Angular Correlation Experiments Research Articles

Quadrupole moments of Cd and Zn nuclei: When solid-state, molecular, atomic, and nuclear theory meet

The nuclear quadrupole moment of the 5/2+ isomeric state of 111Cd, of particular importance to the interpretation of Perturbed Angular Correlation experiments in condensed matter, was determined by combining existing PAC data with high-level ab initio (CCSD(T)) calculations for Cd-dimethyl and hybrid density functional theory for metallic Cd. A revised value of is found, much reduced from earlier estimates. Using the new result together with the values for other Cd isotopes from atomic data, also recently revised, the trend of Q for the 11/2− states in Cd is in perfect agreement with new nuclear covariant density functional theory calculations. Similar theoretical work for metallic Zn and the ZnS molecule, combined with atomic calculations, also results in an equivalent reduction for the reference value of the 67Zn 5/2− ground-state quadrupole moment to .

The influence of coordinated defects on inhomogeneous broadening in cubic lattices

The joint probability distribution function (JPDF) of electric field gradient (EFG) tensor components in cubic materials is dominated by coordinated pairings of defects in shells near probe nuclei. The contributions from these inner shell combinations and their surrounding structures contain the essential physics that determine the PAC-relevant quantities derived from them. The JPDF can be used to predict the nature of inhomogeneous broadening (IHB) in perturbed angular correlation (PAC) experiments by modeling the G2 spectrum and finding expectation values for Vzz and η. The ease with which this can be done depends upon the representation of the JPDF. Expanding on an earlier work by Czjzek et al. (Hyperfine Interact. 14, 189–194, 1983), Evenson et al. (Hyperfine Interact. 237, 119, 2016) provide a set of coordinates constructed from the EFG tensor invariants they named W1 and W2. Using this parameterization, the JPDF in cubic structures was constructed using a point charge model in which a single trapped defect (TD) is the nearest neighbor to a probe nucleus. Individual defects on nearby lattice sites pair with the TD to provide a locus of points in the W1−W2 plane around which an amorphous-like distribution of probability density grows. Interestingly, however, marginal, separable PDFs appear adequate to model IHB relevant cases. We present cases from simulations in cubic materials illustrating the importance of these near-shell coordinations.

Topologically appropriate coordinates for (V z z , η) joint probability distributions

Inhomogeneous broadening (IHB) of hyperfine interactions in materials arises from a distribution of electric field gradients (EFGs) due to randomly distributed defects contributing non-uniformly to the EFG at probe sites. Hyperfine experiments reflect the inhomogeneous distribution of defects through the joint probability distribution function (PDF) of Vzz and η determined by the defect concentration, crystal structure, and defect sites in the crystal. Czjzek showed how to choose coordinates in the (Vzz, η) plane that are consistent with the physical constraints and ordering convention for these EFG parameters. Here we show how to transform to a new set of coordinates that decreases the distortion inherent in Czjzek’s representation. These new coordinates allow one to express the joint PDF for random distributions of defects in a form reasonably approximated by the product of two independent marginal distributions. This paper focuses on these topologically appropriate coordinates, with simple examples drawn from Czjzek’s work and from our simulations of point defects in cubic lattices as well as random amorphous distributions of defects. Detailed simulations have been carried out for IHB in cubic structures and point charge models relevant to perturbed angular correlation (PAC) experiments.

Nuclear structure of 96,98Mo: Shape coexistence and mixed-symmetry states

Excited low-spin states in 96Mo and 98Mo have been studied in γγ angular correlation experiments in order to determine spins and multipole mixing ratios. Furthermore, from a Doppler lineshape analysis effective lifetimes τ in the femtosecond range were obtained. The experimental data show a complex spectrum due to configuration mixing, which is confirmed by Interacting Boson Model calculations based on a Skyrme energy density functional. The M1-transition strengths of transitions depopulating excited 2+ states to the first 2+ state are discussed in terms of the proton–neutron mixed symmetry.

