The relative isobaric yields of fragments produced in a series of heavy-ion-induced multifragmentation reactions have been analyzed in the framework of a modified Fisher model, primarily to determine the ratio of the symmetry energy coefficient to the temperature, $a_{\mathrm{sym}}/T$, as a function of fragment mass $A$. The extracted values increase from 5 to $~$16 as $A$ increases from 9 to 37. These values have been compared to the results of calculations using the antisymmetrized molecular dynamics (AMD) model together with the statistical decay code gemini. The calculated ratios are in good agreement with those extracted from the experiment. In contrast, the values extracted from the ratios of the primary isobars from the AMD model calculation are $~$4 to 5 and show little variation with $A$. This observation indicates that the value of the symmetry energy coefficient derived from final fragment observables may be significantly different than the actual value at the time of fragment formation. The experimentally observed pairing effect is also studied within the same simulations. The Coulomb coefficient is also discussed. DOI: http://dx.doi.org/10.1103/PhysRevC.81.044620 © 2010 The American Physical Society

The reactions ${}^{12}\mathrm{C}{+}^{116}\mathrm{Sn},$ ${}^{22}\mathrm{N}\mathrm{e}+\mathrm{A}\mathrm{g},$ ${}^{40}\mathrm{Ar}{+}^{100}\mathrm{Mo},$ and ${}^{64}\mathrm{Zn}{+}^{89}\mathrm{Y}$ have been studied at $47A\hspace{0.5em}\mathrm{MeV}$ projectile energy. For these reactions the most violent collisions lead to increasing amounts of fragment and light particle emission as the projectile mass increases. This is consistent with quantum molecular dynamics (QMD) model simulations of the collisions. Moving source fits to the light charged particle data have been used to gain a global view of the evolution of the particle emission. Comparisons of the multiplicities and spectra of light charged particles emitted in the reactions with the four different projectiles indicate a common emission mechanism for early emitted ejectiles even though the deposited excitation energies differ greatly. The spectra for such ejectiles can be characterized as emission in the nucleon-nucleon frame. Evidence that the ${}^3\mathrm{He}$ yield is dominated by this type of emission and the role of the collision dynamics in determining the ${}^3\mathrm{H}{/}^3\mathrm{He}$ yield ratio are discussed. Self-consistent coalescence model analyses are applied to the light cluster yields, in an attempt to probe emitter source sizes and to follow the evolution of the temperatures and densities from the time of first particle emission to equilibration. These analyses exploit correlations between ejectile energy and emission time, suggested by the QMD calculations. In this analysis the degree of expansion of the emitting system is found to increase with increasing projectile mass. The double isotope yield ratio temperature drops as the system expands. Average densities as low as $0.36{\rho }_0$ are reached at a time near $100\hspace{0.5em}\mathrm{fm}/c$ after contact. Calorimetric methods were used to derive the mass and excitation energy of the excited nuclei which are present after preequilibrium emission. The derived masses range from 102 to 116 u and the derived excitation energies increase from 2.6 to 6.9 MeV/nucleon with increasing projectile mass. A caloric curve is derived for these expanded $A\sim 110$ nuclei. This caloric curve exhibits a plateau at temperatures near 7 MeV. The plateau extends from $\sim 3.5$ to 6.9 MeV/nucleon excitation energy.

The elastic scattering of ${}^8$B by a ${}^{\mathrm{nat}}$Pb target was measured at an incident energy of 170.3 MeV. Special care was taken with the limited intensity and broad profile of the secondary beam. The measured angular distribution of the differential cross section shows that the Coulomb-nuclear interference peak (CNIP) is not suppressed in this system, in contrast to what was observed in the scattering of neutron halo nuclei by heavy targets at energies around the Coulomb barrier. Analyses of the angular distribution were performed both in terms of the optical model using a single-folding-type potential and the continuum discretized coupled-channels (CDCC) method, which explicitly takes into account the breakup-channel couplings to the elastic scattering. The overall pattern of the differential cross section is well reproduced by the CDCC calculations. The calculations show that the effect of breakup-channel couplings on the elastic scattering is small in the present case.

We discuss experimental evidence for a nuclear phase transition driven by the different concentrations of neutrons to protons. Different ratios of the neutron to proton concentrations lead to different critical points for the phase transition. This is analogous to the phase transitions occurring in ${}^4\mathrm{He}$-${}^3\mathrm{He}$ liquid mixtures. We present experimental results that reveal the $N/A$ (or $Z/A$) dependence of the phase transition and discuss possible implications of these observations in terms of the Landau free energy description of critical phenomena. DOI: http://dx.doi.org/10.1103/PhysRevC.81.044618 © 2010 The American Physical Society

Isotope yield distributions in the multifragmentation regime were studied with high-quality isotope identification, focusing on the intermediate mass fragments (IMFs) produced in semiviolent collisions. The yields were analyzed within the framework of a modified Fisher model. Using the ratio of the mass-dependent symmetry energy coefficient relative to the temperature, asym/T, extracted in previous work and that of the pairing term, ap/T, extracted from this work, and assuming that both reflect secondary decay processes, the experimentally observed isotope yields were corrected for these effects. For a given I=N-Z value, the corrected yields of isotopes relative to the yield of C12 show a power law distribution Y(N,Z)/Y(12C)~A-$\tau${} in the mass range 1$\leqslant${}A$\leqslant${}30, and the distributions are almost identical for the different reactions studied. The observed power law distributions change systematically when I of the isotopes changes and the extracted $\tau${} value decreases from 3.9 to 1.0 as I increases from -1 to 3. These observations are well reproduced by a simple deexcitation model, with which the power law distribution of the primary isotopes is determined to be $\tau${}prim=2.4\ifmmode\pm\else\textpm\fi{}0.2, suggesting that the disassembling system at the time of the fragment formation is indeed at, or very near, the critical point.

The ratio of the symmetry energy coefficient to temperature, $a_{\mathrm{sym}}/T$, in Fermi energy heavy-ion collisions, was experimentally extracted as a function of the fragment atomic number using isoscaling parameters and the variance of the isotope distributions. The extracted values were compared to the results of calculations made with an antisymmetrized molecular dynamics (AMD) model employing a statistical decay code to account for deexcitation of excited primary fragments. The experimental values are in good agreement with the values calculated from the final ground-state products but are significantly different from those characterizing the yields of the primary AMD fragments. DOI: http://dx.doi.org/10.1103/PhysRevC.81.064613 © 2010 The American Physical Society