The neutron cross section is defined for a given type of target particle. For example, the capture cross section of deuterium H is much smaller than that of common hydrogen H. This is the reason why some reactors use heavy water (in which most of the hydrogen is deuterium) instead of ordinary light water as moderator : fewer neutrons are lost by capture inside the medium, hence enabling the use of natural uranium instead of enriched uranium. This is the principle of a CANDU reactor.