RATIO OF DOUBLE-TO-SINGLE IONIZATION OF HE BY N7+ PROJECTILES

We apply the continuum-distorted-wave approximation within the framework of the independent-event model to calculate the ratio (R) of double-to-single ionization cross sections of He by N7+ projectiles. Following Deb and Crothers [J. Phys. B 23, L7999 (1990)], we calculate the double-ionization probability as P1P2, whereas the single-ionization probability is taken to be 2P1(1-P1). Here, P(i) (i = 1,2) is the probability of ionizing the ith electron from He. The values of R found in the 10-40-MeV/u impact-energy range are similar to those obtained in the somewhat different forced-impulse theory of Reading and Ford [Phys. Rev. Lett. 58, 543 (1987)] and by the semiempirical fit of Knudsen et al. [J. Phys. B 17, 3545 (1984)], but are much smaller in absolute value than the corresponding measurements of Heber et al. [Phys. Rev. Lett. 64, 851 (1990); Nucl. Instrum. Methods B 56/57, 15 (1991)].