I am preparing a presentation for my physics class about the LHC and the following question arose:

Every text about the LHC says that it collides protons from a gas of hydrogen whose electrons were previously taken away. Can collisions be achieved with any hydrogen isotope or is it only protium that is being used? If so, how is protium separated from the other isotopes?


1 Answer 1


Given their charge and mass, as soon as you start accelerating particles around a loop with a given magnetic field to deflect them, only particles with the correct mass/charge ratio survive. In effect you have built a giant mass spectrometer - other isotopes of hydrogen are too heavy, and the Lorentz forces are insufficient to deflect them down the tunnel.

As was pointed out by @DMcKee, the process of extracting the protons includes bends in the injector - any particles with the wrong Q/m ratio will be eliminated there, before making it into the main accelerator. You can see that in this diagram (from https://lhc-machine-outreach.web.cern.ch/lhc-machine-outreach/images/complex/Cern-complex.gif)

enter image description here

An of course the first part of the acceleration happens in a LINAC. Although it is a straight line, it selects for the right particles as the RF frequency (and the spacing of the acceleration stages) is tuned for a specific Q/m ratio. So anything that is not a proton will almost certainly never make it to the first bend - anything that did, would "skid out" at that point.

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    $\begingroup$ In practice there are generally bends in the injector so the separation occurs before the beam makes it to the main accelerator at all. $\endgroup$ Nov 14, 2017 at 19:56
  • $\begingroup$ @dmckee of course the moment you look at the diagram which I have now added, you see that there are many stages that select for just one type of particle. $\endgroup$
    – Floris
    Nov 14, 2017 at 20:02
  • $\begingroup$ The protons come out of the ion source (a duoplasmatron) already ionised before they even reach linac2. $\endgroup$
    – dukwon
    Nov 14, 2017 at 21:05
  • $\begingroup$ @dukwon by definition a proton is ionized. The question is whether other (ionized) isotopes of hydrogen can be accelerated in the LHC ; the answer is "no they can't", and there are numerous stages where having the wrong mass/charge ratio will stop you going further. I pointed out a few; it is possible there is a magnetic or electrostatic deflection in the ion source itself, that does the same thing even earlier in the beam line. But the principle is the same at each point. If you don't have the right ratio you can proceed. $\endgroup$
    – Floris
    Nov 14, 2017 at 21:13

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