nonchalance vs unconcern

nonchalance

noun
  • Indifference, unconcern; carelessness; coolness; disregard, detachment. 

unconcern

noun
  • Lack of interest or care; indifference or apathy. 

  • Freedom from worry or apprehensiveness; insouciance or nonchalance. 

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