Byron, FW., Joachain, CJ., & Piraux, B. (1982). On the 2nd Born Approximation for the Ionization of Atomic-hydrogen and Helium By Fast Electrons. Journal of Physics B: Atomic, Molecular and Optical Physics, 15(8), L293-L296. https://doi.org/10.1088/0022-3700/15/8/006 (Original work published 1982)