  We present a comprehensive inelastic neutron scattering study of the magnetic
excitations in twin-free YBa(2)Cu(3)O(6.6) (Tc=61 K) for 5 K < T < 290 K.
Taking full account of the instrumental resolution, we derive analytical model
functions for the magnetic susceptibility chi''(Q,omega) at T = 5 K and 70 K in
absolute units. Our models are supported by previous results on similar samples
and are valid at least up to excitation energies of omega = 100 meV. The
detailed knowledge of chi''(Q,omega) permits quantitative comparison to the
results of complementary techniques including angle-resolved photoemission
spectroscopy (ARPES), as demonstrated in Dahm et al., Nature Phys. 5, 217,
(2009). Based on accurate modeling of the effect of the resolution function on
the detected intensity, we determine important intrinsic features of the spin
excitation spectrum, with a focus on the differences above and below Tc. In
particular, at T = 70 K the spectrum exhibits a pronounced twofold in-plane
anisotropy at low energies, which evolves towards fourfold rotational symmetry
at high energies, and the relation dispersion is "Y"-shaped. At T = 5 K, on the
other hand, the spectrum develops a continuous, downward-dispersing "resonant"
mode with weaker in-plane anisotropy. We understand this topology change as
arising from the competition between superconductivity and the same electronic
liquid-crystal state as observed in YBa(2)Cu(3)O(6.45). We discuss our data in
the context of different theoretical scenarios suggested to explain this state.