First-principles study of Cd impurities localized at and near the (0 0 1) α-Al2O3 surface

A combination of two first-principles electronic structure calculation methods in the framework of density-functional theory was applied to investigate the (001) α-Al2O3 surface reconstruction and its structural, electronic, magnetic, and hyperfine properties when doped with Cd impurities at different depths from the surface. The SIESTA approach was used to obtain the equilibrium positions of all atoms and the Full-Potential Augmented Plane Wave plus local orbital (FP-APW+lo) method was employed in order to obtain the electronic structure at these equilibrium positions and all the other physical properties. For the most stable (001) α-Al2O3 surface, we have demonstrated that the inclusion of the Cd atom at substitutional Al sites at and near the surface produces a ground state magnetic behavior. The largest principal component V33 of the electric-field-gradient (EFG) tensor at the Cd atom localized just above the α-Al2O3 terrace showed the same [001] orientation and a dominating p-character as Cd does when it is localized at bulk α-Al2O3, but exhibits an anomalous V33 magnitude four times larger than its value in bulk. Just below the surface, the non symmetric structural relaxation around the Cd impurity is responsible for the strong change in the asymmetry, magnitude, and orientation of the EFG tensor. The changes in the hyperfine properties have been correlated with the modifications observed on the electronic charge density at the different Cd sites and on the p-states of the Cd-projected partial density of states. Accordingly, the significant differences on the hyperfine parameters showed for different depths suggest that 111Cd probe-atoms used in Perturbed γ–γ Angular Correlation experiments could be used for evaluating geometrical and electronic distortions, particularly for positions quite close to the reconstructed surface, as well as contributing to studies of growth, adsorption, and diffusion of atoms in oxide surfaces and interfaces.

Level-scheme investigation of 33S

An angular correlation experiment was carried out for 33 S at Laboratori Nazionali di Legnaro with the gamma-ray detector array GASP. The reaction used was 24Mg(14N,α p)33S at a beam energy of 40MeV. An analysis of DCO ratios and triple gamma coincidences was performed. So far, a new level depopulated by 3 γ -ray transitions has been found and its spin was determined. The work for further extension of the level scheme is in progress.

The structure of 193Au within the Interacting Boson Fermion Model

Abstract A γγ angular correlation experiment investigating the nucleus 193Au is presented. In this work the level scheme of 193Au is extended by new level information on spins, multipolarities and newly observed states. The new results are compared with theoretical predictions from a general Interacting Boson Fermion Model (IBFM) calculation for the positive-parity states. The experimental data is in good agreement with an IBFM calculation using all proton orbitals between the shell closures at Z = 50 and Z = 126 . As a dominant contribution of the d 3 / 2 orbital to the wave function of the lowest excited states is observed, a truncated model of the IBFM using a Bose–Fermi symmetry is applied to the describe 193Au. Using the parameters of a fit performed for 193Au, the level scheme of 192Pt, the supersymmetric partner of 193Au, is predicted but shows a too small boson seniority splitting. We obtained a common fit by including states observed in 192Pt. With the new parameters a supersymmetric description of both nuclei is established.

Recoil Induced Room Temperature Stable Frenkel Pairs in a-Hafnium Upon Thermal Neutron Capture

Ultrapure hafnium metal (110 ppm zirconium) was neutron activated with a thermal neutron flux of 6:6 · 1012 cm-2s-1 in order to obtain 181Hf for subsequent time differential perturbed angular correlation (TDPAC) experiments using the nuclear probe 181Hf(β-) 181Ta. Apart from the expected nuclear quadrupole interaction (NQI) signal for a hexagonal close-packed (hcp) metal, three further discrete NQIs were observed with a few percent fraction each. The TDPAC spectra were recorded for up to 11 half lives with extreme statistical accuracy. The fitted parameters vary slightly within the temperature range between 248 K and 373 K. The signals corresponding to the three additional sites completely disappear after ‘annealing’ at 453 K for one minute. Based on the symmetry of the additional NQIs and their temperature dependencies, they are tentatively attributed to Frenkel pairs produced by recoil due to the emission of a prompt 5:694 MeV -ray following thermal neutron capture and reported by the nuclear probe in three different positions. These Frenkel pairs are stable up to at least 373 K.

