A linear absorption spectrometer includes: a laser light source that provides mid-infrared laser light; a high finesse optical resonator that includes: a sample cell operating at a temperature from 220 K to 300 K during linear absorption of mid-infrared laser light by radiocarbon and including: a linear absorption optical path length greater than a kilometer; a first zero-pressure difference mirror mount on which a first supermirror is disposed; a second zero-pressure difference mirror mount on which a second supermirror is disposed; an optical switch interposed between the laser light source and the high finesse optical resonator that modulates and communicates mid-infrared laser light to the high finesse optical resonator; a photoreceiver that receives cavity ring down light and includes a noise equivalent power that is less than a shot noise limit of cavity ring down light.