Cadmium acts as an energy-selective neutron absorber in the stated comparison. Its large thermal capture collision cross-section preferentially reduces the thermal neutron field reaching the source material, while its much lower fast-neutron capture probability allows more of the emitted fast neutrons to escape. This limits thermal neutron-induced changes in the source while retaining its qualitative role as a neutron emitter.
The point is selectivity between incoming thermal and outgoing fast components, rather than a claim that cadmium is transparent to every neutron or that capture produces no secondary radiation. This explains the stated material choice at a conceptual level; no source construction, cladding dimensions or irradiation specification follows from it.
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