Un Invariant De Degre 3 Des Algebres Centrales Simples D’exposant 2

We associate to any central simple algebra A of exponent 2 over a field of characteristic ≠2 an invariant with values in the degree 3 unramified cohomology of its Severi-Brauer variety modulo the image of the cohomology of the ground field. The main theorem is that this invariant is nonzero if and only if the index of A is ≥8.