Search Results (20 total)

The existence of the ⁷H nuclear system was investigated via a one-proton transfer reaction with a ⁸He beam at 15.4A  MeV and a ¹²C gas target. The experimental setup was based on the active-target MAYA which allowed a complete reconstruction of the reaction kinematics. The existence of the ⁷H was confirmed with the identification of seven events where the system was formed with a resonance energy of 0.57_-0.21^+0.42  MeV above the ³H+4n threshold and a resonance width of 0.09_-0.06^+0.94  MeV. This study represents an unambiguous proof of the existence of the most neutron-proton unbalanced system presently found.

The breaking of the N=8 shell-model magic number in the ¹²Be ground state has been determined to include significant occupancy of the intruder d-wave orbital. This is in marked contrast with all other N=8 isotones, both more and less exotic than ¹²Be. The occupancies of the 0ℏω νp_1/2 orbital and the 1ℏω, νd_5/2 intruder orbital were deduced from a measurement of neutron removal from a high-energy ¹²Be beam leading to bound and unbound states in ¹¹Be.

Reactions induced by radioactive ^6,8He beams from the SPIRAL facility were studied on ^63,65Cu and ^188,190,192Os targets and compared to reactions with the stable ⁴He projectiles from the Mumbai Pelletron. Partial residue cross sections for fusion and neutron transfer obtained from the measured intensities of characteristic in-beam γ rays for the ⁶He+^63,65Cu systems are presented. Coincidence measurements of heavy reaction products, identified by their characteristic γ rays, with projectilelike charged particles, provide direct evidence for a large transfer cross section with Borromean nuclei ⁶He at 19.5 and 30 MeV and ⁸He at 27 MeV. Reaction cross sections were also obtained from measured elastic angular distributions for ^6,8He+Cu systems. Cross sections for fusion and direct reactions with ^4,6He beams on heavier targets of ^188,192Os at 30 MeV are also presented. The present work underlines the need to distinguish between various reaction mechanisms leading to the same products before drawing conclusions about the effect of weak binding on the fusion process. The feasibility of extracting small cross sections from inclusive in-beam γ-ray measurements for reaction studies near the Coulomb barrier with low intensity isotope separation on-line beams is highlighted.

The population and decay of excited states in ²⁴Mg have been investigated using the ¹²C(¹⁶O,¹²C¹²C)α reaction at beam energies of 135 and 160  MeV. This measurement permitted the decay spectrum to be measured up to ²⁴Mg excitation energies of 70  MeV. Resonance structures were observed up to an excitation energy of 49.4  MeV and spin J=18ℏ. The results are discussed in terms of the termination of a deformed ¹²C+¹²C cluster band.

Generalized isoscaling relationships which may permit one to relate the isotopic distributions of systems that may not be at the same temperature are proposed. The proposed relationships are applied to multifragmentation excitation functions for central Kr+Nb and Ar+Sc collisions.

Intermediate mass fragments (IMFS) (IMF: 3<~Z_IMF<~20) observed in coincidence with two correlated fission fragments following incomplete fusion in ¹²C+²³²Th at E/A=16 and 22 MeV are investigated. IMFs emitted prior to significant deformation of the fissioning system, as well as IMFs emitted near scission, are distinguished based upon their characteristic kinetic energy and angular distributions. The yield distributions of IMFs emitted near scission in these¹²C induced reactions are compared with near-scission IMF yields in spontaneous and low-energy ternary fission. Comparisons are made to both experimental fusion-evaporation data and theoretical predictions of a statistical model. The excitation energy dependence of relative IMF yields for both isotropic and near-scission emission is also presented. Our results for near-scission emission suggest that the production of IMFs near scission is inconsistent with a statistical emission mechanism in which emission barriers follow a standard Z dependence. Dynamical model calculations are used to investigate the role of dissipation, angular momentum, N/Z, and kinetic energy on the fragment formation near scission.

The neutron-rich ^66,68Ni have been produced at GANIL via interactions of a 65.9A MeV ⁷⁰Zn beam with a ⁵⁸Ni target. Their reduced transition probability B(E2;0₁⁺→2⁺) has been measured for the first time by Coulomb excitation in a ²⁰⁸Pb target at intermediate energy. The B(E2) value for ⁶⁸Ni₄₀ is unexpectedly small. An analysis in terms of large scale shell model calculations stresses the importance of proton core excitations to reproduce the B(E2) values and indicates the erosion of the N  =  40 harmonic-oscillator subshell by neutron-pair scattering.

