Search Results (34 total)

Collisions of ¹¹²Sn and ¹²⁴Sn nuclei, which differ in their isospin asymmetry, provide information about the rate of isospin diffusion and equilibration. Although several different probes can provide accurate diffusion measurements, the ratios of the mirror nuclei may be the simplest and most promising one. Ratios of the mass seven mirror nuclei yields are analyzed to show the rapidity, transverse momentum, and impact parameter dependence of isospin diffusion.

Evaporation residue and fission cross sections of radioactive ¹³²Sn on ⁶⁴Ni were measured near the Coulomb barrier. A large subbarrier fusion enhancement was observed. Coupled-channel calculations, including inelastic excitation of the projectile and target, and neutron transfer are in good agreement with the measured fusion excitation function. When the change in nuclear size and shift in barrier height are accounted for, there is no extra fusion enhancement in ¹³²Sn+⁶⁴Ni with respect to stable Sn+⁶⁴Ni. A systematic comparison of evaporation residue cross sections for the fusion of even ^112-124Sn and ¹³²Sn with ⁶⁴Ni is presented.

A new isomer in ¹¹⁶Ag with a half-life of 20(1) s has been discovered through the use of conversion electron, β, and γ spectroscopy of on-line mass-separated radioactivities at the Holifield Radioactive Ion Beam Facility at ORNL. The observed electron peaks at 22.5, 44.42, and 47.33 keV were interpreted as the K,L, and M conversion electron lines resulting from a 47.9-keV E3 transition associated with the decay of a second isomeric level in ¹¹⁶Ag. A new level structure of ¹¹⁶Ag is proposed, with the levels identified as the 0^- ground-state and isomers at 47.9 and 128.8 keV having spin/parities of 3⁺ and 6^- respectively.

The characteristics of intermediate mass fragments (IMFs: 3≤  Z≤  20) produced in midperipheral and central collisions are compared. We compare IMFs detected at midvelocity with those evaporated from the excited projectilelike fragment (PLF^*). On average, the IMFs produced at midvelocity are larger in atomic number, exhibit broader transverse velocity distributions, and are more neutron rich as compared to IMFs evaporated from the PLF^*. These characteristics of midvelocity fragments are consistent with the low-density formation of the fragments. We present in the different kinematical regions studied, the 〈E_⊥〉 for isotopically identified IMFs. For a given Z,〈E_⊥〉 is either constant or decreases slightly with increasing A, in contradiction with a mass-dependent collective expansion in which all IMFs are emitted on average at the same time. Neutron-deficient isotopes of even Z elements manifest higher kinetic energies than heavier isotopes of the same element for both PLF^* and midvelocity emission. This result may be because of the charged-particle decay of long-lived excited states.

The B(E2;0₁⁺→2₁⁺) values for the radioactive neutron-rich germanium isotopes ^78,80Ge and the closed neutron shell nucleus ⁸²Ge were measured at the HRIBF using Coulomb excitation in inverse kinematics. These data allow a study of the systematic trend between the subshell closures at N=40 and 50. The B(E2) behavior approaching N=50 is similar to the trend observed for heavier isotopic chains. A comparison of the experimental results with a shell model calculation demonstrates persistence of the N=50 shell gap and a strong sensitivity of the B(E2) values to the effective interaction.

By combining data from a charged particle ⁵⁸Ni+⁵⁸Ni experiment at 52 MeV/nucleon with an ³⁶Ar+⁵⁸Ni experiment at 50 MeV/nucleon for which free neutrons have been detected, an increase in the neutron to proton ratio of the whole nuclear material at midrapidity has been experimentally observed in the reaction ⁵⁸Ni+⁵⁸Ni at 52 MeV/nucleon. The neutron-to-proton ratio of the quasi-projectile emission is analyzed for the same reactions and is seen to decrease below the ratio of the initial system. Those observations suggest that an asymmetric exchange of neutrons and protons between the quasiprojectile and the midrapidity region exists.

Alpha particles emitted from an excited projectilelike fragment (PLF^*) formed in a peripheral collision of two intermediate-energy heavy ions exhibit a strong preference for emission towards the targetlike fragment. The interplay of the initial deformation of the PLF^* caused by the reaction, Coulomb proximity, and the rotation of the PLF^* results in the observed anisotropic angular distribution. Changes in the shape of the angular distribution with excitation energy are interpreted as being the result of forming more elongated initial geometries in the more peripheral collisions.

Particle correlation functions from central ¹²⁹Xe+¹⁹⁷Au collisions at 50A  MeV have been measured with a large area silicon‐strip∕CsI detector array. A new technique of spin determination from particle correlation functions is proposed. Two examples of correlation functions are studied. The spin of the first excited level of ⁸B at 0.774  MeV is determined as J=1.

