| Electronic stopping occurs when an energetic particle interacts with the electrons of a target material, causing charge exchange, excitation, and ionization of the atomic electrons and a corresponding energy loss for the impinging particle. Charge exchange between the projectile and target, in the form of electron capture and electron stripping, is the dominant mode of energy transfer for low energy projectiles in the keV region. In the case of protons in Helium gas, the difference between the ground state energy level of an ionized H atom and the first ionization energy of a Helium atom is large (11.0 eV), and so the process of electron capture is suppressed at very low energy. This leads to a reduction in the stopping cross section near this threshold, and a resultant deviation from the velocity proportionality which is otherwise characteristic of this low energy regime.;The present work uses time-of-flight techniques to directly measure the stopping cross section of various target gases for the light ions H +, D+, and He+ using projectile energies between 2.4 -- 22 keV/u. Measurements are obtained using a low-energy linear accelerator fed by an RF ion source at the Colorado School of Mines Department of Physics. System accuracy is checked with a projectile-target pair which has been well measured in the past using gaseous targets in the energy regime of interest (He+-N2). Data is then accumulated for several projectile-target pairs (H+-He, D+-He, H+-N 2, D+-N2, H+-Ne, D+ -Ne, He+-H2, He+-He, He +-Ne). Results show that the stopping cross section of H+, D+ in He does exhibit a threshold effect for projectile energies lower than ∼20 keV/u. This work provides an independent measurement of this interaction, for which we find only two previous data sets below the threshold energy, and whose results differ by an order of magnitude below 6 keV/u. This work also provides measurements of several other projectile-target pairs for which there exist only limited experimental results in this very low energy regime, and provides experimental evidence of nuclear stopping effects for D + in He gas. |
