Date: Fri Aug 18 17:52:18 2000
Posted By: Benjamin Monreal, Grad student, Physics, MIT
Area of science: Physics
I'll answer your question first with a diagram. Then I'll talk a bit about
what is going on. Here's a typical muon neutrino beam:
high- hits pions eventually, the pions decay:
energy graphite come pi --> muon + neutrino
proton target out
beam ||| . "
||| . : \|/ |=======|
---->------>bang!--->--------> >poof< ----->|=======|---------> except the
||| * . /|\ |=======| neutrinos,
||| ... the beam passes which
||| through hundreds make it
of feet of iron, through.
lead, dirt, which
into and stops ...
That's how you get a neutrino beam! There are a few points of particle
physics to point out.
- 1) High energy proton beams are pretty standard, that's what we know
how to accelerate . One neutrino experiment I know of, NOMAD, used 450
GeV protons ... you can read about proton accelerators on the Web.
2) Now, 450 GeV protons hitting a target (NOMAD used beryllium, TOSCA will
use graphite, but anything would work.) make a huge spray of garbage.
Pions, gamma rays, antiprotons, etc. However, it's all generally moving
forward in a beamlike manner (the total forward momentum of each
pile-of-collision-debris is 450 GeV, by conservation of momentum) ... and,
right after the target, you can use magnets to steer and focus
this beam. If you want, you can keep just the (say) 100 GeV
pions in the beam, so you can get a pure, "monoenergetic" pion beam out of
the potpourri. Or you can use the whole beam. Notice that later on, you
can't steer neutrinos with magnets, so you'd better decide right now what
direction you want the beam to go!
3) Now, positive pions can decay in a large number of ways, but 99.9877
percent of the time it's going to be to a muon and a muon neutrino. The
presence of the neutrino is an instance of something called "lepton family
number conservation". Typical lifetime is 26 nanoseconds! So you want to
give them several tens of meters to cruise while they're decaying. CHORUS
gives them 300 meters, since there are longer-lived particles in their mix
as well (kaons).
4) And after the pions decay, you've got a beam of the desired neutrinos
... but also lots of high-energy muons, which would make your
ultra-sensitive neutrino detectors light up like the fourth of July. So
you want to stop them; the only way to stop them is by putting stuff in the
way. The neutrinos can pass through this stuff, while the muons will
interact, slow down, and eventually stop and decay. Some experiments run
the beam through the side of a mountain, through a big dirt pile ... one
famous experiment at Brookhaven chopped up a decommisioned Navy battleship,
and built a 14-meter thick wall of solid steel! And some experiments,
looking for neutrino behavior over very large distances, shoot their beam
through some part of the planet ... say, start a beam in Illinois and
detect it in Minnesota, or Geneva to Trieste, or some such.
Oh, and techniques are fairly similar if you want electron-neutrino or
tau-neutrino beams ... you'd want something other than the pion decay to
produce them, but that's easy enough.
A good article on neutrino experiments: here Some
beam-experiment names and locations: BooNE and DONUT at Fermilab, NOMAD and
CHORUS at CERN in Europe, K2K at KEK in Japan, all of which have Web
pages. The Particle Data Group can tell you
more about neutrinos, too.
Good question ... the original answer, after all, was worth a Nobel prize.
Current Queue |
Current Queue for Physics |
Try the links in the MadSci Library for more information on Physics.
MadSci Home | Information |
Random Knowledge Generator |
MadSci Archives |
Mad Library | MAD Labs |
MAD FAQs |
Ask a ? |
Join Us! |
Help Support MadSci
© 1995-2000. All rights reserved.