  Using high sensitivity magneto-optical imaging we find evidence for a jump in
local vortex density associated with a vortex liquid to solid phase transition
just above the lower critical field in a single crystal of Bi2Sr2CaCu2O8. We
find the regions of the sample where the jump in vortex density occurs are
associated with low screening currents. In the field - temperature vortex phase
diagram we identify phase boundaries demarcating a dilute vortex liquid phase
and the vortex solid phase. The phase diagram also identifies a coexistence
regime of the dilute vortex liquid and solid phases and shows the effect of
pinning on the vortex liquid to solid phase transition line. We find the phase
boundary lines can be fitted to the theoretically predicted expression for the
low-field portion of the phase boundary delineating a dilute vortex solid from
a vortex liquid phase. We show that the same theoretical fit can be used to
describe the pinning dependence of the low-field phase boundary lines provided
a dependence of the Lindemann number on pinning strength is considered.
