On 8 m=E4rts, 03:45, Tim Little <t...@[EMAIL PROTECTED]
> wrote:
> On 2008-03-07, Crown-Horned Snorkack <chornedsnork...@[EMAIL PROTECTED]
> wrote:
>
> > At densities which do not get much higher than that, what you have
> > is stable neutron matter. The reason it turns into something else is
> > that the densities have got lower.
>
> Given any density range you like, there will be a time period during
> which the system has that average density.
>
> >> High temperatures? Yes, absolutely.
>
> > But matter cools on expansion.
>
> Not always by much. In particular, there will be a huge phase
> transition as the density reduces enough to permit neutron decay, and
> it slowly transforms to a mix of neutrons, protons, and electrons
> while maintaining constant temperature.
>
I am not quite sure of that. I suspect that a stable neutron matter
would have small, but nonzero, concentration of protons and electrons.
Besides, protons are distinguishable from neutrons and therefore free
from Fermi repulsion from them. It is only when electrons build up to
a sufficient chemical potential that they prevent further protons from
forming.
If you have a small decrease of density, you will have a small
decrease of electron chemical potential - and beta decay will then act
to cause a small increase in proton concentration.
Of course, this depends on the speed of density reduction, and density
itself. Beta decay is a weak interaction process and it has maximum
speed. Also, if you have rapid expansion at high densities, you might
have a buildup of antineutrino chemical potential...
> That will be long before the temperature and density decrease enough
> for discrete nuclei to condense out.
>
Yes. But if you increase density and pressure from the zero pressure,
increasing electron chemical potential, nuclei which are beta unstable
at zero pressure become beta stable. Beginning with nuclei like
rhenium or tritium - but there are plenty of nuclei which are beta
unstable at low pressures and become beta stable before free neutron
becomes beta stable.
This means that as you decrease the density of neutron matter, which
starts with low proton concentration, it goes on to be neutron rich
and proton poor even when discrete nuclei condense out. When the
density further decreases, the neutron-rich and proton-poor nuclei
will undergo beta decay. But by the time zero pressure is approached,
the density and temperature could be low enough that the nuclei cannot
convert to the nuclei which are most stable at low pressures - what
you get is rather daughter nuclei of the nuclei which were most stable
at high pressures.
|
