  In the canonical formalism of statistical physics, a signature of a first
order phase transition for finite systems is the bimodal distribution of an
order parameter. Previous thermodynamical studies of nuclear sources produced
in heavy-ion collisions provide information which support the existence of a
phase transition in those finite nuclear systems. Some results suggest that the
observable Z1 (charge of the biggest fragment) can be considered as a reliable
order parameter of the transition. This talk will show how from peripheral
collisions studied with the INDRA detector at GSI we can obtain this bimodal
behaviour of Z1. Getting rid of the entrance channel effects and under the
constraint of an equiprobable distribution of excitation energy (E*), we use
the canonical description of a phase transition to link this bimodal behaviour
with the residual convexity of the entropy. Theoretical (with and without phase
transition) and experimental Z1-E* correlations are compared. This comparison
allows us to rule out the case without transition. Moreover that quantitative
comparison provides us with information about the coexistence region in the
Z1-E* plane which is in good agreement with that obtained with the signal of
abnormal uctuations of configurational energy (microcanonical negative heat
capacity).
