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Y Rast Phenomena in Fashion 531 '-N'-'ATV'-'-'·RE=----V...::.O.;;.L..,;.306-'-8_D_E_CE_M_BE_R_I98_3_________ NEWS AND VIEWS------------------- Yrast phenomena in fashion The invention of a word new in the past decade signals a resurgence of interest in nuclear physics. But this time the objectives, if attainable, could be important. LEXICOGRAPHERS have been dismayed, mildly sausage-shaped (an oblate of nucleons in which collective motion but word-game players no doubt delighted, spheroid)? And what if the rotation of the dominates, is new. It also accounts for the by the appearence on the scene of the word nucleus as a whole is not around the re­ now fashionable search for collective yrast, one of the rare contributions of the maining axis of symmetry but, rather, in a vibrational states, some of which surpris­ Swedish language to international usage. perpendicular direction? That nuclei in the ingly consist simply of pure radial oscilla­ (The word is apparently the superlative of ground state can be aspherical has been tions whose frequency points directly to the the adjectiveyr, which means "dizzy".) Its known for decades - isotopes of Yb or Hf compressibility of nuclear matter. But col­ currency within the past decade reflects the are familiar examples. The underlying ex­ lectivity is not all, and experience has interest which now centres on a recently un­ planation lies in the Fermi statistics that shown that the rotation of some nuclei can­ fashionable field of physics - the study of control the collective behaviour of the not be explained by a rotation of the object heavy nuclei by means of particle probes nucleons. as a whole but rather requires that the whose energies are modest by the standards The route to the recognition of the in­ angular momenta of individual nucleon or­ set in high-energy physics laboratories, a terest of such nuclei rotating about an axis bits, considered separately, should be few hundred MeV or thereabouts. Only which is not the surviving axis of symmetry aligned. last month, the general assembly of the involved the recognition that collisions be­ In the years ahead, according to Mayer­ European Science Foundation was con­ tween nuclei, perhaps to form a compound Kuckuk, the field will be further enlivened gratulating itself on what had been ac­ nucleus whose eventual product would be by the further development of novel complished, at least in the construction of aspherical, could leave a nucleus in such a nuclear probes. The availability of anti­ specialized machines, in Europe in the past state that its rotational energy was com­ proton beams should make the nuclear decade. parable with the excitation energy equivalent of positronium possible. The That the time is ripe for a resurgence of separating one nucleonic state from use of strange kaons (strange particles in nuclear physics should not be a surprise. another. A glance at one of the usual the quark sense) should make possible for During the 1940s, interest in the properties physics journals will show that people are the first time measurements of nucleon of heavy nuclei was stimulated by the ap­ regularly using rotating nuclei with angular momenta near the centre of the nucleus. plications of nuclear fission then in vogue. momenta of 50, 80 or more quanta. Relativistic ion beams (nowhere yet pro­ (Thirty years ago, neutron cross-sections Last month's report to the European duced) may yield remarkably new kinds of with heavy nuclei were still national Science Foundation by the working group energy transfer. One goal now firmly in secrets.) But the experience of that decade on nuclear physics was introduced by its people' s minds is to use old-fashioned was itself sufficient to confirm what many chairman, Professor T. Mayer-Kuckuk nuclear physics to test field theories of people had expected, that the formal treat­ from the University of Bonn, by means of strong interactions hitherto thought ac­ ment of any but the simplest nuclei as an enthusiastic slide-show of the interest of cessible only by the high-energy physics systems of several interacting particles these nuclei. One of his arguments is that a people. Making a bag of quarks might even would be even more difficult than the rotating nucleus, small though it may be, is provide a microscopic simulation of the Big calculation of the electronic structure of all not in principle very different from more Bang, if ever there was such a thing. but the simplest atoms. familiar objects made of nuclear matter The European Science Foundation's Three developments have brought the and rotating rapidly, the neutron stars committee has been concerned not merely field to life - the recognition that, with which make pulsating stars for example. with what might be accomplished but with quarks and all that, nucleons are even more Certainly it seems to be on the cards that the arrangements that exist for keeping the complicated but much more interesting the further study of rotating nuclei will field alive. Mayer-Kuckuk has made a than had been thought, heavy nuclei turn yield some information about the collective survey of all facilities in Europe, with out to have previously unexpected but in­ motion of particles in the much larger ob­ telephone and telex numbers as triguing properties and there have been in­ jects in the sky. appropriate, together with a calculation of strumental developments of great poten­ On the nuclear scale, it is already clear which countries in Europe support how tial, both in the production of new kinds of that the rotational state of a nucleus and many people in the field. West Germany probes for nuclear structure and in the the state of its internal nucleonic excitation has 20 nuclear physicists per million measurement of what happens. are not independent. In classical language, inhabitants, while Britain is a long way The experience of the past few years has the orbits of nucleons are affected by cen­ down the list with a mere eight. shown that the rotational properties of trifugal and Coriolis forces (responsible, On balance, however, it seems that the heavy nuclei are arresting, well worth the among other things, for cyclonic and an­ field will prosper and that the new word invention of a word such as yrast. That a ticyclonic patterns of terrestrial weather) yrast will soon be on everybody's tongue. nucleus, being an object, should be capable caused by the rotation. On one view, that What can it mean? Half-way between a of rotating about some axis is natural which models the internal conditions of a noun and an adjective to judge from its enough, while its small dimensions ensure nucleus analogous to a superconducting usage, the simplest occurrence (as in "yrast that the rotation will be quantized, with state in which the motion of particles is tru­ trap") refers to the case in which two states permissible values of the total angular ly collective, there is even a chance that suf­ of a nucleus, at least one of which is momentum determined by some integral ficiently energetic rotation could cause a excited, have different angular momenta quantum number and by Planck's cons­ phase transition to a different state. but nearly equal energies, so that it tant. This view of the standard heavy nucleus becomes convenient to look for transitions But what if the nucleus is constitutional­ as a model of co\lective motion in a from one to the other. Why not? ly asymmetrical, not spherical but, say, relatively simple system, a hundred or two John Maddox 0028·0836/ 83 / 490531-01 SOI .OO C> 1983 Macmillan Journals ltd .
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