Stopping power of low-energy deuterons in He-3 gas

The stopping power of atomic and molecular deuterons in 3He gas was measured over the range E d = 10 to 100 keV using the 3He pressure dependence of the 3He(d,p) 4He reaction yield. At energies above 30 keV, the observed stopping power values are in good agreement with a standard compilation. However, near 18 keV the experimental values drop by a factor 50 below the extrapolated values of the compilation. In a simple model, the behavior is due to the minimum 1s → 2s electron excitation of the He target atoms (= 19.8 eV, corresponding to E d = 18.2 keV), i.e. it is a quantum effect, by which the atoms become nearly transparent for the ions.