Isotopic composition of fragments in nuclear multifragmentation

In this Rapid Communication we present results from the analysis of the isotopic yields of fragments emitted in two selected reactions: the decay of the quasiprojectile in Au 1Au peripheral collisions at 35 MeV/nucleon, and the disassembly of the unique source formed in Xe1Cu central reactions at 30 MeV/nucleon. We find that the relative yields of neutron-rich isotopes increase with the excitation energy of the emitting sources. In the framework of a statistical multifragmentation model which reproduces fairly well the experimental observables, such behavior can be explained with the increase of the N/Z ratio of the hot primary fragments. This corresponds to the statistical evolution of the fragmentation mechanism as a function of the excitation energy, from the decay into few small fragments with a heavy residue to complete multifragmentation. PACS number~s!: 25.70.Pq, 24.60.2k Nuclear fragmentation and its connection to the behavior of nuclear matter at high excitation energy is the subject of intensive theoretical and experimental investigations @1#. Some general properties of this process are already established: at relatively small excitation energies ( E* < 2‐3 A MeV) there is a formation and decay of a longlived compoundlike nucleus system. This process can be described by evaporation/fissionlike models. At higher excitation energies ~close to the binding energy! there is a complete fast disintegration of the system into fragments, this process can take place in a finite breakup volume. In this case statistical models based on the hypothesis of a nuclear phase transition ~simultaneous decay! happen to be very successful @2,3#. In this Rapid Communication we present recent data on the isotope production in heavy-ion collisions at intermediate energies @4,5# with the aim of studying the isotopic content of fragments for different sources and excitation energies during the transition from the low energy decay to the multifragmentation. We will show that the behavior of the experimental isotopic yields, as a function of source size, isospin, and excitation energy, can be connected to the corresponding evolution of the N/Z ratio of the hot primary fragments in the breakup volume, which is an important ingredient in the disintegration process. In fact, there are both experimental and theoretical studies which show that the knowledge of the chemical composition of hot fragments helps in establishing the freeze-out conditions. ~1! Information about the density of the freeze-out volume can be obtained from the analysis of the velocity correlation functions and kinetic energies of the emitted fragments. Among the different methods @6,7# to extract the temperature of the system, the most popular is based on the statistical properties of the double isotope ratios. With this technique a nuclear caloric curve, as an experimental evidence of a nuclear liquid-gas phase transition @8#, was obtained. In any case, a correction for secondary decay of hot fragments produced in the freeze-out volume @9# is needed, and depending on their N/Z ratio, the secondary decay can proceed differently.