Isoscaling in Peripheral Nuclear Collisions around the Fermi Energy and a Signal of Chemical Separation from its Excitation Energy Dependence

The isoscaling is investigated using the fragment yield data from fully reconstructed quasi-projectiles observed in peripheral collisions of 28Si with 124,112Sn at projectile energies 30 and 50 MeV/nucleon. The excitation energy dependence of the isoscaling parameter beta_prime is observed which is independent of beam energy. For a given quasi-projectile produced in reactions with different targets no isoscaling is observed. The isoscaling thus reflects the level of N/Z-equilibration in reactions with different targets represented by the initial quasi-projectile samples. The excitation energy dependence of the isoscaling parameter beta_prime, corrected for the trivial 1/T temperature dependence, does not follow the trend of the homogeneous system above 4 MeV/nucleon thus possibly signaling the onset of separation into isospin asymmetric dilute and isospin symmetric dense phase.Comment: 4 pages, 4 figures, RevTeX, to appear in Physical Review

Heavy Residue Isoscaling as a Probe of the Symmetry Energy of Hot Fragments

The isoscaling properties of isotopically resolved projectile residues from peripheral collisions of 86Kr (25 MeV/nucleon), 64Ni (25 MeV/nucleon) and 136Xe (20 MeV/nucleon) beams on various target pairs are employed to probe the symmetry energy coefficient of the nuclear binding energy. The present study focuses on heavy projectile fragments produced in peripheral and semiperipheral collisions near the onset of multifragment emission E*/A = 2-3 MeV). For these fragments, the measured average velocities are used to extract excitation energies. The excitation energies, in turn, are used to estimate the temperatures of the fragmenting quasiprojectiles in the framework the Fermi gas model. The isoscaling analysis of the fragment yields provided the isoscaling parameters "alpha" which, in combination with temperatures and isospin asymmetries provided the symmetry energy coefficient of the nuclear binding energy of the hot fragmenting quasiprojectiles. The extracted values of the symmetry energy coefficient at this excitation energy range (2-3 MeV/nucleon) are lower than the typical liquid-drop model value ~25 MeV corresponding to ground-state nuclei and show a monotonic decrease with increasing excitation energy. This result is of importance in the formation of hot nuclei in heavy-ion reactions and in hot stellar environments such as supernova.Comment: 11 pages, 9 figures, submitted to Phys. Rev.

Isoscaling Studies of Fission - a Sensitive Probe into the Dynamics of Scission

The fragment yield ratios were investigated in the fission of 238,233U targets induced by 14 MeV neutrons. The isoscaling behavior was typically observed for the isotopic chains of fragments ranging from the proton-rich to the most neutron-rich ones. The observed high sensitivity of neutron-rich heavy fragments to the target neutron content suggests fission as a source of neutron-rich heavy nuclei for present and future rare ion beam facilities, allowing studies of nuclear properties towards the neutron drip-line and investigations of the conditions for nucleosynthesis of heavy nuclei. The breakdowns of the isoscaling behavior around N=62 and N=80 manifest the effect of two shell closures on the dynamics of scission. The shell closure around N=64 can be explained by the deformed shell. The investigation of isoscaling in the spontaneous fission of 248,244Cm further supports such conclusion. The Z-dependence of the isoscaling parameter exhibits a structure which can be possibly related to details of scission dynamics. The fission isoscaling studies can be a suitable tool for the investigation of possible new pathways to synthesize still heavier nuclei.Comment: 7 pages, 3 figures, RevTex, final version, to appear in Phys. Rev. C as a regular articl

Production of cold fragments in nucleus-nucleus collisions in the Fermi-energy domain

The reaction mechanism of nucleus-nucleus collisions at projectile energies around the Fermi energy is investigated with emphasis on the production of fragmentation-like residues. The results of simulations are compared to experimental mass distributions of elements with Z = 21 - 29 observed in the reactions 86Kr+124,112Sn at 25 AMeV. The model of incomplete fusion is modified and a component of excitation energy of the cold fragment dependent on isospin asymmetry is introduced. The modifications in the model of incomplete fusion appear consistent with both overall model framework and available experimental data. A prediction is provided for the production of very neutron-rich nuclei using a secondary beam of 132Sn where e.g. the reaction 132Sn+238U at 28 AMeV appears as a possible alternative to the use of fragmentation reactions at higher energies.Comment: LaTeX, 15 pages, 5 figures, minor modifications, accepted for publication in Nuclear Physics

