  We analyze Chandra temperature maps for a sample of clusters with high
quality radio halo data, to study the origin of the radio halos. The sample
includes A520, A665, A754, A773, A1914, A2163, A2218, A2319, and 1E0657-56. We
present new temperature maps for all but two of them (A520 and A754). All these
clusters exhibit distorted X-ray morphology and strong gas temperature
variations indicating ongoing mergers. Some clusters, e.g., A520, A665,
1E0657-56, exhibit the previously reported spatial correlation between the
radio halo brightness and the hot gas regions. However, it is not a general
feature. While most mergers are too messy to allow us to disentangle the
projection effects, we find clear counterexamples (e.g., A754 and A773) where
the hottest gas regions do not exhibit radio emission at the present
sensitivity level. This cannot be explained by projection effects, and
therefore argues against merger shocks -- at least those relatively weak ones
responsible for the observed temperature structure in most clusters -- as the
main mechanism for the halo generation. This leaves merger-generated turbulence
as a more likely mechanism. The two clusters with the clearest radio brightness
- temperature correlation, A520 and 1E0657-56, are both mergers in which a
small dense subcluster has just passed through the main cluster, very likely
generating turbulence in its wake. The maximum radio brightness and the hot gas
are both seen in these wake regions. On the other hand, the halos in 1E0657-56
and A665 (both high-velocity mergers) extend into the shock regions in front of
the subclusters, where no strong turbulence is expected. Thus, in high-velocity
(M=2-3) mergers, both shock and turbulence acceleration mechanisms may be
significant.
