Exists Neutronium On Earth? Why Not Create?

[FeDeX_LaTeX]

Hi, I was wondering whether or not there exists any neutronium on Earth. Could you not simply create neutronium by taking two neutrons and putting them together? If not, why not? I also heard that neutronium cannot exist outside the core of a neutron star... why? I'm assuming they mean that it won't exist because other particles (i.e. protons) would be attracted to the neutrons, so it would no longer be neutronium.

 

[Janus]

Neutrons by themselves are unstable and decay into a proton and electron with a half-life of about 15 min. Without protons in the right ratio, as in a nucleus, to stabilize the neutrons, your neutronium will decay away. It's only when there is enough pressure from gravity to prevent this decay that you can have stable neutronium.