We have all heard of particle accelerators, if only for the “discovery” of the Higgs boson a few years ago.
And there are just two questions laymen like us ask themeselves about these machines:

1. What the heck are accelerators anyway, and how do they work?
2. What happens if I put my hand in a functioning particle accelerator?

Okay, maybe I’m the only one asking the second question.

© 2017 Robert Hradil, Monika Majer/ProStudio22.ch

Anyway, regarding question #1, CERN’s official website puts it this way:

An accelerator propels charged particles, such as protons or electrons, at high speeds, close to the speed of light. They are then smashed either onto a target or against other particles circulating in the opposite direction. By studying these collisions, physicists are able to probe the world of the infinitely small.
When the particles are sufficiently energetic, a phenomenon that defies the imagination happens: the energy of the collision is transformed into matter in the form of new particles, the most massive of which existed in the early Universe. This phenomenon is described by Einstein’s famous equation E=mc2, according to which matter is a concentrated form of energy, and the two are interchangeable.

As for the second question, from a quick search it turns out that I’m not the only fool asking it: on the internet, as soon as one claims to be working on an accelerator, he or she is overwhelmed by the requests of those who can’t wait to get hit by a bundle of protons in the hope of acquiring superpowers.

Even some physicists at CERN have tried to answer the age-old question of the hand-in-the-accelerator, in the first 4 minutes of this video.
And yet, either because they might know everything about physics but they’re not doctors, or because it is too childish and frivolous a question, these luminaries seem rather clueless and their attempts to answer can be summed up in a quite embarrassed “we don’t know”. (How come they didn’t prepare a better answer? Has no one ever asked them stupid questions before?)

Well, no harm done, because on this delicate issue the only scientist whose answer I would blindly trust is not at CERN, but in Russia.
His name is Anatoli Bugorski, and he didn’t put his hand in a particle accelerator… he stuck his whole head in one.

© Sergey Velichkin – TASS

On 13 July 1978 Bugorski was inspecting the U-70 synchrotron at the Institute of High Energy Physics in the city of Protvino.
Inaugurated in 1967, this circular accelerator with a perimeter of 1.5 km had held the world record in beam energy for five years (the “70” refers to the amount of gigaelectronvolts it can reach in its final phase); and still today it is the most powerful accelerator in all of Russia.

Inspecting such a monster is not quite like opening the hood of your car anche and checking the engine – although, for this writer, these two activities are equally esoteric. To perform maintenance that morning, Burgoski had to actually slip into the accelerator circuit. Therefore, before heading downstairs, he told the control center to turn off the beam in the next five minutes.
He arrived at the experimental room a minute or two in advance, but did not pay it too much attention because he found the door was open; furthermore, the safety signal was off, meaning that the machinery was no longer in operation. “How efficient, my good old pals, up at the station! – he must have thought – Knowing I was coming down here, they’ve already turned everything off.”

The boys in the control room.

All being nice and quiet, he decided to open the accelerator.
What he didn’t know was that the door had been left ajar by mistake; and that the light bulb inside the warning sign had just burned out.

© Sergey Velichkin – TASS

As soon as he peeked inside the corridor, his head was instantly pierced by a beam of protons of 2×3 millimeters in size, shot at a dizzying speed.
Burgoski saw a flash “brighter than a thousand suns”, as the beam entered the back of the skull and, in a fraction of a second, burned a straight line through his brain, coming out near his left nostril.
The whole thing was too fast for Burgoski to feel any pain; but not fast enough (at least, this is what I like to imagine) to prevent him from mentally cursing the security staff.

Burgoski was taken to the infirmary, and within a short time the left half of his face was swelling. The beam had pierced his middle ear, and the wound continued to slowly burn the nerves in his face. But the real problem was radiation.
In that thousandth of a second, Bugorski had been exposed to a radiation of 200,000 / 300,000 röntgen, which is 300 times greater than the lethal amount.

To the scientists, this unfortunate graduate student had suddenly become an interesting case study.
Burgoski’s days were numbered, so he was rushed to a clinic in Moscow to be placed under observation during what were presumably the last couple of weeks of his life. Scholars came from all over just to watch him die, because no one before him had ever been the victim of such a concentrated beam of radiation, as this article in The Atlantic reminds us:

unique to Bugorski’s case, radiation was concentrated along a narrow beam through the head, rather than being broadly distributed from nuclear fallout, as was the case for many victims of the Chernobyl disaster or the bombing of Hiroshima. For Bugorski, particularly vulnerable tissues, such as bone marrow and the gastrointestinal track, might have been largely spared. But where the beam shot through Bugorski’s head, it deposited an obscene amount of radiation energy, hundreds of times greater than a lethal dose.

Yet, in spite of any prediction, Burgoski did not die.
Following the accident he completely lost hearing from his left ear (replaced by a tinnitus), suffered epileptic seizures and facial hemiparesis for the rest of his life. But apart from this damage, which was permanent but not lethal, he recovered his health completely: within 18 months he was back to work, he completed his doctorate and continued his career as a scientist and experimental coordinator.
He was forbidden from talking about his accident until the end of the 1980s, due to Russia’s secrecy policy surrounding anything even remotely connected with nuclear power. Burgoski is still alive and well at the age of 77.

Maybe his story does not exactly answer our childish question about what would happen if we stuck our hand inside an accelerator, but it can give us a rough idea.
As summarized by Professor Michael Merrifield in the aforementioned video: