I’ve often heard that it would take so many light years of lead to block half of the neutrinos that might otherwise pass through it.
Neutronium is substantially denser than lead.
How much neutronium is required to block half the neutrinos?
Reading various sources, it looks like high energy neutrinos can travel like a 100 m before getting absorbed. The low energy neutrinos created by the neutron star itself can pass right through.
And, now we wait for an actual physicist to set me straight!
The sheer gravitational compression required for neutronium to be stable means that the interior of the neutron star is a high potential energy environment, which would presumably mean that neutrinos would be more likely to interact than a simple density calculation would indicate.
That, and it’s possible that the interior might be a quark gluon plasma rather than individually distinct neutrons. Who knows how that would affect things.
The interaction rate should be comparable to what you would calculate from a straight density scaling of normal matter.
It’s comparable to regular matter in this regard. The neutrons (and protons) piling up on top of one another in either a nucleus or a neutron star form an approximate “Fermi gas” whose energies scale as (density)2/3. Neutron star densities are just a little higher than nuclear densities, and so, too, the neutron (and proton) energies are just a little higher than in a nucleus. Typical kinetic energies will be below 100 MeV, so not that violent in either case.
If the larger neutron stars have sizeable quark-gluon plasma cores, things don’t change significantly for neutrinos above a GeV or so in energy, but below that, the interaction rate would increase somewhat.
Ah, you’re right. I forgot that in this case gravity is simply substituting for the strong nuclear force. On the other hand, do the neutrinos gain enough energy just from falling into the star’s gravity well to significantly increase their interaction rate?
I estimate this would be a 10%-ish effect.
(ETA: For this: estimate the gravitational potential difference between the center of the star and a point very far away [infinity]; determine the fractional blueshift the neutrino would get falling through that potential difference; note that that’s the same as the fractional difference in energy for this approximately massless neutrino; and then finally note that the interaction rates in question scale approximately linearly in energy.)