Numero magico (fisica)

Nuclei stabili e instabili in funzione del numero di protoni e neutroni

In chimica nucleare e in fisica nucleare, un numero magico è un numero di nucleoni (protoni o neutroni) in corrispondenza del quale i nuclei risultano particolarmente stabili (oppure in corrispondenza del quale nuclei instabili presentano un'instabilità assai minore, una quasi-stabilità). Ciò accade perché, secondo il modello attualmente più accettato (detto a shell), in questo modo i nucleoni sono sistemati in livelli completi all'interno del nucleo atomico. I numeri sono:

2, 8, 20, 28, 50, 82, 126.

È stato ipotizzato, ma ciò deve essere ancora confermato, che il numero 184 sia anch'esso un numero magico[1].

Inoltre nuclei che hanno sia il numero di neutroni, sia quello di protoni uguale ad uno dei numeri magici di cui sopra, sono ancora più stabili e vengono chiamati "nuclei doppiamente magici". Di tutte le combinazioni possibili dei numeri magici per formare nuclei doppiamente magici, 3 (4 per altri autori) si riscontrano in natura e sono: elio-4 (2,2); ossigeno-16 (8,8); calcio-40 (20,20); piombo-208 (82,126). Altri autori considerano anche il calcio-48 (20,28) un nucleo doppiamente magico, anche se è un nucleo instabile. Lo 100Sn radioattivo fu scoperto in tempi relativamente recenti, nel 1994[2].

Note

  1. ^ (EN) Study of α-decay channels and half-lives of Superdeformed Superheavy Nuclei Archiviato il 23 ottobre 2016 in Internet Archive.
  2. ^ Phil Walker, Doubly Magic Discovery of Tin-100, in Physics World, vol. 7, giugno, 1994.

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