doomsday machine vs neutron bomb

doomsday machine

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