Are there nuclei of only neutrons?

[haael]

My question: do there exist particles similar to atomic nuclei, but made only of neutrons? If not, then why?

I suspect that this would be beta-unstable, but it should live for some time and get observed somehow.

 

[Vanadium 50]

The dineutron is unbound. There was thought that the tetraneutron might be bound but all searches have come up negative.

 

[mathman]

The closest thing to a stable "nucleus" made up of neutrons is a neutron star.