On Thu, 18 Nov 2010, D and N wrote: > Most of you have probably heard the news out of CERN that hydrogen > anti-matter has finally been /captured /in a container for about a second. > > The reports have varied, but the mention of the Canadian team, including > physicists from Triumf, rang a bell. Congratulations to your colleagues, > Pete! > > If you could, enlighten us on the real story. > > Natalia
Sure. I've been out of the loop with this experiment for a while, and as is common with big impact papers these days, there was a tight embargo on the news until the public announcement. People on authors list gave no hint how well things were progressing. Art Olin managed to sneak a mini-seminar into a regular science division meeting yesterday, simultaneous to the announcement; his talk was scheduled but there was no indication he had anything new to reveal. I would have gone to see what he had to say, anyway, except that it was at 10am. Just as I missed the ALPHA Canada group picture a couple of years ago, which was taken at about 10:15. Oh, well, Makoto emailed me my copy of the paper this morning, and there's my name in the acknowledgements. I haven't seen any of the group around since then, as I have a few questions myself on the details. Basically, as I described last time, the trick to this process is convincing the antihydrogen (Hbar, in physics jargon) to have quite low energy after being formed (equivalent to heat for large numbers of atoms, or simply velocity for numbers down in the double digits or less), so it can be held by the very weak force generated by a strong magnetic field acting on the magnetic dipole of the atom. The paper points out the problems which the initial group had a decade ago (I can't tell if the paper is publically accessible, as we have a blanket site-licence subscription to such websites here) as they were feeding antiprotons (Pbars) into the reaction vessel at a few eV, which was still too energetic to capture. The paper then goes on to describe a trick whereby the Pbars, which are initially collected in a standard charged particle trap, after some clever tricks to remove much of their energy, are gently nudged into oscillating over through a low potential barrier into the region where the antielectrons aka positrons (e+) are held, in a similar trap. Counterintuitively, a "chirping" (frequency dropping) oscillation applied to the field strength in the Pbar trap, across the frequency range corresponding to the oscillation time of the Pbars as they bounce back and forth within the confines of the trap, induces the antiprotons to match the dropping frequency by travelling a little farther in each oscillation, gaining a small amount of energy as they do so. Eventually, in a couple of hundred microseconds, they've acquired sufficient energy to just slip out of the trap, and across into the neighbouring space where the e+ are held. The positrons, of which there are generally more (in this case two million per cycle vs 30,000 Pbars), as they are easier to generate and collect, coming in this case from a radioactive decay source, are much colder than the Pbars, having been cooled by repeatedly allowing the fastest ones to escape their trap in brief intervals, separated by intervals where the remaining particles "rethermalize", ie reforming the upper tail of the statistic distribution of their energies. (It is planned in future to collect more Pbars per instance, so that this trick can be applied to them as well, thus increasing overall production efficiency. This is possible because the collection time can be extended somewhat. The duration for the trapped Hbars can also be extended far beyond the milliseconds of the current specimens, as well, by the way. The short capture time was simply a feature of the proof-of-principle nature of this particular trial.) The trick at this point is for the Pbars and e+ to combine into neutral atoms without zinging off out of the trap (which has now shifted into neutral particle magnetic trap mode). I'm not sure exactly how this part works, as the deionization is highly exothermic. I rather suspect that the reason they can do it is that they are only catching one Hbar for every ten cycles, so there is only one Hbar there at a time. The exothermic nature may only be a problem when there are a large number of atoms each giving off the energy of formation as radiation, which then gets scattered and degraded off the neighbouring atoms, being absorbed as kinetic energy. With only a small number of candidate atoms, the radiation may simply escape to deposit its heat in the containment vessel instead. The other possibility is that there is a considerable amount of Hbar being formed in each cycle, with all the atoms initially acquiring sufficient energy to escape, but that they then thermalize, off each other, before escaping, so that one in ten times one of them is knocked just the right way to leave it with a low velocity relative to the lab frame, and thus it gets snagged by the trap. I will get this cleared up the next time I see one of the authors wandering about, maybe next week. -Pete _______________________________________________ Futurework mailing list [email protected] https://lists.uwaterloo.ca/mailman/listinfo/futurework
