Search Results (25 total)

Fission and binary fragmentation of the excited nuclear systems of Z=116 and 124 were investigated using the reactions induced by ⁸⁰Se beams on ²⁰⁸Pb and ²³²Th targets at bombarding energies ranging from 470 to 630 MeV. The mass and kinetic energy of the binary reaction products were reconstructed by measuring their velocities by the time-of-flight method and the angles of emission using multiwire proportional chambers. Total neutron multiplicities were measured in coincidence with the fragments, using an array of neutron detectors. The fragment mass-energy correlation was studied for the two systems. The average total kinetic energy (TKE) of fragments for the ⁸⁰Se+²⁰⁸Pb system agrees with earlier measurements and with Viola's systematics in the mass symmetric region for compound nucleus fission, whereas for the ⁸⁰Se+²³²Th system, the TKE values are significantly lower. This is also consistent with higher values of total neutron multiplicities observed for the case of ⁸⁰Se+²³²Th at comparable available energies. From an extrapolation of the measured total neutron multiplicities for the mass symmetric region to zero compound nucleus excitation energy, the average number of prompt neutrons expected to be emitted in the spontaneous fission of the superheavy Z=116 has been estimated to be ν_tot^sf=10±2, which is consistent with the value derived for the same compound nucleus populated in the ⁵⁶Fe+²³²Th reaction in an earlier work. In the case of the ⁸⁰Se+²³²Th system, similar analysis was carried out by taking the average TKE from Viola's systematics for estimating the available energy for particle emission corresponding to compound nucleus fission. In this way, by extrapolating the observed neutron multiplicities to zero compound nucleus excitation energy, a value of ν_tot^sf=15±2 was obtained for the spontaneous fission of the superheavy Z=124 nucleus. The increase in the average number of neutrons emitted in fission as a function of the atomic number of the nucleus in the superheavy mass region was confirmed by comparing the results of the present work with published data.

The sizes, temperatures, and free neutron-to-proton ratios of the initial interaction zones produced in the collisions of 40 MeV/nucleon ⁴⁰Ar+¹¹²Sn and 55 MeV/nucleon ²⁷Al+¹²⁴Sn are derived using total detected neutron plus charged particle multiplicity as a measure of the impact parameter range and number of participant nucleons. The size of the initial interaction zone, determined from a coalescence model analysis, increases significantly with decreasing impact parameter. The temperatures and free neutron-to-proton ratios in the interaction zones are relatively similar for different impact parameter ranges and evolve in a similar fashion.

Experimental analyses of moderate-temperature nuclear gases produced in the violent collisions of 35 MeV/nucleon ⁶⁴Zn projectiles with ⁹²Mo and ¹⁹⁷Au target nuclei reveal a large degree of α particle clustering at low densities. For these gases, temperature- and density-dependent symmetry energy coefficients have been derived from isoscaling analyses of the yields of nuclei with A≤4. At densities of 0.01 to 0.05 times the ground-state density of symmetric nuclear matter, the temperature- and density-dependent symmetry energies range from 9.03 to 13.6 MeV. This is much larger than those obtained in mean-field calculations and reflects the clusterization of low-density nuclear matter. The results are in quite reasonable agreement with calculated values obtained with a recently proposed virial equation of state calculation.

The binary fragmentation of the excited superheavy system of Z=116 formed in the reaction 372 MeV ⁵⁶Fe+²³²Th has been investigated. The fragment masses and kinetic energies were determined through measurement of the fragment velocities by the time-of-flight method. Neutron spectra were measured in coincidence with the fragments at several angles with respect to the fragment direction, which were analyzed to deduce the total as well as the pre-scission neutron multiplicities. We analyzed the correlations between fragment mass and kinetic energy, gated and nongated by the coincidence with neutrons, to learn about the dynamics of the reaction with respect to the two-body exit channels. The events in the near-mass-symmetric valley region appear to receive a significant contribution from the asymmetric mode of fission of the superheavy compound nucleus Z=116, which can be due to the influence of the closed proton and neutron shells of Z=50 and N=82 in the light fragments. From the observed number of pre-scission neutrons, it is inferred that the time scales of the fissionlike reactions leading to near-mass-symmetric splits are rather large, of the order of several times 10^-20  s. The average number of prompt neutrons emitted in the spontaneous fission of such a superheavy nucleus is ν=(12±1), as deduced from the neutron measurements.

