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Il Nuovo Cimento A (1965-1970)

, Volume 36, Issue 2, pp 113–170 | Cite as

Evidence for two-dimensional ising structure in atomic nuclei

  • M. H. Mac Gregor
Article

Summary

Although the unpaired nucleons in an atomic nucleus exhibit pronounced shell-model-like behavior, the situation with respect to the paired-off « core region » nucleons is considerably more abscure. Several recent « multi-α knockout » and « quasi-fission » experiments indicate that nucleon clustering is prevalent throughout the core region of the nucleus; this same conclusion is suggested by nuclear-binding-energy systematics, by the evidence for a « neutron halo » in heavy nuclei and by the magnetic-moment systematics of low-mass odd-A nuclei. A number of arguments suggests, in turn, that this nucleon clustering is not spherical or spheroidal in shape, as has generally been assumed, but instead is in the form of two-dimensional Ising-like layers, with the layers arrayed perpendicular to the symmetry axis of the nucleus. The effects of this two-dimensional layering are observed most clearly in low-energy-induced fission, where nuclei with an even (odd) number of Ising laysers fission symmetrically (asymmetrically). This picture of the nucleus gives an immediate quantitative explanation for the observed asymmetry in the fission of uranium, and also for the transition from symmetric to asymmetric and back to symmetric fission as the atomic number of the fissioning nucleus increases fromA=197 up toA=258. These results suggest that, in the shell model formulation of the atomic nucleus, the basis states for the paired-off nucleon core region should be modified so as to contain laminar nucleon cluster correlations.

Подтверждение для двумерной структуры Иэинга в атомных ядрах

Реэюме

Хотя неспаренные нуклоны в атомных ядрах обнаруживают явно выраженное поведение, определяемое оболочечной моделью, ситуация, относяшаяся к нуклонам, спаренным вне « области остова », является не вполне ясной. Некоторые недавние зксперименты по « множественному выбиванию альфа-частиц » и ?кваэи-делению « укаэывают, что нуклонная кластериэация превалирует в области остова ядра. Укаэанное эаключение следует иэ систематики знергий свяэи ядер, иэ сушест-вования « нейтронного гало » в тяжелых ядрах и иэ систематики магнитных моментов легких ядер с нечетным А. Выдвигаются ряд аргументов в польэу того, что нуклонная кластериэация не является сферической или сфероидальной по форме, как, вообше говоря, предполагается, а имеет форму двумерных слоев иэинговского типа. Причем, слои расположены перпендикулярно оси симметрии ядер. Эффекты, свяэанные с обраэованием зтих двумерных слоев наблюдаются более ясно при индуцированном делении при ниэких знергиях, когда ядра с четным (нечетным) числом слоев Иэинга делятся симметрично (асимметрично). Эта картина для ядра дает непосредственное количественное общяснение для наблюденной асимметрии при делении урана, а также для перехода иэ симметричного в асимметричное и обратно в симметричное деление, когда атомный номер деляшегося ядра увеличивается отA=197 доA=258. Эти реэультаты предполагают, что в оболочечной модели атомного ядра основные состояния для нуклонов, спаренных вне области остова, должны быть модифици-рованы таким обраэом, чтобы содержать корреляции ламинарных нуклонных кластеров.

Riassunto

Sebbene i nucleoni non appaiati in un nucleo atomico mostrino un pronunciato comportamento tipo modello a strati, la situazione nei confronti dei nucleoni accoppiati della regione del nocciolo è considerevolmente più oscura. Molti recenti esperimenti sul knockout a molte particelle α e sulla «quasi fissione» indicano che l’aggruppamento dei nucleoni è prevalente per tutta la regione centrale del nucleo; questa stessa conclusione è suggerita dalla sistematica dell’energia di legame, dalla prova di un «alone di neutroni» nei nuclei pesanti e dalla sistematica del momento magnetico dei nuclei a bassa massa e conA dispari. Per contro, un certo numero di argomenti suggerisce che questo aggruppamento di nucleoni non è sferoidale o sferico di forma, come si suppone generalmente, ma è invece della forma di strati bidimensionali del tipo di Ising, con gli strati ordinati perpendicolarmente all’asse di simmetria del nucleo. Gli effetti di questa stratificazione bidimensionale sono osservati più chiaramente nella fissione indotta a bassa energia in cui i nuclei con un numero pari (dispari) di strati di Ising fissionano simmetricamente (asimmetricamente). Questo quadro del nucleo dà un’immediata spiegazione quantitativa dell’asimmetria osservata nella fissione dell’uranio ed anche della transizione da fissione simmetrica ad asimmetrica e di nuovo a simmetrica quando il numero atomico del nucleo di fissione aumenta daA=197 fino adA=258. Questi risultati suggeriscono che, nella formulazione del modello a strati del nucleo atomico, gli stati di base per i nucleoni appaiati della regione del nocciolo dovrebbero essere modificati in modo da contenere correlazioni laminari di nucleoni a grappoli.

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Copyright information

© Società Italiana di Fisica 1976

Authors and Affiliations

  • M. H. Mac Gregor
    • 1
  1. 1.Lawrence Livermore LaboratoryUniversity of CaliforniaLivermore

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