The boron isotope 10 B captures neutrons in an (n,)--neutron in, alpha particle outreaction whose cross section for thermal neutrons is 4.010 3 b. The

The boron isotope ¹⁰B captures neutrons in an (n,α)--neutron in, alpha particle out—reaction whose cross section for thermal neutrons is 4.0×10³ b. The density of ¹⁰B is 2.2×10³ kg/m³ . What thickness of ¹⁰B is needed to absorb 99 percent of an incident beam of thermal neutrons?