The kinetic-energy variation of emitted light clusters has been employed as a clock to explore the time evolution of the temperature for thermalizing composite systems produced in the reactions of 26A, 35A, and 47A MeV ⁶⁴Zn with ⁵⁸Ni, ⁹²Mo, and ¹⁹⁷Au. For each system investigated, the double-isotope ratio temperature curve exhibits a high maximum apparent temperature, in the range of 10–25 MeV, at high ejectile velocity. These maximum values increase with increasing projectile energy and decrease with increasing target mass. The time at which the maximum in the temperature curve is reached ranges from 80 to 130 fm/c after contact. For each different target, the subsequent cooling curves for all three projectile energies are quite similar. Temperatures comparable with those of limiting temperature systematics are reached 30 to 40 fm/c after the times corresponding to the maxima, at a time when antisymmetrized molecular dynamics transport model calculations predict entry into the final evaporative or fragmentation stage of deexcitation of the hot composite systems. Evidence for the establishment of thermal and chemical equilibrium is discussed.

Calorimetric and coalescence techniques have been employed to probe equilibration for hot nuclei produced in heavy-ion collisions of 35 to 55 MeV/nucleon projectiles with medium mass targets. Entrance channel mass asymmetries and energies were selected so that very hot composite nuclei of similar mass and excitation would remain after early stage preequilibrium particle emission. Intercomparison of the properties and deexcitation patterns for these different systems provides evidence for the production of hot nuclei with decay patterns relatively independent of the specific entrance channel.

An extensive experimental survey of the features of the disassembly of a small quasiprojectile system with A~36, produced in the reactions of 47 MeV/nucleon ⁴⁰Ar  +  ²⁷Al, ⁴⁸Ti, and ⁵⁸Ni, has been carried out. Nuclei in the excitation energy range of 1–9 MeV/nucleon have been investigated employing a new method to reconstruct the quasiprojectile source. At an excitation energy ∼5.6 MeV/nucleon many observables indicate the presence of maximal fluctuations in the deexcitation processes. These include the normalized second moments of the Campi plot and normalized variances of the distributions of order parameters such as the atomic number of the heaviest fragment Z_max and the total kinetic energy. The evolution of the correlation of the atomic number of the heaviest fragment with that of the second heaviest fragment and a bimodality test are also consistent with a transition in the same excitation energy region. The related phase separation parameter, S_p, shows a significant change of slope at the same excitation energy. In the same region a Δ-scaling analysis for of the heaviest fragments exhibits a transition to Δ = 1 scaling, which is predicted to characterize a disordered phase. The fragment topological structure shows that the rank-sorted fragments obey Zipf's law at the point of largest fluctuations, providing another indication of a liquid gas phase transition. The Fisher droplet model critical exponent τ ∼ 2.3 obtained from the charge distribution at the same excitation energy is close to the critical exponent of the liquid gas phase transition universality class. The caloric curve for this system shows a monotonic increase of temperature with excitation energy and no apparent plateau. The temperature at the point of maximal fluctuations is 8.3±0.5  MeV. Taking this temperature as the critical temperature and employing the caloric curve information we have extracted the critical exponents β,γ, and σ from the data. Their values are also consistent with the values of the universality class of the liquid gas phase transition. Taken together, this body of evidence strongly suggests a phase change in an equilibrated mesoscopic system at, or extremely close to, the critical point.

