  We study an analytically solvable model for decoherence of a two spin system
embedded in a large spin environment. As a measure of entanglement, we evaluate
the concurrence for the Bell states (Einstein-Podolsky-Rosen pairs). We find
that while for two separate spin baths all four Bell states lose their
coherence with the same time dependence, for a common spin bath, two of the
states decay faster than the others. We explain this difference by the relative
orientation of the individual spins in the pair. We also examine how the Bell
inequality is violated in the coherent regime. Both for one bath and two bath
cases, we find that while two of the Bell states always obey the inequality,
the other two initially violates the inequality at early times.
