  The validity of the Brink-Axel hypothesis, which is especially important for
numerous astrophysical calculations, is addressed for 116,120,124Sn below the
neutron separation energy by means of three independent experimental methods.
The $\gamma$-ray strength functions (GSFs) extracted from primary
$\gamma$-decay spectra following charged-particle reactions with the Oslo
method and with the Shape method demonstrate excellent agreement with those
deduced from forward-angle inelastic proton scattering at relativistic beam
energies. In addition, the GSFs are shown to be independent of excitation
energies and spins of the initial and final states. The results provide the
most comprehensive test of the generalized Brink-Axel hypothesis in heavy
nuclei so far, demonstrating its applicability in the energy region of the
pygmy dipole resonance.
