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