The reaction systems, ⁶⁴Zn+⁵⁸Ni, ⁶⁴Zn+⁹²Mo, ⁶⁴Zn+¹⁹⁷Au, at 26, 35, and 47  A  MeV, have been studied both in experiments with a 4π detector array, NIMROD, and with antisymmetrized molecular dynamics model calculations employing effective interactions corresponding to soft and stiff equation of state (EOS). Direct experimental observables, such as multiplicity distributions, charge distributions, energy spectra and velocity spectra, have been compared in detail with those of the calculations and a reasonable agreement is obtained for both EOS’s. No conclusive preference for either EOS has been observed. Neither of the above direct observables nor the strength of the elliptic flow are also sensitive to changes in the in-medium nucleon-nucleon cross sections. A detailed analysis of the central collision events revealed that multifragmentation with cold fragment emission is a common feature predicted for all reactions studied here. A possible multifragmentation scenario is presented; after the preequilibrium emission ceases in the composite system, cold light fragments are formed in a hotter gas of nucleons and stay cold until the composite system underdoes multifragmentation. For reaction with ¹⁹⁷Au at 47A  MeV a significant radial expansion takes place. For reactions with ⁵⁸Ni and ⁹²Mo at 47A  MeV semitransparency becomes prominent. The differing reaction dynamics drastically change the kinematic characteristics of emitted fragments. This scenario gives consistent explanations for many existing experimental results in the Fermi energy domain.

Fusion-evaporation reactions induced by 110  MeV ¹¹B and radioactive ¹¹C on ⁸⁷Rb targets have been studied by measuring evaporation residue–light particle coincidences. The proton to α particle ratio in each reaction has been derived and compared with predictions from statistical model calculations. These calculations account rather well for the experimental data, when a small empirical adjustment of the emission barrier is performed, in agreement with earlier results. No evidence is found for predicted temperature and isospin modification of the binding energies. The possibility of a further study of isospin and temperature dependent effects in fusion-evaporation reactions with radioactive beams is discussed.

A wide variety of observables indicate that maximal fluctuations in the disassembly of hot nuclei with A∼36 occur at an excitation energy of 5.6±0.5  MeV∕nucleon and temperature of 8.3±0.5  MeV. Associated with this point of maximal fluctuations are a number of quantitative indicators of apparent critical behavior. The associated caloric curve does not appear to show a flattening such as that seen for heavier systems. This suggests that, in contrast to similar signals seen for liquid-gas transitions in heavier nuclei, the observed behavior in these very light nuclei is associated with a transition much closer to the critical point.

Intermediate mass fragment (Z>2) emission in ¹²⁴Sn,¹²⁴Xe+¹²⁴Sn,¹¹²Sn reactions at 28  MeV∕nucleon were studied using neutron ion multidetector for reaction oriented dynamics, a 4π charged particle detection system. A number of observables, such as isotopic yield distributions, energy spectra of light charged particles and intermediate mass fragments, isotopic and isobaric yield ratios, and average neutron to proton ratios are investigated. These observables show significant dependence on the isospin N∕Z of the reacting system. It is observed that the formation of neutron-rich clusters are correlated with the excess neutrons in the composite system and depends on the temperature of the emitting source. The origin of light particles and fragments was studied through observations of rapidity distribution as a function of collision violence. With increasing centrality, the heavier ⁶He isotope is found to be emitted closer to the midrapidity region than the lighter ³He isotope. The emission of heavy fragments from the midrapidity region becomes increasingly favorable for fragments with higher charge Z. The results suggest that the midrapidity region is not only neutron rich but also a rich source of heavy fragment (cluster) formation.

Differential and total cross sections for the exclusive breakup of a ⁶Li projectile into α+d and α+p fragments from a ²⁰⁸Pb target were measured at several energies around the Coulomb barrier. The α+d exclusive cross sections are well reproduced by full continuum discretized coupled channel calculations with excitation up to 11.5 MeV of the α+d (⁶Li^*) system. The total exclusive cross sections are much smaller than the inclusive ones for the production of α particles. This large difference gives very clear experimental evidence for the stripping breakup mechanism. Such a process, relevant in the reaction dynamics even at the Coulomb barrier with loosely bound projectiles such as ⁶Li, needs to be carefully considered and described theoretically.

γ Rays in the πν^-1 nucleus ²⁰⁸Bi have been studied at the Gammasphere using deep-inelastic reactions induced by a 305-MeV ⁴⁸Ca beam on a thick ²⁰⁸Pb target. Previously unknown yrast γ-ray cascades above the 10^- millisecond isomer in ²⁰⁸Bi were identified in cross coincidence with known γ rays from complementary potassium products. Yrast and near-yrast levels up to 5.6 MeV in ²⁰⁸Bi have been located, and they are interpreted in light of earlier charged particle spectroscopy results, and with the help of shell model calculations.

