Boron:

Boron has been involved as a neutron absorber in several materials further to concrete. For instance,  borated  graphite,  a  mixture  of elemental boron and graphite, has been used in fast-reactor shields. The Boral, consisting of boron carbide (B₄C) and aluminum, epoxy resins and resin-impregnated wood laminates incorporating boron must have been used for local shielding reasons.  Boron has also been added to steel for shield structures to decrease secondary gamma- ray production.   Within special conditions, where a shield has consisted of a heavy metal and water, it has been useful to add a soluble boron compound to the water.