Search Results (12 total)

The crystal structure and magnetic properties of UPdSi and UNiSi hydrides synthesized at T=923 K and hydrogen pressure p=130 bars were studied. Hydrogenation results in modification of the lattice symmetry from the orthorhombic (TiNiSi structure type) to hexagonal (ZrBeSi type) and in the increase of magnetic ordering temperatures in both compounds. UPdSiH_x orders antiferromagnetically at T_N=46 K compared to T_N=31 K for UPdSi. UNiSiH_x is a ferromagnet with T_C=98 K, whereas UNiSi is an antiferromagnet with T_N=85 K. The observed changes are attributed to the increased uranium-uranium spacing in the hydrides.

Experimental and theoretical x-ray magnetic circular dichroism (XMCD) studies of the intermetallic compounds UCoAl and UPtAl at the uranium M₄ and M₅ edges are reported. UPtAl is a 5f-electron ferromagnet, whereas UCoAl exhibits, at low temperatures and in a magnetic field of 0.65 T, a metamagnetic transition from paramagnetic to ferromagnetic ordering of U moments. Applying the XMCD sum rules to the experimental spectra, expectation values of the orbital and spin magnetic moments of the uranium 5f electrons were calculated. The results show that the orbital-to-spin moment ratio is of comparable value, μ_L/μ_S≈-2, for both compounds; however, the moments in UCoAl are strongly reduced with respect to UPtAl. Furthermore, the μ_L/μ_S ratio for UCoAl appears to be conserved both in the ferromagnetic and paramagnetic states, although the μ_S value in the former case is at least five times larger. The theoretical counterparts of the experimental data were obtained from the electronic structure calculated using the FLAPW method within the local spin density approximation. The calculated x-ray absorption and XMCD spectra show a satisfactory agreement with the experimental data and reproduce well the shape and structure of the spectral lines at the M₄ and M₅ edges.

The nuclear science community considers the construction of the Rare Isotope Accelerator (RIA) facility as a top priority. The RIA includes a 1.4 GV superconducting linac for production of 400 kW cw heavy ion beams. The initial acceleration of heavy ions delivered from an electron cyclotron resonance ion source can be effectively performed by a 57.5 MHz 4-m long room temperature RFQ. The principal specifications of the RFQ are (i) formation of extremely low longitudinal emittance, (ii) stable operation over a wide range of voltage for acceleration of various ion species needed for RIA operation, and (iii) simultaneous acceleration of two-charge states of uranium ions. cw operation of an accelerating structure leads to a number of requirements for the resonators such as high shunt impedance, efficient water cooling of all parts of the resonant cavity, mechanical stability together with precise alignment, reliable rf contacts, a stable operating mode, and fine tuning of the resonant frequency during operation. To satisfy these requirements a new resonant structure has been developed. This paper discusses the beam dynamics and electrodynamics design of the RFQ cavity, as well as some aspects of the mechanical design of the low-frequency cw RFQ.

A microscopic description of the excitation of isoscalar giant monopole resonance (ISGMR) and quadrupole resonance (ISGQR) in ²⁸Si, ⁴⁰Ca, ⁵⁸Ni, and ¹¹⁶Sn by 240 MeV bombarding energy α particles is provided based on self-consistent Hartree-Fock– (HF-) random-phase-approximation (RPA) approach and the distorted-wave Born approximation (DWBA). The folding model is used to obtain optical potentials from the HF ground-state density and a density dependent Gaussian nucleon-α interaction (V_αn). The parameters of V_αn are determined by fitting experimentally measured angular distributions for the case of elastic scattering. Angular distributions of inelastically scattered α particles for ISGMR and ISGQR excitations of the target nucleus are obtained using the folding model DWBA and both microscopic (RPA) and hydrodynamical (collective model) transition densities (found from HF ground state densities). A possible overestimation of the energy weighted sum rules and shifts of centroid energies due to the collective-model-based DWBA reaction description is reported.

We study the important effects of Fermi surface distortion on the isoscalar giant monopole resonance (ISGMR), within a Fermi-liquid drop model, by considering consistently the effects on nuclear incompressibility coefficients and the boundary conditions needed to determine the energy of the ISGMR. There is a significant difference between the static nuclear incompressibility K, derived as a stiffness coefficient with respect to an adiabatic change in the bulk density, and the dynamic one K^′ associated with the zero sound velocity. We show that the enhancement in the energy of the ISGMR, the lowest breathing mode, which is due to the renormalization of K into K^′ is strongly suppressed by the effects of the Fermi surface distortion on the boundary condition. This is not the case for higher breathing modes such as the overtone. We also discuss, in particular, the effects of the Fermi surface distortion on energy weighted sums for the monopole mode and on the constrained and the scaling incompressibility coefficients and their relation to the liquid drop one.

