From: Graeme Lindsell <gal502@huxley.anu.edu.au>
Subject: Neutron Beams
Organization: NCR Corporation -- Law Department
Date: Sun, 11 Oct 1992 15:22:56 GMT


From Graeme Lindsell <gal502@huxley.anu.edu.au>

  Someone asked about neutron beams...there are several sites that  produce
thermal neutrons by a process known as "spallation".
 
 A proton beam of decently high energy (~800Mev) is directed at a target
with
high atomic number: at the ISIS source at Rutherford-Appleton Laboratories 
near Oxford, the target is currently U238 ("depleted uranium") though the 
first time I was there it was tantalum, due to a failure of their first 
U238 target. Spallation comes from (I think) a mining term "to spall" which
means to
knock chips off something, i.e. a rock, and this basically what the proton
beam does: it has enough energy to knock neutrons (and larger fragments)
out of 
the nucleus of a high atomic number element. 

 The neutrons coming off the target have too high an energy to be of much 
use in most instruments, so they are "thermalized" by collision with a cold
material which interacts strongly with neutrons: at ISIS again, there is a 
hot source above the target thet containsmethane at 100K, and there is a
cold
source beneath it that has hydrogen at 4K. After being slowed down by 
interactions with the sources the neutrons emerge with an average energy
equivalent to the thermal energy of the moderator ie 4K for cold source,
100K for hot. This corresponds to a kinatic energy of the order of milli- 
electron Volts, and of velocities of hundred's of meter's a second: pretty
slow for an elementary pariticle. It also corresponds to a wavelength of
around an Angstrom ie the distances between atoms in most condensed matter,

which is what most of these neutrons are used to analyse.

 Getting to the purpose of this newsgroup, I would think the direct
military
utility of these beams would be about nil ( the indirect ulitity of
advancing
condensed matter research could be fairly high ). On the other hand I was
told 
that ISIS was producing a beam of over 100MeV (million eV) neutrons that
are
going straight out of the end of the beam stop, out of the building, and 
continuing off into space. No one can measure the flux of these because
100Mev
neutrons don't like interacting with matter much more than neutrinos do: so
it
sounds like zero military use again. ISIS isn't small: it fills up  a large
factory space and the synchroton accelerating the protons is next door.
 

 Its been some time since I was there, and I am a condensed matter
physicist
and not a particle physicist, so many of my numbers may be wrong. I am sure
that the target was U238 however, not low atomic number elements like Be
and
Li as several others have reported; these might also work, since I think
that Li & Be aren't very stable.

Graeme Lindsell a.k.a gal502@huxley.anu.edu.au
Research School of Chemistry, Australian National University       



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Graeme Lindsell                      Email: gal502@huxley.anu.edu.au
Research School of Chemistry         Phone: (06) 249 3575
Australian National University       Fax:   (06) 249 0750
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