Even at very low temperatures neutrons tend to move quickly. That they actually got them cold enough to stay inside a bowl is pretty amazing. Every time one of those neutrons decayed it would bust up into a proton and an electron which would have a bunch of kinetic energy and tend to heat up the rest of the neutrons.
The above image is about the best image anybody is ever going to see of the element Francium. I figure if you did get enough neutrons together it would look similar to what Francium looked like after they gathered a bunch of those atoms together. Francium only has a half life of 22 minutes.