Ehrhardt, H., Fischer, M., Jung, K., Byron, FW., Joachain, CJ., & Piraux, B. (1982). Triply Differential Cross-sections for the Ionization of Helium By Fast Electrons. Physical Review Letters, 48(26), 1807-1810. https://doi.org/10.1103/PhysRevLett.48.1807 (Original work published 1982)