Studies of reaction dynamics in the Fermi energy domain

An overview of recent results on reaction dynamics in the energy region 20 - 50 A.MeV is given. The results of the study of projectile multifragmentation using the detector array FAUST are presented. Reaction mechanism is determined and thermodynamical properties of the hot quasiprojectile are investigated. Preliminary results on fragment isospin asymmetry obtained using the 4$\pi$ detector array NIMROD are given. Procedure for selecting centrality in two-dimensional multiplicity histograms is described. Possibility to extract thermodynamical temperature from systematics of isotope ratios is investigated. Reaction mechanism leading to production of hot sources is discussed. Furthermore, the possibilities for production of rare isotopes are discussed and recent experimental results obtained using recoil separator MARS are presented.Comment: 18 pages, 11 figures, contribution to proceedings of the conference DANF'2001, Casta-Papiernicka, Slovakia published by World Scientifi

Tracing the evolution of the symmetry energy of hot nuclear fragments from the compound nucleus towards multifragmentation

The evolution of the symmetry energy coefficient of the binding energy of hot fragments with increasing excitation is explored in multifragmentation processes following heavy-ion collisions below the Fermi energy. In this work, high-resolution mass spectrometric data on isotopic distributions of projectile-like fragments from collisions of 25 MeV/nucleon 86Kr and 64Ni beams on heavy neutron-rich targets are systematically compared to calculations involving the Statistical Multifragmentation Model. The study reveals a gradual decrease of the symmetry energy coefficient from 25 MeV at the compound nucleus regime (E*/A < 2 MeV) towards 15 MeV in the bulk multifragmentation regime (E*/A > 4 MeV). The ensuing isotopic distributions of the hot fragments are found to be very wide and extend towards the neutron drip-line. These findings may have important implications to the composition and evolution of hot astrophysical environments, such as core-collapse supernova.Comment: 5 pages, 4 figures, submitted to Phys. Rev.

Isotopic Scaling of Heavy Projectile Residues from the collisions of 25 MeV/nucleon 86Kr with 124Sn, 112Sn and 64Ni, 58Ni

The scaling of the yields of heavy projectile residues from the reactions of 25 MeV/nucleon 86Kr projectiles with 124Sn,112Sn and 64Ni, 58Nitargets is studied. Isotopically resolved yield distributions of projectile fragments in the range Z=10-36 from these reaction pairs were measured with the MARS recoil separator in the angular range 2.7-5.3 degrees. The velocities of the residues, monotonically decreasing with Z down to Z~26-28, are employed to characterize the excitation energy. The yield ratios R21(N,Z) for each pair of systems are found to exhibit isotopic scaling (isoscaling), namely, an exponential dependence on the fragment atomic number Z and neutron number N. The isoscaling is found to occur in the residue Z range corresponding to the maximum observed excitation energies. The corresponding isoscaling parameters are alpha=0.43 and beta=-0.50 for the Kr+Sn system and alpha=0.27 and beta=-0.34 for the Kr+Ni system. For the Kr+Sn system, for which the experimental angular acceptance range lies inside the grazing angle, isoscaling was found to occur for Z<26 and N<34. For heavier fragments from Kr+Sn, the parameters vary monotonically, alpha decreasing with Z and beta increasing with N. This variation is found to be related to the evolution towards isospin equilibration and, as such, it can serve as a tracer of the N/Z equilibration process. The present heavy-residue data extend the observation of isotopic scaling from the intermediate mass fragment region to the heavy-residue region. Such high-resolution mass spectrometric data can provide important information on the role of isospin in peripheral and mid-peripheral collisions, complementary to that accessible from modern large-acceptance multidetector devices.Comment: 8 pages, 6 figures, submitted to Phys. Rev.

Heavy Residue Isoscaling as a Probe of the Process of N/Z Equilibration

The isotopic and isobaric scaling behavior of the yield ratios of heavy projectile residues from the collisions of 25 MeV/nucleon 86Kr projectiles on 124Sn and 112Sn targets is investigated and shown to provide information on the process of N/Z equilibration occurring between the projectile and the target. The logarithmic slopes $\alpha$ and $\beta^{'}$ of the residue yield ratios with respect to residue neutron number N and neutron excess N--Z are obtained as a function of the atomic number Z and mass number A, respectively, whereas excitation energies are deduced from velocities. The relation of the isoscaling parameters $\alpha$ and $\beta^{'}$ with the N/Z of the primary (excited) projectile fragments is employed to gain access to the degree of N/Z equilibration prior to fragmentation as a function of excitation energy. A monotonic relation between the N/Z difference of fragmenting quasiprojectiles and their excitation energy is obtained indicating that N/Z equilibrium is approached at the highest observed excitation energies. Simulations with a deep-inelastic transfer model are in overall agreement with the isoscaling conclusions. The present residue isoscaling approach to N/Z equilibration offers an attractive tool of isospin and reaction dynamics studies in collisions involving beams of stable or rare isotopes.Comment: 15 pages, 4 figures, submitted to Phys. Lett.