Natural cadmium consists of eight isotopes, 106Cd (1.3%), 108Cd (0.9%), 110Cd (12.5%), 111Cd (12.8%), 112Cd (24.3%), 113Cd (12.2%), 114Cd (28.7%) and 116Cd (7.5%). Two of them are radioactive isotopes with very long half-life (113Cd – 7.7 x 1015 y and 116Cd – 2.9 x 1019 y).In nuclear industry cadmium is commonly used as a thermal neutron absorber due to very high neutron absorption cross-section of 113Cd. 113Cd has specific absorption cross-section. There is a cadmium cut-off energy (Cadmium edge) in the absorption cross-section. Only neutrons of kinetic energy below the cadmium cut-off energy (~0.5 eV) are strongly absorbed by 113Cd. Therefore cadmium is widely used to absorb thermal neutrons in a thermal neutron filters.
Source: JANIS (Java-based Nuclear Data Information Software); The JEFF-3.1 Nuclear Data Library