Interaction of relativistic H- ions with thin foils.

The response of relativistic H- ions to thin carbon foils was investigated for beam energies ranging from 226 to 800 MeV. For the foil thicknesses studied, ranging from 15 to 300 mu-g/cm2, an appreciable fraction of the H- beam survives intact, some H- ions are stripped down to protons, and the remainder is distributed over the states of H(O). This experiment is different from the low-energy studies in that the projectile velocity is comparable to the speed of light, leading to an interaction time typically less than a femtosecond. The present results challenge the theoretical understanding of the interaction mechanisms. An electron spectrometer was used to selectively field ionize the Rydberg states 9 < n < 17 at beam energies of 581 and 800 MeV. The yield of low-lying states was measured by Doppler tuning a Nd:YAG (where YAG represents yttrium aluminum garnet) laser to excite transitions to a Rydberg state that was then field ionized and detected. Data are presented for production of n = 2,3 at 226 MeV, n = 2,3 at 500 MeV, n = 2,3,4 at 581 MeV, n = 2 at 716 MeV, and n = 1,2,3,4,5 at 800 MeV. A simple model is developed to fit the yield of each state as a function of foil thickness. Although the simple model is successful in predicting the general features, the data are suggestive of a more complex structure, in the yield of a state as a function of the foil thickness. The optimum thickness to produce a given state increases with the principal quantum number of the state, suggesting an excitation process that is at least partially stepwise. The results of a Monte Carlo simulation are compared with the experimental data to estimate the distribution of the excited states coming out of a foil.