One of the major questions which has been raised with the rotating neutron star model of pulsars, is whether cosmic rays can be produced by the pulsar phenomenon through the acceleration of the surface material of neutron stars. It is therefore very instructive to review the calculations which have been made on the surface structure and cooling histories of neutron stars.

Figure 1 shows a diagram of the interior of a neutron star, one of a sequence of models calculated by Cohen et al. (1970). In this diagram the radii of the various parts of the model are to scale. The total radius of the model is 13.7 km. Over a fairly large range of distance downwards from the surface, ions and electrons also put in an appearance, and below that there is a still narrower strip where protons coexist with the ions, electrons, and neutrons. Below this the ions disappear, and at still greater depths mu mesons put in an appearance. Finally, near the center of the star, the calculations indicated that other hyperons probably appear.

The region containing the ions can be expected to form a crystalline solid, except in the outer fringes of the atmosphere, where thermal effects and the relatively small pressure will vaporize any crystals.