A MEASUREMENT OF CROSS SECTION FOR CHARGE EXCHANGE IN H+-H-COLLISIONS

The cross section for the charge-exchange reaction H+ +H - -> H+H has been measured, using an inclined-beams technique, over an energy range from 0·25 to 10 kev (in the frame of reference in which the target H- ion is at rest). A proton beam collides with an H- beam at an intersection angle of 20°and hydrogen atoms formed from proton charge exchange in the above reaction are detected. This technique provides a relative collision energy much lower than the energy of either beam and ensures separation together with efficient detection of the reaction products in each beam after the collision. The measured cross section, which falls from about 2·1 × 10-14 cm2 at 0·25 kev to 2·5 × 10-15 cm2 at 10 kev, is compared with the theory derived by Bates and Lewis in 1955 and three points of discrepancy are identified. At high energies the calculated cross section decreases as E-1/2, whereas the experimental energy dependence is closer to E-1. The experimental cross section is between two and three times larger than the calculation and it shows structure in the region of 1 kev.