Spin-dependent structure functions for Xe129 and Xe131 from figure 6 from:
    Klos et al, "Large-scale nuclear structure calculations for spin-dependent WIMP scattering with chiral effective field theory currents"
    https://arxiv.org/abs/1304.7684

u = p^2 b^2 / 2 with 
    p the momentum transfer
    b = 2.2853 fm for 129Xe
        2.2905 fm for 131Xe

Since u is dimensionless, 
    

Curve traced by Jelle on 12 January 2018

'up' and 'down' correspond to theoretical uncertainties.


