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Matter beams

  1. Aug 30, 2004 #1
    I have a few questions about matter beams.

    1. Is a matter beam simply a stream of particles? Can I make an alpha, electron or neutron beam by simply encasing a radioactive source in lead, punch out a hole and filtering out the rest? Can air rushing out of a tire in a vacuum chamber be considered as superposed nitrogen and O2 beams in the same sense as particle beams?

    2. How would an electron beam from a such a source be different than that of a cathode ray? Is it only a difference in energy and momentum?

    3. Is it possible to easily tune the "linewidth" (delta E of the many particles) of a matter beam? It this useful?

    4. Can an actual or theoretical matter beam be coherent (like photons in a laser)?

    5. Can we define a polarization to a matter beam? How is it related to its spin? (A circularly polarized photon has its spin oriented (anti-)parallel to its propagation vector. Are the spins of matter beam particles similarly oriented?)

    6. Can we control neutron (or other neutral particle) beams with devices acting as mirrors, lenses, filters, scatterers etc.?

    7. Can we make a superfluid He beam?

    8. Can we make beams of exotic particles? (other than ions, e, protons, neutrons, photons, and alpha particles)?
  2. jcsd
  3. Aug 31, 2004 #2


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    yes. (to all questions).
    Up to a few details.

    For instance, I don't know how to make a neutrino mirror :-)
    But we use neutron mirrors, for instance. It works well for cold neutrons, less and less for more energetic neutrons.
    At CERN there (used to be?) a muon and a pion beamline. Just collection of "junk" and filtering.

  4. Aug 31, 2004 #3


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    All you have to do is to block one of the beams produced by a Stern-Gerlach experiment. (Thanks to whomever I stole this idea from. I read this somewhere else in this forum a few days ago). This will produce a beam of silver atoms (neutral spin-1/2 particles) with all their spins in the direction that's perpendicular to the plane that the two beams are in. (This means that the spins are also perpendicular to the velocity vector).
  5. Aug 31, 2004 #4


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    Particle beams, as used in the setting of a physics research, tend to be made up of particle of only one specie (so your air/gas leaking out of a tire isn't typically considered as a particle beam). It can be a continuous stream of particles, or a series of particles that come in bunches.

    The old "cathode ray", as used in those vacuum tubes, are electrons produced via thermionic emission. This is not the only means to produce electrons. Photoinjectors in many accelerators produces electrons via photoemission. Needle cathodes produces electrons via field emission, etc... The quality of the beam produce can vary tremendously, depending on what is used for and how it is accelerated.

    One can "tuned" the energy spread and momentum distribution (technical term for this is "emittance") along the direction of motion, called the axial direction. However, the transverse momentum and energy spread are more cumbersome. In fact, in this direction, the lower limit of the transverse momentum is governed by the photoemission process itself at cathode, so it is why this is called the "intrinsic" emittance. Electrons produced by thermionic emission have a larger energy and momentum spread.

    Is this useful? Depending on the application. If you are producing x-ray for medical application, it isn't that crucial to have as fine of an energy and momentum spread. However, for FEL and particle colliders, this is of utmost importance, since a large energy and momentum spread can degrade the beam very quickly for what it is intended to do.

    Not in the quantum mechanical sense because these are treated like classical particles. However, away from particle accelerators, you CAN have a coherent situation since we have seen BE atomic gas, and even fermionic condensation. These cannot happen without the particles in the beam interacting with each other to form a coherent state.

    Polarized beam are produced via circularly polarized light and a cathode that can produced polarized electrons. Currently, GaAs is the leading candidate. The polarization is usually in the axial direction (along the direction of propagation).


    Read response to #4.

    Those particle jets that we detected in a particle collider are "beams", no? These can be anything from those CP-violating mesons to exotic baryons.

  6. Aug 31, 2004 #5
    Is it somehow possible to "shoot" a beam of BE gas at something else (a slit, double-slit, or matter target) and see what happens? Or would giving it the necessary kinetic energy warm it too much? What about if we shoot a slit at it?

    Would the polarization of such a boson beam behave as that of photons? With the mutually perpendicular rotating E end B fields? How would it differ?

    Would it be a property of fermions to be able to have their spin either co-axial or perpendicular (Stern-Gerlach) to their velocity vector? Because photons can't do this (AFAIK).

    I suppose I was wondering whether these are easy enough to produce that we can shoot them at targets and slits on a regular basis. From vanesch, I understand that we have the capability, but it is a matter of justification.
  7. Sep 1, 2004 #6


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    You need to remember how DIFFICULT it is to maintain a collection of BE gas. They have to be cooled to such low temperatures and have to be suspended by EM fields to keep them away from the walls of the vacuum chamber since contact with that would destroy any coherence. So, to "shoot" them at something already means that you are imparting energy into it, which would inevitably imparts an energy distribution and temperature! This is even before it has to pass though an obstacle like a slit, which would, I'm sure, destroy the quantum coherence of those that actually hit the side of the slit.

    If all you want to see is interference pattern generated by something that has undergone a BE condensation, then look at the superconducting quantum interference device (SQUID)! You see condensed cooper pairs making the same Fraunhoffer patterns.

    Why would any generic boson behave the same way as photons, beyond having a coherent BE condensation? The only common characteristics between them is the integer spin. Then all the boson rules apply. Other than that, they can differ like night and day. One can be a "point" boson while another is a composite boson. One can be neutral while the other has a net charge. They don't have to be the same just because they share a set of similar properties.

  8. Sep 6, 2004 #7
    Trying to understand the relation between spin and polarization. It seems to be much simpler with photons.
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