 Naturally occurring beryllium , save for slight contamination by cosmogenic radioisotopes , is essentially beryllium @-@ 9 , which has a nuclear spin of 3 / 2 . Beryllium has a large scattering cross section for high @-@ energy neutrons , about 6 barns for energies above approximately 10 keV . Therefore , it works as a neutron reflector and neutron moderator , effectively slowing the neutrons to the thermal energy range of below 0 @.@ 03 eV , where the total cross section is at least an order of magnitude lower – exact value strongly depends on the purity and size of the crystallites in the material . 
