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@xxxxxxxxxxxx
