Matter At Nuclear Densities (Dense States of Cosmic Matter)
A single isolated neutron decays into a proton, an electron and anti -neutrino with a half-life of 12 minutes. This is an example of ? decay. However, in matter in which the electrons are moving at nearly the velocity of light, the reverse process can take place: an energetic electron collides with a proton to form a neutron, emitting A neutrino in the process. The density at which this starts to take place depends on the constitution of the matter: it is about I010kgm-3 for hydrogen and about 1012kgm-3 for iron. Normally,the neutron-rich nuclei so formed would undergo radioactive lesion, expelling the electron again, but at sufficiently high densities the presence of other free electrons prevents this. There is simply nowhere for the expelled electron to go; once again we are seeing Pauli’s Exclusion Principle at work inside a star.
As the density of the matter increases, the formation of more neutrons proceeds and consequently nuclei with more and more bizarre neutron-to-proton ratios are formed. However, because there are now so many neutrons compared to protons in each nucleus, the binding forces of each nucleus become weaker. Eventually, when the density is as high as about 4 X 1014kgm-3, the .neutrons start to ‘drip’ out of the nucleus, in a process .called NEUTRON-DRIP. At higher densities, after neutron-drip has occurred, the matter consists of essentially three components: the nuclei are able to arrange themselves in the form of a crystal lattice of electrons, nuclei and free neutrons. At densities of about 1017 kg m-3 the nuclei have completely dissolved
The matter consists almost entirely of free neutrons together with 0.5 per cent protons and electrons. Such a neutron fluid may well be a SUPERFLUID, having the property of zero viscosity (absolutely no resistance to flow) similar to liquid helium. The few protons present may also be superfluid, and, in addition, make the fluid SUPERCONDUCTING (no resistance to electric current). There are so few electrons present that the main pressure in material at nuclear densities is due to NEUTRON DEGENERACY PRESSURE. This is exactly analogous to electron degeneracy pressure except that the particles exerting the pressure are the free neutrons. The equation of state is similar to the case of electron degeneracy, with the neutron mass replacing the electron mass.