Evidence for shape coexistence in98Mo

A $\ensuremath{\gamma}\ensuremath{\gamma}$ angular-correlation experiment has been performed to investigate the low-energy states of the nucleus ${}^{98}$Mo. The new data, including spin assignments, multipole mixing ratios, and lifetimes reveal evidence for shape coexistence and mixing in ${}^{98}$Mo, arising from a proton intruder configuration. This result is reproduced by a theoretical calculation within the proton-neutron interacting boson model with configuration mixing, based on microscopic energy density functional theory. The microscopic calculation indicates the importance of the proton particle-hole excitation across the $Z=40$ subshell closure and the subsequent mixing between spherical vibrational and the $\ensuremath{\gamma}$-soft equilibrium shapes in ${}^{98}$Mo.

Study of PRIMAVERA steel samples by positron annihilation spectroscopy technique II – Lifetime measurements

In the present article, a positron annihilation lifetime technique was used for the study of VVER-440/230 weld materials, manufactured in the frame of the international PRIMAVERA project on microstructural investigation of the irradiated WWER-440 reactor pressure vessel steel. The present results complement our previous report of positron angular correlation experiments and provide in-depth characterization of vacancy type defects behavior under irradiation and thermal treatment. The results give new insight into the previously published atom probe tomography and angular correlation of annihilation radiation studies. The measurements do not show any association of phosphorus or its segregation to the open volume defects investigated by positron annihilation spectroscopy. The embrittlement effects related to the phosphorus seem to be effectively annealed-out during 475 °C thermal treatment and the post annealing microstructure and mechanical properties of the material are consequently affected mostly by agglomerations of vacancy clusters coarsened during thermal treatment.

γγangular-correlation analysis of200Hg after cold-neutron capture

We report on a $\ensuremath{\gamma}\ensuremath{\gamma}$ angular-correlation experiment investigating ${}^{200}$Hg after cold-neutron capture. The experiment was performed using eight high-purity germanium detectors mounted on an array installed at the PF1B neutron beam line of the research reactor of the Institut Laue-Langevin in Grenoble, France. The geometry of the array allows $\ensuremath{\gamma}\ensuremath{\gamma}$ angular-correlation analyses that can be used for the determination of level spins and multipole mixing ratios in ${}^{200}$Hg. We present multipole mixing ratios for secondary $\ensuremath{\gamma}$ rays and investigate the nature of the ${}^{200}$Hg neutron-capture state by analyzing the observed primary $\ensuremath{\gamma}$ rays of this nucleus.

Investigation of Ni/Ce–ZrO 2 catalysts in the autothermal reforming of methane

Nickel catalysts supported on α-Al 2O 3, CeO 2, ZrO 2 and Ce–ZrO 2 were investigated in the autothermal reforming of methane. Ce–ZrO 2 supports formed a solid solution and presented better oxygen storage capacity per unit of mass of Ce when compared to CeO 2. Diffuse reflectance UV–Vis spectroscopy spectra and temperature-programmed reduction profiles, showed the presence of Ni 2+ in tetrahedral and octahedral geometries for catalysts supported on mixed oxides. Temperature-programmed surface reaction experiments showed that the catalytic activity for autothermal reforming is proportional to the amount of metallic sites on the surface. However, when operating under severe coking conditions, catalysts with a higher oxygen storage capacity were more stable in the autothermal reforming of methane. Time-differential angular correlation experiments provided an atomic view on how the mobility of oxygen on CeZrO 2 is enhanced by the presence of Ni, which increases the stability of the catalyst.

