fusion bomb vs neutron bomb

fusion bomb

noun
  • An atomic bomb that uses nuclear fusion as a source of energy; a hydrogen bomb 

neutron bomb

noun
  • An atomic bomb that produces a greater amount of neutrons, and a lesser amount of blast; the intention being to create equal lethal neutron radiation and blast radii which increases the effectiveness of the weapon against armour while reducing collateral damage. 

How often have the words fusion bomb and neutron bomb occurred in a corpus of books? (source: Google Ngram Viewer )