Uranium or lead, heavy nuclei that shield well against gamma radiation would do little for neutrons.

Neutrons needs something with light atom like hydrogen - water or maybe polyethylene and lots of it to slow down. So a small nuclear explosion which emits a neutron pulse might work pretty well. It would hellish if it somehow exploded over a population center, though. Didn't get hit by the ICBM (good). Everyone's bones melted (bad).