Of Particular Significance

How to Make a Neutrino Beam

POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 09/23/2011

Here’s a layperson’s explanation of how to make a beam of neutrinos, roughly speaking (the details depend on the individual experimental facility.) 

First, make a beam of protons, just the way you would if you were wanting to load up the Large Hadron Collider.  (That’s a story of its own, but I’ll take the proton beam for granted here.)

Next, smash the proton beam into a “target”, just a thin slab of material.  The protons will hit atomic nuclei in the material and shatter them, not only breaking them apart into their protons and neutrons but creating many other particles in the process, including pions (examples of hadrons) of both positive and negative electric charge.  All of these particles  come flying out the back of the target slab, giving us a beam of protons, neutrons, pions, and a few other stray particles.

A pulse containing many protons hits a slab of material (target) and even more particles exit out the back. A magnet separates the neutral, positively- and negatively-charged particles, with lower-momentum particles bending more. Most particles stop in the wall, but a gap is left through which positively charged particles (mostly positively-charged pions) of a certain range of momentum and energy may pass.

Now put the beam near a magnet.  A magnet will cause the paths of charged particles to bend.  The direction of bending depends on the particle’s electric charge; the amount of bending depends on the particle’s energy.  So the neutrons go straight on; the negatively charged pions bend one way; and the protons and positively charged pions bend the other way.   Let most of these particles just run into the wall; where you leave a doorway, the particles that go through will have roughly similar energies and the same electric charge.  In this way, by putting the doorway in the right place, you can get a beam of mostly positively charged pions with similar energies.

The pions will begin to decay, one by one turning into an anti-muon and a neutrino.  Before long, your beam has positively charged muons, with a few as-yet-undecayed pions and stray protons left over, and neutrinos.

The positively-charged pions passing through the each may decay (inset) to an antimuon and a neutrino. After most have decayed, another magnet sweeps all charged particles aside, leaving a nearly pure neutrino beam. This beam goes straight through the walls and the rock on its way to a distant neutrino detector.

Now put the beam near another magnet.  The neutrinos, being electrically neutral, go straight ahead.   The positively charged particles — the muons, and any leftover pions and protons, will bend to one side.  Let them run into the wall.  What remains?  A neutrino beam.  Not a very narrow beam, to be sure, but, if you started with a lot of protons, a very powerful one.

By controlling the directions of the initial protons and the intermediate pions, you can get this beam to go in any direction you choose.  For instance, you can build the beam at CERN, and point it at Gran Sasso and the OPERA experiment.   Not a narrow beam at all — 2 kilometers wide by the time it travels the 730 kilometers to Gran Sasso.  But it’s enough to do the job.

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30 Responses

  1. Hello! Very nice post.
    One question . Let’s suppose that I want to create a positive pion beam in order to do some relevant-to-pions measurement.
    I dump the initial proton beam into a target and then use an analysing magnet to choose positively charged particles of a specific momentum range.
    Let’s suppose that the charged particles remaining are protons, positive pions and positive kaons.
    How do I isolate the pions then?
    To make the question more specific, let’s suppose that all three kinds of particles have momenta in the 2-3 GeV/c range.
    If we allow them to fly in a long distance, kaons will decay prior to the pions thus we will have protons and pions. And since the kaons are far less than the pions, let’s suppose that they are insiginificant. Thus the key question, is: how do I isolate pions from protons in a 2-3 GeV/c beam without changing the pion energy?

  2. Do mechanisms to produce such a neutron beam exist naturally in the sun. Could solar neutrino beams point many neutrinos out of the plane of the ecliptic and account for low neutrino levels detected on earth?

  3. nutrieno wave can neutralize nuclear strikes, But the “muon storage ring” generator needed to propose the neutrino beam would need to be 1000 kilometres wide. It would also require 50 gigaWatts of power to operate – the same as used by the entire UK – and would cost an estimated $100 billion to construct. what do u say friends

  4. this nutrieno beam can be used against the latest nuclear weapons andcounter them in future i hope im not wrong please frgv me if im wrong

  5. I know that neutrinos are created in beta decay… besides particle accelerators and radio-active decay are there any other ways to produce neutrinos?

    1. This is partly a question of which measurements you are trying to do; a good detector for one measurement may not be ideal for another. You can bet that the best neutrino experimentalists in the world spend a lot of their time trying to invent better detectors. I’m nowhere near expert enough to suggest anything beyond what they are already doing, or developing.

      But the real limitation on neutrino experiments is that neutrinos interact rarely with ordinary matter. We can’t change that. We can only try to have bigger detectors (which means more detected neutrinos) or more precise measurements (so that we can extract a bit more information with the same number of neutrinos). Details are experiment-specific, but those are really the principles.

  6. Whoops, forgot the decimal point! Should be 0.010 m beamwidth at 1 km. Or, even better yet, 0.001 m beamwidth. Is there any solid state detector, or something that is smaller or more practical than IceCube?

    1. Again, if you want the beam 1000 or 10000 times narrower than the beam CERN sends to OPERA, you need 1000 or 10000 times the energy per pion (until such point as you can’t even make the pion beam narrow enough.) That in turn means you need a proton beam 1000 or 10000 times more energetic than what CERN uses. I doubt you will see that in our lifetimes.

  7. Very interesting. So, the question in my mind is: How many GeV would one need in order to accelerate particles so that the beam width of generated neutrinos is 10 m at 1 km?

    1. Everything scales linearly: if you have 2 km width at 700 km distance, as for OPERA you’ll *already* have 2000/700 = 3 m width at 1 km distance. The beam forms something like a cone.

      If you want 2 m width instead of 2 km width at 700 km distance, you’d need pions with 1000 times the energy. (Unless I am forgetting something…)

    1. Oh, you can bet they make it as narrow as is feasible. Part of the problem is that not all pions decay in exactly the same way; the neutrinos go off at various angles. Higher energy neutrino beams can be made narrower, but sometimes you need lower energy to do a measurement, and any given accelerator has a maximum energy for the initial protons, which sets an upper limit on the neutrino energies.

  8. What is the second magnet good for — can’t we let the full beam run into the wall? All particles except for the neutrinos should be absorbed and we get the same result. Or am I missing something?

    1. Ah! Excellent question. Partly it was to make the picture clearer, but partly it is because if you don’t sweep the anti-muons away, some of them will decay while heading in the same direction as your neutrinos and produce neutrinos and anti-neutrinos of the wrong type, polluting what is nearly a pure muon-type neutrino beam with electron-type neutrinos and muon-type antineutrinos. Also, collisions of the anti-muons and protons with the wall will also potentially make more neutrinos and anti-neutrinos that you don’t want, and some of them will get into your muon-type neutrino beam if you haven’t swept the protons and anti-muons to the side.

      Strictly speaking, you’ll always get a little bit of pollution, but generally you want to minimize it.

    1. I’ve no idea; something inexpensive, I’m sure, but with large atomic nuclei, I would think. Maybe steel, maybe lead. Really doesn’t matter too much as far as I know.

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