Sunday, May 06, 2012

Getting Hit by the LHC Beam

This post has a video asking what would happen if you were hit by the beam of the Large Hadron Collider. It wouldn't be good, and as this commenter noted, there is the example of someone who was hit by a beam, albeit of much lower energy: Russian scientist Anatoli Bugorski.

In 1978 Bugorski was struck in the head by the U-70 synchrotron. It didn't kill him, but it did mess him up pretty good:
"The left half of Bugorski's face swelled up beyond recognition, and over the next several days, started peeling off, revealing the path that the proton beam (moving near the speed of light) had burned through parts of his face, his bone, and the brain tissue underneath. As it was believed that he had received far in excess of the radiation dose that would normally kill a person, Bugorski was taken to a clinic in Moscow where the doctors could observe his expected demise. However, Bugorski survived and even completed his Ph.D. There was virtually no damage to his intellectual capacity, but the fatigue of mental work increased markedly. Bugorski completely lost hearing in the left ear and only a constant, unpleasant internal noise remained. The left half of his face was paralyzed, due to the destruction of nerves. He was able to function well, except for the fact that he had occasional complex partial seizures and rare tonic-clonic seizures."
That synchrotron was a child compared to the LHC: its maximum proton energy was 76 GeV, with 17 trillion protons per pulse and a pulse repetition frequency of 0.11 Hz.

Put that all together, and the beam has 23 kilowatts of power.

But the LHC is much larger beast: 8 TeV protons with 110 billion proton per bunch and 2808 bunches circulating in the 27-km circumference ring. That means, if I did the math correctly, the beam contains 396 megajoules and a power of 4.4 trillion watts.

That would definitely sting. As CERN notes (pg 57), the energy in the beam is about the same as a  high-speed French train of 400 tonnes traveling at 150 km/hr, or enough to melt 500 kg of copper.

Sure, it's concentrated in a tiny beam of a few millimeters wide (constricted to 16 microns at the collision points (pg 34), 1/3rd the thickness of a human hair), so perhaps it just burns a small hole in your hand. (Any scattering of the protons in the beam off the protons and neutrons in your hand would, I would think, be at very small angles due to their immense energy -- that's what happened to Bugorski with much less energetic protons.) But the beam is essentially continuous, so when you pull your hand (or head) away from it you will be irradiated during all the time it takes you to do that, which would burn your hand or head over the entire path. But if it shredded (say) your hand, might you still live? I don't know.

Note that the beam that struck Bugorski was (if I understand it correctly) more of a one-shot deal: the synchroton has a pulse frequency of only 0.11 Hz, so there were 9 seconds between pulses -- plenty of time for him to pull (or collapse) out of the way of the next bunch.

The effect of the Moon
One thing I didn't realize is that gravity has to be taken in account when analyzing LHC collisions -- specifically, the effect of lunar tides on the surrounding landform:
"The phenomenon of tides in the ocean due to the influence of the Moon (and to a lesser extent that of the Sun) is well known. They cause the level of water on the edge of the sea to rise and fall with a cycle of some 12 hours. The ground is also subject to the effect of lunar attraction because the rocks that make it up are elastic. At the new Moon and when the Moon is full, the Earth’s crust rises by some 25 cm in the Geneva area under the effect of these ‘ground tides’. This movement causes a variation of 1 mm in the circumference of the LHC (for a total circumference of 26.6 km) and this produces changes in beam energy. Thus, physicists must take the Moon into account in their measurements." (pg 31)
And all physics students are taught that you can ignore gravity in microscopic physics since (say) the gravitational force between two electrons is about 10-43 times smaller than the electromagnetic force (and smaller still for particles subject to the strong force). Not the full story!

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