fission bomb vs neutron bomb

fission bomb

noun
  • An atomic bomb that uses nuclear fission as a source of energy 

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 fission bomb and neutron bomb occurred in a corpus of books? (source: Google Ngram Viewer )