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

The stopping power of atomic and molecular deuterons in He-3 gas was measur ed over the range E-d = 10 to 100 keV using the He-3 pressure dependence of the He-3(d,p)He-4 reaction yield. At energies above 30 keV, the observed s topping power values are in good agreement with a standard compilation. How ever, near 18 keV the experimental values drop by a factor 50 below the ext rapolated 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 whi ch the atoms become nearly transparent for the ions.