[Yum] Multiple Repository Hell

Robert G. Brown rgb at phy.duke.edu
Wed Mar 12 15:56:21 UTC 2003


On Wed, 12 Mar 2003, Troy Dawson wrote:

> Hmm ... sorta.
> First we start with hydrogen, and strip the electrons off with a flash of 
> electricity.  So we have a core with just a proton and an neutron.  And send 
> that off the accelorator.  But then we have to get the neutron off because 

Not exactly "hydrogen" (usually considered a single proton) but
deuterium, yeah, exactly.  You NEED the proton, because one can't push a
neutron with an EM field, because it is, well, neutral!  So you
piggyback the neutron along with the proton it is nuclear-glued to.

However, that particular nuclear bond is relatively weak.  You get
energy OUT of fusion processes because the nuclear bond energy depth
goes UP as you add more protons and neutrons (they are more tightly
bound, which is negative energy, so give off more positive energy to get
into the bound state), up until you make something like iron nuclei, at
which point it goes the other way (so fission processes from heavier
nucleii also release energy).  The neutron in deuterium is just barely
bound.

So you whonk it into something, and hope to transfer just enough
transverse relative momentum to break the two apart, while still leaving
the forward momentum of the separated neutrons dominant and mostly
unaffected.  Just what and how you whonk it into likely depends on lots
of things, e.g. the energy range of the neutron beam you are producing.
Carbon films, metal deposit films, tubes of paraffin or polystyrene
(neutron rich material)?  I don't keep up to date on the details,
though.

> But your guess is actually a sorta close.  And I think if it was a gum 
> wrapper, I would have been just as disapointed.

"Guess"?  I'm wounded.  Sure, I'm only a theorist, and nuclear physics
isn't my bag, baby, but I have watched nuclear grad students here spend
hours vapor depositing some particular metal on a piece of film,
listened to them lament when a film blows and opens a beam pipe up to
air (so it has to be pumped out again and can be messed up in other
ways, which takes a long time and delays their disserations for ANOTHER
six months as they lose their accelerator slot:-).  I've even spent a
few nights of my life baby-sitting Duke's backyard accelerator -- just
enough to know that it wasn't my baby, baby..:-)

Heck, one of the two things I actually did for TUNL (our nuclear lab)
back when I was deciding not to be a nuclear physicist was (using a
computational tool called TRANSPORT -- mandatory caps for fortran IV
code -- to do geometrical beam optics.  Transport was written by other
humans, although I did find bugs and fix them and add features.  With it
I computed the placement and settings of their quadrupole and sextapole
magnetic lenses and their 90 degree bending magnets for their high
resolution neutron experimental facility.  I apparently did a good
enough job that when the leg was finally completed (twenty-plus years
ago, mind you:-) they turned it on, twiddled the knobs a bit, and found
their beam right on target with beam currents at or above their
expectations and requirements.  They were even happy, I think.  I got
petted on the head and given a bone.

So I still vaguely remember how accelerators work, although some of the
details have doubtless gone south over twenty plus years...

I actually think that the most impressive part of accelerators is the
accelerators themselves, especially the sources (which are in a lot of
cases FILLED with all sorts of ad hoc engineering and cleverness and
even look like "real physics".  By comparison the experimental end is
pretty boring.  A beam pipe leading right up to the target, a target (a
plain old chunk of machined metal) suspended by something prosaic (like
monofilament fishing line).  Various detectors for the crap that comes
out.  Lots of shielding and indirect controls so that the crap doesn't
affect your DNA -- much ;-) -- while you are operating all of same.

Although you guys at fermilab probably have some pretty nifty detectors,
which can also be amazing from the engineering perspective, especially
the electronics and data collection stuff, especially compared to what
was around when I was hanging out near nuclear target:-).  

Of course, you probably have some pretty nifty accelerators, and
sources, and bending magnets, and magnetic lensing, and control
apparatus as well...:-)   Money buys cool toys.

Nuclear physics is fun and interesting, no doubt.  Still, I like doing
what I do now more, and tend to be first author in a string of two
authors on all of my papers, instead of 83rd author in a string of 126
authors, more typical of big-lab nuclear papers...:-)

   rgb

-- 
Robert G. Brown	                       http://www.phy.duke.edu/~rgb/
Duke University Dept. of Physics, Box 90305
Durham, N.C. 27708-0305
Phone: 1-919-660-2567  Fax: 919-660-2525     email:rgb at phy.duke.edu






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