The energy loss of deuterons in He-3 gas was measured at E-d = 15 to 100keV using the He-3 pressure dependence of the He-3(d,p)He-4 cross-section at a given incident energy. At the highest energies, the observed energy loss is in good agreement with a standard compilation. However, with decreasing energy the experimental values drop steadily below the theoretical values and near E-d = 18keV they drop sharply (within 1keV) reaching the domain of nuclear stopping power. This threshold behavior is due to the minimum 1s --> 2s electron excitation of the He target atoms, i.e. it is a quantum effect. Some consequences are discussed.
- NUCLEAR ASTROPHYSICS