very small part of his body.

For the record, Anatoli Petrovich Bugorski is still alive. He resigned from his career as a physicist and is now a pensioner.