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Can Any Anti-Particle Make a Probe?

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Can Any Anti-Particle Make a Probe? NEWS AND VIEWS Can any anti-particle make a probe? Positron probes (of the solid state) are already a booming business and may soon be applied to the study of liquids. Can anti-proton probes be far away? EACH new particle of matter discovered from zero to some maximum. So what is new? Two things, both in the may most immediately be of interest to The trick of making a mono-energetic same issue of Physical Review Letters (2 those with theories to check (or invent, as beam of reasonable but still low energy September). First, E. Gramsch, KG.Lynn, the case may be), but one thing is certain: usually entails some interaction with a J. Throwe and I Kanazawa, from the somebody will eventually use the particle solid within which the energy of incident Brookhaven National Laboratory, have as a probe for some purpose or another. positrons is first reduced to the average extended the use of positrons as probes And that is likely to be true even if the thermal energy, whereupon some propor­ from solids to liquids, and with surprising newly discovered particle is inherently tion of the incident particles is afterwards results (67, 1,282; 1991). The incentive unstable, or if it interacts with and is de- re-emitted by scattering, but with an en­ has been to see whether positron probes stroyed by some component of the envi- ergy that is a measure of the work function can throw light on the structure of liquids, ronment in which it finds itself. What - the potential difference between the for which purpose they have studied matters is merely that the particle should interior and the exteriorofthe solid. Then, positron annihilation in gallium and bis­ last for longer than the timescale of the usually, there must follow devices for ac­ muth at the melting transition. phenomena studied. And the instability celerating and shaping the beam. All that One expectation is readily confirmed. may then be an advantage, making plain is intricate stuff. At the transition from solid to liquid, the either where the particle was before it And there is worse to come. Positrons diffusion length of positrons in either Ga ceased to exist or some other attribute of fired into a solid are usually thermalized or Bi is abruptly decreased (by a factor 10 its state of being. within a few picoseconds, whereafter sev- in the former and 3 in the latter). This generalization is best justified by eral things can happen. The endpoint of a Simplemindedly, this means that, in a liq­ the growth, in the past three decades, of positron's existence is the formation of uid, there are fewer special sites at which the use of the positron as a probe, mostly positronium, but until that happens it can positrons may hide from contact with elec­ of the solid state. It is an improbable <level- diffuse. And what, then, is measured? The trons. The surprise is that, in molten gal­ opment. Although positrons are inher- standard diagnostic appears to be the Dop­ lium, the diffusion length appears to in­ ently stable, with almost as much right as pier shift from the frequency of 511-keV crease exponentially with increasing tem­ ordinary negatively charged electrons to caused by the momentum of the positron, perature, and not linearly as in bismuth. continue indefinitely to exist, the whole itself dominated (in metals, at least) by the The explanation offered is that positrons world knows that positrons readily anni- energy ofelectrons in the conduction band. in liquid gallium are trapped in temporary hilate with electrons, so that their lifetime But since there are many incident positrons, potential wells formed by fluctuations of in the real world is best measured in, say, the trick is to measure the shape of a ionic structure and then, when that disap­ picoseconds (10-12 seconds). By way of photon-emission band centred on 511 ke V pears, hop to the next nearest such fluctua­ compensation, the mutual annihilation of and then to make sense of it. tion. What matters is that the wavelength a positron and an electron yields two pho- Even so, positrons not immediately of the positron is still quite long, but the tons with a characteristic energy that peaks captured are likely to be trapped at simple picture cries out for a little calculation. at 51 ·l ke V - next to the 21-cm line of ionic vacancies in the crystal lattice of a And what else? If the charge-counter­ hydrogen, the radiation frequency at which metal. This process is efficient because the part of the electron can be a probe, why not cosmologically inclined astrophysicists are quantum wavelength of a positron ther­ also the charge-counterpart of the proton, always on the look-out, in observations of malized to, say, room temperature is still the anti-proton? It is too soon to say that the galactic centre, for example. many times the typical lattice spacing in that has been done, but a Japanese group Hitherto, the solid state has been the solids. And, once trapped in a vacancy, the seems to be on the way. M. Iwasaki et al. chief beneficiary. The principles are quite wave function of the positron will col­ from the University of Tokyo used a beam simple, at least on Page One. The yard- lapse geometrically as it substitutes for the of anti-protons from the TRISTRAN ac­ stick of stability for a positron in the real missing ion, the chances of its forming celerator at the National Laboratory for world is the lifetime of positronium - the positronium will decrease and, when it High-Energy Physics at Tsukuba to look hydrogen 'atom' in which a positron has does link up with an electron from the for long lived anti-protons trapped in liq­ replaced the proton. Naturally, the singlet solid, the momenta of both particles will uid helium (67, 1,246; 1991). state (in which the spins are anti-parallel) be so small that two gamma rays eventu­ The outcome has been well-flagged, is the least stable, decaying in 125 pico- ally emitted in the annihilation will almost both theoretically and in experiments with seconds. Any intrinsically more rapid phe- exactly compensate in momentum, so that negative pions. In principle, it should be nomenon is in principle observable. their Doppler shift will be only small. possible for an anti-proton to replace an Naturally, there are complications, That explains the interest of positrons electron in a helium atoms, when its life­ practical and of interpretation. Making as probes in the exploration of defects in time should be much longer than the usual nearly mono-energetic beams of positrons solids and of the detailed structure of sur­ few picoseconds. What the Japanese find whose energy compares with those char- faces. Different levels in a solid can be is that the annihilation of 3.6 per cent of acteristic of the processes in solids is not probed by different choices of incident the particles stopped in liquid helium is child's play; the favoured techniques do energy.Butitisonlyrighttoacknowledge delayed, in some cases for as long as 30 not rely on high-energy physics, but on that the hardy band of men and women microseconds - a factor of a million or artificially produced isotopes such as 22Na working in this field are among those who so. For the time being, the trick seems to or 58Ni, which are positron emitters. But, most valiantly extract meaning from fuzzy work only with helium, but it can only be as with ~-emitters, these materials yield data. Each of them deserves some kind of a matter of time before somebody makes a particles with a Fermi spectrum ranging medal. probe of it. John Maddox NATURE · VOL 353 · 19 SEPTEMBER 1991 207 © 1991 Nature Publishing Group.
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