Search Results (35 total)

The low-lying yrast states in the ⁵¹Ca and ⁵²Sc nuclei were investigated to obtain information on the evolution of the p_1/2 and f_5/2 neutron single-particle orbitals in neutron-rich nuclei near proton number Z=20. Level structures associated with neutron excitations into these two orbitals and with proton excitations across the Z=20 shell gap were identified. Shell-model calculations with the recently proposed GXPF1A interaction account reasonably well for the fp-shell states. The energy separation between the νp_1/2 and νf_5/2 orbitals in the Ca isotopes appears to be overestimated by the GXPF1A Hamiltonian.

The positive-parity yrast states in the ⁸⁹Rb, ⁹²Y, and ⁹³Y nuclei were studied using γ-ray spectroscopy with heavy-ion induced reactions. In the multinucleon transfer reactions ²⁰⁸Pb+⁹⁰Zr (590 MeV) and ²³⁸U+⁸²Se (505 MeV), several γ-ray transitions were identified in these nuclei by means of coincidences between recoiling ions identified with the PRISMA spectrometer and γ rays detected with the CLARA γ-ray array in thin target experiments. Level schemes were subsequently determined from triple-γ coincidences recorded with the GASP array in a thick target experiment, in the reactions produced by a 470 MeV ⁸²Se beam with a ¹⁹²Os target. The observed level schemes are compared to shell-model calculations.

The neutron-rich Fe isotopes from A=61 to 66 were studied through multinucleon transfer reactions by bombarding a ²³⁸U target with a 400 MeV ⁶⁴Ni beam. Unambiguous identification of prompt γ rays belonging to each nucleus was achieved using coincidence relationships with the ions detected in a high-acceptance magnetic spectrometer. The new data extend our knowledge of the level structure of Fe isotopes, which is discussed in terms of the systematics of the region and compared with large-scale shell-model calculations.

Multinucleon transfer reactions in ⁴⁰Ca+⁹⁶Zr and ⁹⁰Zr+²⁰⁸Pb have been measured at energies close to the Coulomb barrier in a high-resolution γ-particle coincidence experiment. The large-solid-angle magnetic spectrometer PRISMA coupled to the CLARA γ array has been employed. Trajectory reconstruction has been applied for the complete identification of transfer products. Mass and charge yields, total kinetic energy losses, γ transitions of the binary reaction partners, and comparison of data with semiclassical calculations are reported. Specific transitions in ⁹⁵Zr populated in one-particle transfer channels are discussed in terms of particle-phonon couplings. The γ decays from states in ⁴²Ca in the excitation energy region expected from pairing vibrations are also observed.

A precise fusion excitation function has been measured for ⁴⁰Ca+⁹⁴Zr at near- and sub-barrier energies, and the fusion barrier distribution has been extracted. Comparing with the existing data for ^40,48Ca+^90,96Zr shows that couplings to inelastic excitations determine the fusion cross sections and the shape of the barrier distributions near the barrier. At lower energies, the two systems possessing neutron-transfer channels with large positive Q-values (⁴⁰Ca+^94,96Zr) have remarkably similar cross sections, and both show a large enhancement with respect to the other systems and to coupled-channels calculations including up to three quadrupole and octupole phonons in the Zr targets.

Mass-energy and angular distributions of fission fragments for the ⁴⁸Ca+^144,154Sm→^192,202Pb, ⁴⁰Ca+¹⁵⁴Sm→¹⁹⁴Pb reactions have been measured. Fusion suppression and the presence of quasifission at energies near and below the Coulomb barrier have been observed for the reactions with the deformed target ¹⁵⁴Sm. In the case of the spherical ¹⁴⁴Sm target no evidence of quasifission has been found. Quasifission cross sections have been extracted from total fission-like events by analysis of their mass and angular distributions.

The excited states of the neutron-rich nucleus ₁₅³⁷P₂₂ have been populated in grazing reactions, using a beam of ³⁶S ions (at 215 MeV) delivered onto a thin ²⁰⁸Pb target. Emitted γ rays from excited projectile-like nuclei were detected using the CLARA array of 25 escape-suppressed Ge clover detectors in coincidence with reaction products detected and identified with the PRISMA magnetic spectrometer. A level scheme is presented for ³⁷P together with proposed spin assignments. The level structure of ³⁷P is discussed within the context of shell-model calculations by using an improved sdpf effective interaction.

Excited states of N=22 ³⁶Si, populated in deep-inelastic processes produced by the interaction of a 215 MeV beam of ³⁶S ions with a ²⁰⁸Pb target, were studied in the present work. γ rays from the binary fragments detected using CLARA, an array of 25 Ge Clover detectors, were measured in coincidence with projectile-like fragments detected by PRISMA, a large solid angle magnetic spectrometer. Two new γ-ray photopeaks at energies of 1442 and 842 keV were observed and tentatively assigned to the 4⁺→2⁺ and 6⁺→4⁺ transitions, respectively. The systematics of the level structures of N=22 isotones are presented, and a comparison is made of the behavior of Si, Mg, and S isotopes. The level structure of ³⁶Si is also compared with the results of sdpf shell model calculations.

Fusion-evaporation cross sections were measured in the two systems ⁴⁸Ca+^90,96Zr in an energy range from well below to well above the Coulomb barrier. The sub-barrier fusion of ⁴⁸Ca+⁹⁰Zr is reproduced by coupled-channels calculations including the lowest quadrupole and octupole vibrations of ⁹⁰Zr, and using a Woods-Saxon potential with a standard diffuseness parameter a = 0.68 fm. However, the fusion cross sections are overestimated above the barrier. The low-energy slope of the excitation function for ⁴⁸Ca+⁹⁶Zr is steeper. This implies a larger diffuseness parameter a = 0.85 fm. Fusion cross sections are well fit in the whole energy range, and the effect of the strong octupole vibration in ⁹⁶Zr is predominant. The extracted fusion barrier distributions are reasonably well reproduced by calculations for both systems. A comparison with previous data for ⁴⁰Ca+^90,96Zr is made in an attempt to clarify the role of transfer couplings in sub-barrier fusion.

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.

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.

Multinucleon transfer reactions in ⁴⁰Ca+²⁰⁸Pb have been studied at bombarding energies close to the Coulomb barrier. Projectilelike fragments have been identified in nuclear mass and charge with a time-of-flight spectrometer. Angular and total kinetic energy loss distributions and inclusive cross sections have been compared with those of semiclassical models. The analysis shows that a successive transfer mechanism of single nucleons does not account for the data, and a direct nucleon pair transfer has to be included in the description. Nucleon evaporation effects are taken into account.

Excitation functions for ^208–211Fr isotopes produced in the ¹⁸O+¹⁹⁷Au fusion-evaporation reaction have been measured at E_lab=75–130 MeV via characteristic α decays by means of an electrostatic deflector and a semiconductor detector. Data have been compared with calculations giving barrier-passing (capture) cross sections and probabilities of the compound nucleus decay into different channels according to the standard statistical model.

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.

Light and heavy reaction products in the ⁵⁸Ni+²⁰⁸Pb system were measured at E_lab=328.4 MeV. Light products were identified with a time-of-flight magnetic spectrometer and heavy fragments with a multiwire parallel plate detector. From the kinematic coincidence the survival probability of the heavy fragments against fission was derived. Data are well described by semiclassical model calculations including, in addition to all one particle transfers, a proton pair-transfer mode with a macroscopic form factor.

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.