What would happen to a person that was inside the Large Hadron Collider when it is turned on? 
Possible Duplicate:
What would happen if you put your hand in front of the 7 TeV beam at LHC? 

Not a terribly scientific question, but one that I'm sure many people have thought about :)
 A: First of all, no one could be inside the collider itself: the beam pipe and other component are simply not big enough and in any case are kept under a high quality vacuum, so you'd have other things on your mind.
The concern is that they would be in the tunnel or one of the experimental halls  where they would encounter non-trivial radiation levels when the beam was turned on. In some places (such as near beam dumps) the peak level could be lethal in a very short time. In most places it will be hot enough that you really want to leave, but cool enough that you'd have time to notice the warning beacons and take some action to get out of there before your were facing a short term risk of death.
A considerable amount of effort goes into insuring that this does not happen. 
Things like 


*

*Locking the enclosures and making an exhaustive search when you are coming out of maintenance mode and toward run mode, and interlocking the beam on all the doors (i.e. these portals must be secure before the beam can come on).

*Allowing entry into the enclosures only in teams and carrying a physical interlock token (key) when entry must be made during a run. The interlock prevents the beam from coming on, and as long as you have your key in your pocket you know you are safe.

*Having cameras that survey all parts of the enclosures which can be viewed from the control rooms, and using these for a second survey of the enclosures.

*Having flashing beacons in the enclosures to alert anyone who somehow is there that all is not right, and big red panic buttons to kill the beam if you find yourself in that position.

*Making everyone allowed into those parts of the site without escort take a training course that covers these things.



The facility where I worked on my dissertation had an electron beam machine where the prompt bremstrahlung level would be pretty bad. The LHC is proton machine and as long as the beam steering is good the tunnel itself is probably a little cooler, but I still wouldn't want to spend any time there while the beam was on: I intend to get to my old age with little enough professional dose that I don't need to blame any cancer that I get on my work.
A: To complete the answer of @dmckee I will add that the only place you could be in the way of an accelerator beam would be at the beam dump. . This is  solid matter where beams from accelerators are dumped when they are shut down, in order to absorb the energy at one go, which can be megawatts. 
I remember walking after a shift by the SPS beam dump: you could even hear the beam dumped !  and that was at much lower energies than the LHC.
The LHC with its very high energy content has a meticulously designed beam dump to avoid excess radiation in the environment too.

The nominal LHC beam contains an unprecedented stored energy of 350 MJ, contained in 2808 bunches with a beam sigma of the order of 0.3 mm.

The answer is that if you were in the path of the beam a hole would be bored through you and kill you as efficiently as a dagger, probably with little blood because the vessels would be seared, before any radiation could affect you.

The extremely high destructive power of such a beam imposes an external dump, where the beam must be extracted completely from the LHC, diluted to reduce the peak energy density and then absorbed in a dedicated system.

Reading how they do it, in the link above, is interesting.
