Generalized isoscaling of isotopic distributions

At incident energies in excess of about E/A530 MeV, a rapid collective expansion of the combined system may occur during the later stages of a central collision between heavy nuclei @1,2#. At densities less than about 1 3 of the saturation density, such systems disassemble into a mixture of fragments and light particles; the duration of fragment emission is of the order of 100 fm/c @3,4#. Even though the emission time is short, statistical models such as bulk multifragmentation models, which assume equilibrium at a single breakup density and temperature, have been used successfully to describe many experimental observables such as fragment multiplicities, charge distributions, and energy spectra of the emitted fragments @2,5–7#. These descriptions require careful, though not necessarily obvious, choices for the source size, excitation energy, and collective velocity of expansion @2,6–8#. Many of these statistical models display a phase transition in nuclear matter with subsaturation density @9,10#; such models have been employed to extract the caloric curve, i.e., the relationship between excitation energy and temperature for the nuclear liquid-gas phase transition @5,11–17# and to address whether finite nuclear systems may display negative heat capacities in analogy to those deduced for finite metallic clusters @18#. The success of thermal models raises the fundamental