The Asymmetry Parameter is needed in the calculation of Effective Attenuation Lengths (EAL). Follow this link to a very handy site that gives
photoionization cross-sections and asymmetry parameters for all the elements. This data comes from references 1 and 2.
Reference:
[1] J.J. Yeh, Atomic Calculation of Photoionization Cross-Sections and Asymmetry Parameters, Gordon and Breach Science Publishers, Langhorne, PE (USA), 1993.
[2] J.J. Yeh and I. Lindau, Atomic Data and Nuclear Data Tables 32 (1985) 1-155.
