Neutronium/neutron-degenerate matter: critical mass for stability?

[cephron]

I've read (in wikipedia, at least) that neutron-degenerate matter is unstable in small quantities, and would decay into protons, electrons and antineutrinos.

Is there any generally accepted estimate for the critical mass at which a ball of neutronium would be stable?
(Trying to calculate minimum possible surface gravity of a stable ball)

 

[syhprum]

As I understand it neutron-degenerate matter can only exist in a stable form as neutron stars that have a minimam mass about twice that of the Sun.