Using symmetric ¹¹²Sn+¹¹²Sn, ¹²⁴Sn+¹²⁴Sn collisions as references, we probe isospin diffusion in peripheral asymmetric ¹¹²Sn+¹²⁴Sn, ¹²⁴Sn+¹¹²Sn systems at an incident energy of E/A=50  MeV. Isoscaling analyses imply that the quasiprojectile and quasitarget in these collisions do not achieve isospin equilibrium, permitting an assessment of isospin transport rates. We find that comparisons between isospin sensitive experimental and theoretical observables, using suitably chosen scaled ratios, permit investigation of the density dependence of the asymmetry term of the nuclear equation of state.

Isotopic yields for light particles and intermediate mass fragments have been measured for central ¹¹²Sn+¹¹²Sn, ¹¹²Sn+¹²⁴Sn, ¹²⁴Sn+¹¹²Sn, and ¹²⁴Sn+¹²⁴Sn collisions at E∕A=50  MeV and compared with predictions of stochastic mean field calculations. These calculations predict a sensitivity of the isotopic distributions to the density dependence of the asymmetry term of the nuclear equation of state. However, the secondary decay of the excited fragments modifies significantly the primary isotopic distributions and these modifications are rather sensitive to theoretical uncertainties in the excitation energies of the hot fragments. The predicted final isotope distributions are narrower than the experimental data and the sensitivity of the predicted yields to the density dependence of the asymmetry term is reduced.

Evaporation residue cross sections have been measured with neutron-rich radioactive ¹³²Sn beams on ⁶⁴Ni in the vicinity of the Coulomb barrier. The average beam intensity was 2×10⁴ particles per second and the smallest cross section measured was less than 5 mb. Large sub-barrier fusion enhancement was observed. Coupled-channel calculations taking into account inelastic excitation significantly underpredict the measured cross sections below the barrier. The presence of several neutron transfer channels with large positive Q values suggests that multinucleon transfer may play an important role in enhancing the fusion of ¹³²Sn and ⁶⁴Ni.

Projectilelike fragments (PLF, 15<~Z<~46) formed in peripheral and midperipheral collisions of ¹¹⁴Cd projectiles with ⁹²Mo nuclei at E/A=50 MeV have been detected at very forward angles, 2.1°<~θ_lab<~4.2°. Calorimetric analysis of the charged particles observed in coincidence with the PLF reveals that the excitation of the primary PLF is strongly related to its velocity damping. Furthermore, for a given V_PLF*, its excitation is not related to its size, Z_PLF*. For the largest velocity damping, the excitation energy attained is large, approximately commensurate with a system at the limiting temperature.

Fusionlike events in ⁵⁸Ni+¹²C at 34.5 MeV/nucleon are isolated by use of the statistical discriminant analysis method applied to a set of 20 global variables in complete events with at least 90% of the total charge of the system. Two-fragment reduced-velocity correlation functions are measured for the emission of intermediate mass fragments (IMF’s) and compared to many-body trajectory calculations. Alpha particle emission sequence has been deduced as a function of excitation energy. At the highest excitations, α particles are emitted simultaneously with IMF’s on a time scale of about 200–300 fm/c. In less excited events, they are emitted on a longer time scale. This hybrid deexcitation mechanism, sequential and prompt, is corroborated by comparing the charge of the heaviest and the second heaviest fragments to predictions of GEMINI and SMM simulations.

Angular distributions of oxygen produced in the breakup of ¹⁷F incident on a ²⁰⁸Pb target have been measured at angles from 75° to 113° and 39° to 79° for beam energies of 98 and 120 MeV, respectively. The data are dominated by the proton stripping mechanism and are well reproduced by dynamical calculations. The measured breakup cross section is approximately a factor of 3 less than that of fusion at 98 MeV. The influence of breakup on fusion is discussed.

The defining characteristics of fragment emission resulting from the noncentral collision of ¹¹⁴Cd ions with ⁹²Mo target nuclei at E/A=50 MeV are presented. Charge correlations and average relative velocities for midvelocity fragment emission exhibit significant differences when compared to standard statistical decay. These differences associated with similar velocity dissipation are indicative of the influence of the entrance channel dynamics on the fragment production process.

Investigation of intermediate-velocity particle production is performed on entrance channel mass asymmetric collisions of ⁵⁸Ni+C and ⁵⁸Ni+Au at 34.5 MeV/nucleon. Distinctions between prompt preequilibrium ejections, multiple neck ruptures, and an alternative phenomenon of delayed aligned asymmetric breakup are achieved using source reconstructed correlation observables and time-based cluster recognition in molecular dynamics simulations.

The formation and deexcitation of fusionlike events selected in events with a total charge equal or greater than 16 in ²⁴Mg+¹²C system has been investigated at 25, 35, and 45 MeV/nucleon with a large multidetector array. Central single-source events are selected by use of the statistical discriminant analysis method applied to a set of 26 global variables. The fusion cross section has been extracted for the three bombarding energies and compared to other experimental data and to theoretical predictions. The total multiplicity is found to first increase to a maximum value and then decrease with increasing beam energy. It is shown that this behavior is connected to the opening of multifragmentation channels at 45 MeV/nucleon and the disappearance of channels with only light charged particles.

