A figure ( GIF file of 9.8KB or PS file of 138KB ) presents the nucleon skin thicknesses calculated according to the above definition. We put an open (solid) circle if the proton (neutron) skin exists for each nucleus. The diameter of the circle is proportional to the skin thickness. The staircase-like lines represent two-nucleon drip lines from our calculations.
Among even-even nuclei inside the proton drip line, only nine have the proton skin (the heaviest one is 46Fe). Proton skins thicker than 1 fm are found in 12O (1.3 fm), 6Be (1.2 fm), and 22Si (1.1 fm), all of which are single-magic spherical nuclei.
On the other hand, the neutron skin has non-zero thickness in 39% of the 1029 nuclei which we have calculated. In middle-weight nuclei 134Sn and 136Sn, between which the present experimental neutron-rich frontier passes, the neutron skin is as thick as 1.02 fm and 1.19 fm, respectively.
The growth of the skin along isotope and isotone chains is monotonous and have no irregularity irrespective of proton or neutron and both inside and outside the proton drip line. This is in a shart contrast to the manner of the growth of the halo.
