  We present details about an experimental method based on the Landau Zener
model which allows to measure very small tunnel splittings $\Delta$ in
molecular clusters Fe8. The measurements are performed with an array of
micro-SQUIDs. The observed oscillations of Delta as a function of the magnetic
field applied along the hard anisotropy axis are explained in terms of
topological quantum interference of two tunnel paths of opposite windings.
Transitions between M = -S and (S - n), with n even or odd, revealed a parity
(symmetry) effect which is analogous to the suppression of tunneling predicted
for half integer spins. This observation is the first direct evidence of the
topological part of the quantum spin phase (Berry phase) in a magnetic system.
The influence of intermolecular dipole interactions on the measured tunnel
splittings $\Delta$ are shown.