Excited states have been observed for the first time in the neutron deficient nuclei ¹⁶⁶Os and ¹⁶⁴Os. The nuclei were produced using the reactions ¹⁰⁶Cd(⁶³Cu,p2n)¹⁶⁶Os, ¹¹²Sn(⁵⁸Ni,2p2n)¹⁶⁶Os, and ¹⁰⁶Cd(⁶⁰Ni,2n)¹⁶⁴Os at beam energies of 292, 286, and 257 MeV, respectively. The γ rays emitted by ¹⁶⁶Os and ¹⁶⁴Os were identified by correlating the associated recoil evaporation residues with their subsequent characteristic α decays. The deduced level schemes indicate that ¹⁶⁶Os and ¹⁶⁴Os continue the trend of decreasing deformation moving away from the N=104 midshell. The level energy systematics of the low-spin states are presented.

The nucleus ⁶⁹As was studied using the ⁴⁰Ca(³²S,3p)⁶⁹As reaction at a beam energy of 105 MeV. An extension of the band built on the g_9/2 orbital was observed to exhibit a band crossing at a rotational frequency of 0.511 MeV with an associated alignment of 7ħ. This alignment is interpreted as being due to a pair of g_9/2 neutrons. Total Routhian surface calculations have been carried out which confirm that the shape of this nucleus changes from oblate at low spin to a triaxial prolate shape at intermediate spin.

Ternary fission in the reaction ¹²C+²³²Th at E_lab  =  22A MeV reveals evidence for dynamical decay. The relative emission probability of intermediate-mass fragments (IMF: 3≤Z≤20) as a function of the initial excitation of the composite system is examined. While IMFs emitted pre-scission exhibit behavior consistent with statistical emission, near-scission IMFs, characterized by unique angular and energy distributions, clearly exhibit a behavior consistent with dynamical decay.

Ratios of the populations of ground and excited states of ⁴He, ⁵Li, and ⁸Be and double ratios constructed from the yields ^3,4He, ^6,7,8Li, ^11,12,13C isotopes were measured for central Kr+Nb collisions at E/A=35, 70, 100, and 120 MeV. These ratios were analyzed to estimate an apparent temperature for emission. Consistent and approximately constant apparent temperatures were obtained from the excited states of ⁴He, ⁵Li, and ⁸Be nuclei and from thermometers based upon the yields of carbon isotopes. In contrast, apparent temperatures obtained from thermometers based upon the ratios using helium isotopes increase monotonically with incident energy.

As a test for the time scale of neutron emission, two-neutron intensity interferometry was applied to the reaction ⁴⁰Ar+¹⁶⁵Ho at E/A=25 MeV. After appropriate corrections for neutron scattering, the correlation function showed a lifetime that was about a factor of two shorter than the lifetime predicted for the statistical decay of an equilibrated nucleus. Preequilibrium emission may explain some of this difference.

The resolution of reaction filters for intermediate-energy heavy-ion collisions based upon charged-particle and neutron multiplicity measurements with 4π coverage is compared for ¹¹²Sn+¹¹²Sn and ¹²⁴Sn+¹²⁴Sn collisions at E/A=40 MeV. The quality of event characterization is inferred from the azimuthal correlations of α particles emitted at intermediate rapidity and from the Z distributions of projectilelike fragments emitted at forward angles. For central and midimpact parameter collisions, the associated charged-particle multiplicity appears to provide better impact-parameter selection than the associated neutron multiplicity.

Sidewards directed fragment flow has been extracted for ⁸⁴Kr+¹⁹⁷Au collisions at E/A  =  200 MeV, using techniques that are free of reaction plane dispersion. The fragment flow per nucleon increases with mass, following a thermal or coalescencelike behavior, and attains roughly constant limiting values at 4≤A≤12. Comparisons of the impact parameter dependences of the measured coalescence-invariant proton flow to Boltzmann-Uehling-Uhlenbeck calculations clearly favor a momentum dependent nuclear mean field.

