The ratio of D‐3He to D‐D reactions (RD3He/RDD) is proportional to fuel ρR for D2 and D‐T filled targets. For D2 filled targets, the ratio varies as (RD3He/RDD)=0.14ρR, for ρR≲0.02 g/cm2. For present day D2 filled target experiments, it is necessary to detect 104 D‐3He reactions against a background source of 108 D‐D reactions. For D2 filled targets, detection of the 14.7‐MeV proton of the D‐3He reaction can be accomplished with sheets of the solid state nuclear track detector CR‐39 and metal foils. Spatial coincidence can be used as a means of eliminating background due to imperfections in the track detector and background due to protons in the CR‐39 which are elastically scattered by D‐D neutrons.