The transfer-induced fission channel has been studied in the collision of 340 MeV ²⁸Si on ²³²Th as a function of the atomic number of the projectilelike fragments (PLF’s) by using a 4π detector array. It is found that the energy loss of the transfer reaction increases as a function of the net charge transfer ΔZ from the projectile to the target nucleus, going from quasielastic to deep-inelastic regimes. The average excitation energy of the targetlike fragment (TLF) is derived from the measured energy loss, whereas its angular momentum has been obtained from the angular distribution of fission fragments. It is found that the populated TLF nuclei with Z_TLF=90–96 (ΔZ=0–6) have average excitation energies up to about 100 MeV and angular momenta up to about 40ħ. The measured ratio of transfer-fission yield to PLF singles, Y_f, first increases with increasing net charge transfer up to ΔZ=4 and then shows a plateau around the values Y_f=0.4–0.6 followed by a decrease for higher ΔZ transfers. This ratio can be identified as the cumulative fission probability of the populated nuclei for net charge transfers up to ΔZ<~6, where a two-body mechanism for the first reaction step is supported by the experimental data. This result suggests a significant survival probability against fission of these TLF nuclei, in marked disagreement with the standard statistical model predictions. The observed survival probability implies that there is a strong hindrance to fission in the early stages of deexcitation, as also indicated by the large fission times (t_f=10–100 zs) derived from earlier neutron measurements in fusion-fission reactions. The importance of such effects in the population of nuclei in the heavy and superheavy mass regions by transfer reactions is discussed.

The emission of two coincident energetic (E>~8 MeV) gamma rays has been observed in the 187 MeV ³⁷Cl+¹²⁰Sn reaction by using the cluster detectors of the EUROBALL III array. Those events are attributed to the decay of the double giant dipole resonance built on highly excited states.

Neutrons and isotopically resolved light charged particles have been detected in coincidence with evaporation residues produced in the reaction E/A=11 MeV ⁶⁰Ni+¹⁰⁰Mo. Multiplicities of evaporated particle-unstable clusters have been determined from correlations in the emission of these light particles. The decay of the short-lived ⁵He and ⁸Be (E^*=3.04 MeV) states was found to be affected by the Coulomb field of the compound nucleus in accordance with theoretical estimates. The contributions to the measured kinetic-energy distributions of stable fragments from the sequential decay of the unstable clusters was examined. Overall, the contributions from secondary fragments do not greatly influence the spectral shapes and specifically the location of the spectral peaks are not significantly shifted down in energy due to the presence of these secondary fragments. Therefore contrary to the suggestion of Charity et al. [Phys. Rev. C 56, 873 (1997)], the lower peak energy of the experimental α-particle spectrum as compared to standard statistical-model calculations cannot be attributed to sequential α particles from ⁵He and other clusters. Only for the extreme “subbarrier” regions of the α-particle, deuteron, ^6,7Li, and ⁸Be spectra was the sequential contribution found to be dominant. Statistical-model calculations incorporating large initial deformations are shown to provide enhancements in the yield of low-energy fragments which are roughly appropriate for all the detected isotopes. This suggests that the origin of the sub-barrier enhancements may be a result of evaporation from highly deformed systems which are either produced dynamically during the fusion process or by thermal shape fluctuations.

The total average spins of fission fragments were measured as a function of fragment mass in ¹²C, ¹⁶O+²⁰⁹Bi and ¹²C, ¹⁶O+²³²Th reactions at bombarding energies near and above the Coulomb barrier. The mass dependence of the fragment spin for the ¹²C, ¹⁶O+²³²Th reactions is observed to be in marked contrast to that for the ¹²C, ¹⁶O+²⁰⁹Bi reactions. The present results have been analyzed within the framework of the statistical model taking into account the finite relaxation times for the equilibration of the collective-spin modes. The observed features in ¹²C, ¹⁶O+²³²Th reactions can be ascribed to incomplete equilibration of the collective-spin modes where the mass relaxation at extreme mass asymmetries terminates before full statistical equilibration of the collective-spin modes is reached.

The energy deposition associated with inelastic α particle scattering on ²⁰⁹Bi at 240 MeV has been determined using the TAMU neutron ball. A comparison of the reconstructed average excitation energies with the beam energy losses demonstrates that only part of the missing beam energy is usually deposited as thermal excitation in the target nucleus. Requiring an additional coincidence with a light charged particle or fission fragment leads to selection of a significant higher average excitation energy.