The production of intermediate mass fragments (IMF’s) from the four reactions 55A MeV ^124,136Xe + ^112,124Sn is studied with an experimental apparatus which is highly efficient for the detection of both charged particles and neutrons. The IMF’s are more localized in the midvelocity region than are the light charged particles, and the detected multiplicity of IMF’s depends linearly on the charge lost from the projectile and increases with the neutron excess of the system. Remnants of the projectile, with very little velocity reduction, are found for most of the reaction cross section. Isotopic and isobaric fragment yields in the projectile-velocity region indicate that charge-to-mass ratio neutralization is generally not achieved but is approached when little remains of the projectile. For all systems, the fragments found in the midvelocity region are substantially more neutron rich than those found in the velocity region dominated by the emission from the projectile. This observation can be accounted for if the midvelocity source (or sources) is either more neutron rich or smaller, with the same neutron-to-proton ratio, than the source with the velocity of the projectile. Taken together, the observations of this work suggest that the intermediate mass fragments are, to a large extent, formed by multiple neck rupture of the overlap material, a process which might enhance the neutron-to-proton ratio of the primary source material and/or limit the size of the sources. This scenario is reminiscent of low-energy ternary fission and one predicted by Boltzmann-Uehling-Uhlenbeck calculations. However, these calculations predict too much velocity damping of the projectile remnant. The calculations improve, in this regard, when the in-medium nucleon-nucleon cross sections and the cost of creating low density material are reduced. © 1996 The American Physical Society.

Multiplicities of intermediate-mass fragments (IMFs), neutrons, and charged particles were measured for ¹¹²Sn+¹¹²Sn and ¹²⁴Sn+¹²⁴Sn at E/A  =  40 MeV. Significantly different scalings of the mean IMF multiplicities with neutron and charged-particle multiplicities are observed for the two reactions. These differences can be qualitatively understood in terms of fragment emission from an expanding evaporating source for which the initial rates of cooling by neutron and light-charged-particle emission depend on the neutron and proton numbers of the source according to statistical expectations.

Fusion excitation functions for the reactions ^144,148,154Sm and ¹⁸⁶W + ¹⁶O and ¹⁴⁴Sm + ¹⁷O have been measured with high precision, both in the cross sections and the small energy intervals, thus allowing meaningful fusion barrier distributions to be extracted. In this representation it is clearly seen that the excitation functions are not smooth and featureless; each is unique and is shown to depend on the details of the structure of the interacting nuclei. The effects of excitation of the collective single phonon states in ¹⁴⁴Sm are evident. For the ¹⁷O projectile, the role of additional coupling to neutron stripping channels with positive Q values can be seen. As expected, the barrier distributions associated with ¹⁵⁴Sm and ¹⁸⁶W are dominated by deformation effects. However, the data appear to display sensitivity to additional couplings, even though they involve relatively weak inelastic and transfer channels.

Fusion excitation functions for ¹⁴⁴Sm + ¹⁶O and ¹⁷O have been measured to high precision. The extracted fusion barrier distributions show a double-peaked structure which is interpreted in terms of coupling to inelastic excitations of the target. The effect of the neutron stripping channel is evident in the reaction with ¹⁷O. These barrier distributions show clearly the signatures of specific inelastic and transfer channels.

The effects of deformation on sub-barrier fusion were first demonsrated almost fifteen years ago. Since that first analysis, it has become generally accepted that quadrupole deformation parameters deduced from fusion are significantly lower than those deduced from Coulomb excitation. Following a detailed analysis of the excitation function for ¹⁵⁴Sm+¹⁶O we find the excitation function is best fitted with deformation parameters which are consistent with Coulomb excitation data when a sufficiently complete model is used. The shape of the deduced barrier distribution strongly favors the inclusion of a positive hexadecapole moment.

The commonly held view that fusion excitation functions are featureless and do not provide a good test of models is challenged by high-precision measurements for the reaction ¹⁵⁴Sm+¹⁶O. The data yield a well-defined distribution of barrier heights using a recently proposed, novel analysis technique. The measured distribution is that expected from the quadrupole deformation of ¹⁵⁴Sm and models using significantly different distributions cannot reproduce the measured excitation functions.