neutron bomb vs plutonium 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. 

plutonium bomb

noun
  • A plutonium-fired nuclear bomb. 

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