How Do the Nucleons Pack in an Atomic Nucleus?

Abstract

A nuclear structure model of “ring plus extra nucleon” is proposed. For nuclei larger than 4 He, protons (P) and neutrons (N) are basically bound alternatively to form a Z P + Z N ring. The ring folds with a “bond angle” of 90° for every 3 continuous nucleons to make the nucleons packed densely. Extra N(‘s) can bind to ring-P with the same “bond angle” and “bond distance”. When 2 or more P’s are geometrically available, the extra N tends to be stable. Extra P can bind with ring N in a similar way when the ratio of N/P < 1 although the binding is weaker than that of extra N. Even -Z rings, as well as normal even-even nuclei, always have superimposed gravity centers of P and N; while for odd -Z rings, as well as all odd- A ( A : number of nucleon) nuclei, the centers of P and N must be eccentric. The eccentricity results in a depression of binding energy ( E B ) and therefore odd and even Z dependent zigzag features of E B / A . This can be well explained by the shift of eccentricity by extra nucleons. Symmetrical center may present in even -Z rings and normal even-even nuclei. While for odd -Z ring, only antisymmetric center (every P can find an N through the center and vice versa ) is possible. Based on this model, a pair of mirror nuclei, P X + n N X and P X N X + n , should be equivalent in packing structure just like black-white photo and the negative film. Therefore, an identical spin and parity was confirmed for any pair. In addition, the E B / A difference of mirror nuclei pair is nearly a constant of 0.184 n MeV. Many other facts can also be easily understood from this model, such as the neutron halo, the unusual stability sequence of 9 Be, 7 Be and 8 Be and so on.