We present an analytical estimate for the upper limit of the shell correction δK_A to the compressibility coefficient K_A of finite nuclei. A simple model with a spherical harmonic oscillator potential was adopted and the self-consistency condition was taken into account. We find that the magnitude of the shell correction term δK_A is comparable to that associated with the current experimental uncertainties in determining the energy of the isoscalar giant monopole resonance.

The method of Albergo, Costa, Costanzo, and Rubbino to determine the temperature and free nucleon density of a disassembling hot nuclear source from fragment yields is modified to include the effects of radial collective flow, generated in the system from compression. We find that the freeze-out density increased substantially whereas the extracted temperature is modified only a little.

We derive a relation between the temperature T of the disassembling excited nucleus and the double ratio R of fragment yields within the macrocanonical description by adopting Maxwell-Boltzmann statistics and the Wigner-Seitz approximation for the Coulomb interaction between fragments and imposing only the condition of thermal equilibrium. Under the same assumptions, this relation between T and R, which differs from that of Albergo et al. by a Coulomb term, is also derived within the canonical description by considering realistic values for the multiplicities of fragments.

Observations of heavy remnants emitted at forward angles with high velocities and high associated particle multiplicities have been used to select central collisions of 35A MeV ⁶³Cu with ¹⁹⁷Au. The data indicate that these remnants, both fission fragments and evaporation residuelike products, result from the deexcitation of nuclei with A∼225–240 having excitation energies of ∼ 800–1300 MeV. Similar particle multiplicities are observed for both evaporative and fission decay channels. Modeling the results with hybrid codes which treat entrance channel dynamics followed by sequential statistical decay requires the inclusion of some delay in the fission channel to produce heavy remnants with mass A>~130, but the trend of the predicted velocities of these heavy remnants is different from that of the experiments. Calculations with a dynamic model based on the molecular dynamics approach have also been performed and lead to similar results. He and Li isotope yield ratios and the apparent temperatures derived from those ratios are similar to those previously reported for excited nuclei in this mass region. Temperatures derived from other yield ratios are also similar once a self-consistent treatment, taking into account population and decay of known excited states, is applied. The derived temperatures show little variation with excitation energy, suggesting that a limiting temperature may be reached at relatively low excitation energy, although the interpretation of this result and the determination of the actual initial value of this temperature is model dependent. Comments on the application of the double isotope yield ratio technique to extraction of the nuclear caloric curve are made.

The structure of the low-lying levels in the mirror nuclei ⁵⁷Ni and ⁵⁷Cu is described within the extended unified model. The problem of single-particle energies in ⁵⁶Ni is treated in detail. ‘‘Bare’’ single-particle energies are extracted from existing experimental data for the energy levels in ⁵⁷Ni and ⁵⁷Cu by carefully considering the influence of the coupling to excitations of the core. Important contributions arise, influencing especially the results on the spin-orbit splitting. The differences between the Coulomb energy shifts of various orbitals in ⁵⁶Ni are discussed and compared with those resulting from Hartree-Fock calculations carried out using a broad range of Skyrme interactions. The parameters of the Woods-Saxon potential reproducing these neutron ‘‘bare’’ single-particle energies and the charge root-mean-square radius of ⁵⁶Ni are extracted. It is demonstrated that the contributions associated with the Thomas-Ehrman effect and the electromagnetic spin-orbit interaction are important and large enough to account for the differences between the Coulomb energy shifts of the single-particle levels in ⁵⁶Ni. © 1996 The American Physical Society.

We modify the method of Albergo et al. for determining the temperature of an excited nucleus from double ratios of isotope yields and present a statistical model which accounts for the population and decay of excited states of the emitted fragments. Nuclear temperatures are extracted using experimental ratios of isotopic yields of fragments from helium through carbon for the reactions ⁴⁰Ca + ⁵⁸Ni, ⁴⁰Ar + ⁵⁸Ni, ⁴⁰Ca + ⁵⁸Fe, and ⁴⁰Ar + ⁵⁸Fe at 33 MeV/nucleon projectile energy. Using the model we obtain consistent values for the temperature from various isotope combinations within the experimental error when accounting for the population and decay of the excited fragments.

The Kr isotopes with A=72 to 84 are investigated within the interacting boson model (IBM-2) using two different approaches. In the first investigation the single-particle energies of the neutron and proton bosons are assumed equal, ε=ε_ν=ε_π, whereas the second investigation is based on their being different (ε_ν≠ε_π). The results of these calculations are compared with the experimental data and previous theoretical investigations. We also attempt to determine the shape transition in these nuclei.