The ridge structure at ΔE_γ=±30 keV, observed previously in the reaction 187 MeV ³⁷Cl+¹²⁰Sn and attributed to an hyperdeformed nuclear shape, has been studied in a high statistics proton-γ coincidence experiment. The presence of the ridge is confirmed but it is correlated to a set of γ-ray energies different from those proposed in earlier works. Some discrete transitions associated with this structure, which accounts for only ∼3×10^-5 of the fusion evaporation cross section, are proposed.

The spectra of high-energy γ rays in coincidence with evaporation residues ^155,154Er populated in the reaction of 241 MeV ⁶⁴Ni on ⁹²Zr have been compared with results from Monte Carlo statistical model calculations. The shape of the spectrum in coincidence with the ¹⁵⁵Er nuclei is reproduced only by simulations in which the spin distribution is extended towards very high spin values.

The feeding mechanism of the superdeformed bands in ¹⁴⁷Gd and ¹⁴⁸Gd has been studied via the ¹²⁴Sn + ²⁹Si reaction at a beam energy of 157 MeV. Using the BGO inner ball of the GASP array, high-energy γ rays have been detected in coincidence with discrete transitions deexciting normal-deformed (ND) and superdeformed (SD) states in the final product nuclei. The slope of the measured high-energy γ-ray spectra depends strongly on the number of emitted neutrons and is somewhat lower for spectra in coincidence with SD bands than for those in coincidence with ND structures. Both observations are qualitatively reproduced by statistical model Monte Carlo calculations, which point out the importance of angular momentum effects in the emission of energetic γ rays. The present data exclude the enhanced population of SD bands when fed through high-energy E1 transitions.

Neutron-rich sdf shell nuclei have been produced in deep-inelastic processes during collision of 230 MeV ³⁷Cl ions on a thick ²⁰⁸Pb target. The in-beam as well as off-beam γ-γ coincidence data yielded new spectroscopic information on some hard-to-access N=18,19 isotones. A new 27 ns isomer at 5583 keV in ³²Si has been located. A candidate for the negative parity state 4^- has been identified above the 200 ns isomer in ³²Al.

The multiplicities of energetic γ rays have been measured in coincidence with both evaporation residues and fission fragments in the reactions of ¹⁹F on ¹⁸¹Ta at projectile energies of 90, 100, 110, 120, 130, and 140 MeV. The experimental data have been compared with results of statistical calculations, which take into account dynamical fission hindrance. The damping of the fission partial width has been determined as a function of excitation energy in ²⁰⁰Pb.

The alpha particle emission from 90 to 140 MeV ¹⁹F+¹⁸¹Ta fusion-evaporation reactions has been studied. The comparisons of the experimental spectral shapes and multiplicities with statistical model predictions indicate a need to use an excitation energy dependent level-density parameter a=A/K in which K increases with excitation energy. This increase is more rapid than that in lower mass nuclei. The effect of this change in level density on the prescission multiplicities in fission is significant.

The light charged particles emitted from hot ⁴⁰Ca compound nuclei, populated at excitation energy E_x=94 MeV and 〈J〉=20.5ħ by the reaction 130 MeV ¹⁶O on ²⁴Mg, have been studied. Energy spectra of protons and alpha particles, measured in coincidence with evaporation residues and with a selection of multiple-alpha chains, have been compared with the predictions of Monte Carlo statistical model calculations. The comparison shows that the level density of hot nuclei with A≤40, needed to account for the measured quantities, is well predicted by the Fermi-gas model using a level-density parameter a_eff=A/8 MeV^-1 up to excitation energy 〈E_th/A〉∼1.7 MeV. In agreement with theory, light hot nuclei do not show the transition from a_eff=A/8 MeV^-1 to a_eff∼A/13 MeV^-1 evidenced in the A∼160 mass region for the same range of excitation energies E_th/A. The analysis of the alpha-particle spectra shows that the effects associated with the angular-momentum-induced deformations depend on the entrance channel characteristics and, in this case, are very small compared with those evidenced in the past for compound nuclei in the region A=50–70.