Isotope ratios of fragments produced at midrapidity in peripheral and central collisions of ¹¹⁴Cd ions with ⁹⁸Mo target nuclei at E/A=50 MeV are compared. The influence of the size (A), density, N/Z, E^*/A, and E_flow/A of the emitting source on the measured isotope ratios was explored by comparison with a statistical model (SMM). The midrapidity region associated with peripheral collisions does not appear to be neutron-enriched relative to central collisions.

The particle emission at intermediate velocities in mass asymmetric reactions is studied within the framework of classical molecular dynamics. Two reactions in the Fermi energy domain were modeled, ⁵⁸Ni+C and ⁵⁸Ni+Au at 34.5 MeV/nucleon. The availability of microscopic correlations at all times allowed a detailed study of the fragment formation process. Special attention was paid to the physical origin of fragments and emission timescales, which allowed us to disentangle the different processes involved in the midrapidity particle production. Consequently, a clear distinction between a prompt preequilibrium emission and a delayed aligned asymmetric breakup of the heavier partner of the reaction was achieved.

A new 7 μs isomer in the drip line nucleus ¹⁴⁰Dy was selected from the products of the ⁵⁴Fe (315 MeV)+⁹²Mo reaction by a recoil mass spectrometer and studied with recoil-delayed γ-γ coincidences. Five cascading γ transitions were interpreted as the decay of an I^π=8^- {ν9/2^-[514]⊗ν7/2⁺[404]} K isomer (T_1/2=7.0(5) μs) via the ground-state band. The probability of proton emission from ¹⁴¹Ho to the 0⁺ ground state and to the 2⁺ excited state in ¹⁴⁰Dy is discussed.

Two-fragment reduced-velocity correlation functions of intermediate-mass fragments emitted from midrapidity component and quasiprojectile (QP) sources formed in ⁵⁸Ni+¹²C and ⁵⁸Ni+¹⁹⁷Au reactions at 34.5 MeV/nucleon have been studied. For the midrapidity component, they show a stronger Coulomb suppression at low relative velocities than for the QP source, suggesting a shorter emission time for it than for the QP source. Comparing the experimental correlation functions with the prediction of many-body Coulomb trajectory code, the emission times of a QP source formed in both reactions were extracted as a function of the excitation energy. The variation of emission time of the QP source with the excitation energy is independent of the reaction system. It decreases monotonically with the excitation energy in the range of (2–6)A MeV from several hundred fm/c to about 100 fm/c. Above an excitation energy of 6A MeV, it becomes very short and saturates, suggesting that the QP source undergoes a multifragmentationlike breakup. The influence of the quasitarget fragment Coulomb interaction on QP emission time is also considered.

We have measured two-fragment correlation functions of the intermediate mass fragments emitted from quasiprojectile sources and midrapidity component formed in ⁵⁸Ni+¹⁹⁷Au at 34.5 MeV/nucleon. The two-fragment correlation functions of midrapidity component show a stronger Coulomb suppression than the quasiprojectile source. This Coulomb suppression for midrapidity component changes very little with the excitation energy of the quasiprojectile source deduced event by event by calorimetry method. By comparing the experimental correlation functions with an N-body Coulomb trajectory code calculation, the emission time of quasiprojectile sources has been extracted as a function of the excitation energy. The emission time decreases monotonically with the excitation energy in the range of 2–6 A MeV from 550 fm/c to about 150 fm/c. Above excitation energy of 6A MeV, the emission time becomes shorter and constant, suggesting that prompt multifragmentation occurs in these quasiprojectile sources.

Isotopic yields of IMF’s produced in the ⁵⁸Ni+¹²C,²⁴Mg reactions at 34.5A MeV are investigated. Analysis of experimental data from the CRL-Laval 4π multidetector array focuses on events where at least 75% (60%) of the charge and momentum were detected for the ⁵⁸Ni+¹²C (⁵⁸Ni+²⁴Mg) system. Averaged isospin ratios (N/Z) for IMF’s with Z=3 and 4 are plotted as a function of emission angle and parallel velocity in the center-of-mass frame. Results from simulations with the statistical codes SMM and GEMINI, assuming an equilibrated source, are compared to the experimental ratios. The ratios seem to indicate the presence of a midrapidity necklike structure that would produce IMF’s richer in neutrons than the two main emitters, even for very central collisions.

Isotopic distributions for light particles and intermediate mass fragments have been measured for ¹¹²Sn+¹¹²Sn, ¹¹²Sn+¹²⁴Sn, ¹²⁴Sn+¹¹²Sn, and ¹²⁴Sn+¹²⁴Sn collisions at E/A  =  50 MeV. Isotope, isotone, and isobar yield ratios are utilized to estimate the isotopic composition of the gas phase at freeze-out. Analyses within the equilibrium limit imply that the gas phase is enriched in neutrons relative to the liquid phase represented by bound nuclei. These observations suggest that neutron diffusion is commensurate with or more rapid than fragment production.