backpack nuke vs neutron bomb

backpack nuke

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