Relating PAC damping to EFG fluctuation rates through the PAC relaxation peak

A perturbed angular correlation (PAC) experiment that measures dynamic damping also needs information about the fundamental quadrupole frequency to relate the damping as a function of temperature to the EFG fluctuation rate. When the experiment is unable to access slow electric field gradient (EFG) fluctuations that show the fundamental quadrupole frequency directly, one needs additional information to determine the hyperfine field parameters and thereby the connection between observed damping and EFG fluctuation rates. One way to solve this problem is to estimate the hyperfine parameters from the fluctuation rate for maximum damping (i.e. at the relaxation peak) or from the rate of maximum damping. This work relates both the maximum damping rate and the fluctuation rate at the relaxation peak to EFG magnitudes (or quadrupole frequencies) for five dynamic N-state symmetric models of fluctuating EFGs.

Impurity Centers in Oxides Investigated by γ-γ Perturbed Angular Correlation Spectroscopy and <i>Ab Initio</i> Calculations

In this chapter Perturbed Angular Correlation (PAC) experiments on binary oxides are described. These experiments provide local-scale fingerprints about the formation, identification, and lattice environment of defect complexes at the PAC probe site. The potential of the PAC observations in conjunction with ab initio calculations is shown. Measurements of the electric-field gradient at impurity sites using 111Cd and 181Ta probes are reviewed. Special attention is paid to oxides with the bixbyite structure. The case of In2O3 is particularly analyzed. Results obtained with HfO2, in form of coarse grain or nano particles, are described. The potential results that can be obtained from Density Functional Theory ab initio calculations in doped systems are shown describing the main results observed in many impurity-host systems.

Gfactors of the low-lying states inPd106: Examination of the vibrational character ofPd106

The transient field (TF) technique in inverse kinematics was used to measure the g factors of the low lying 2{sub 1}{sup +}, 4{sub 1}{sup +}, and 2{sub 2}{sup +} states in {sup 106}Pd. The g factor of the 4{sub 1}{sup +} state was determined and the g(2{sub 1}{sup +}) and g(2{sub 2}{sup +}) factors were remeasured. The values of g(2{sub 1}{sup +}) and g(2{sub 2}{sup +}) had been determined earlier in integral perturbed angular correlation (IPAC) experiments and the value of the former served to calibrate the TF. The three g factors, g(2{sub 1}{sup +}), g(2{sub 2}{sup +}), and g(4{sub 1}{sup +}), agree with each other and with the collective Z/A value. The uncertainties in the g(2{sub 2}{sup +}) and g(4{sub 1}{sup +}) factors remain fairly large in spite of long measuring times because of the weak excitation of the two-phonon states. The lifetimes of the 2{sub 2}{sup +} and 4{sub 1}{sup +} states were newly determined from line-shape fits.

Hg196and the “magical quartet” of the extendedUν(6/12)⊗Uπ(6/4) supersymmetry

We present the results of a $\ensuremath{\gamma}\ensuremath{\gamma}$ angular correlation experiment investigating the nucleus $^{196}\mathrm{Hg}$ and compare these with a theoretical description of $^{196}\mathrm{Hg}$ within the ${\mathrm{U}}_{\ensuremath{\nu}}(6/12)\ensuremath{\bigotimes}$${U}_{\ensuremath{\pi}}(6/4)$ extended supersymmetry. To populate excited $^{196}\mathrm{Hg}$ states, we used the Cologne FN Tandem accelerator inducing the reaction $^{194}\mathrm{Pt}$$(\ensuremath{\alpha},2n)^{196}\mathrm{Hg}$ and analyzed the $\ensuremath{\gamma}$ decays of levels up to an excitation energy of $2.4$ MeV with the HORUS cube spectrometer. The new results on this mercury isotope allow a comparison between the experimental data and the supersymmetrical predictions and show good agreement. This way we can add $^{196}\mathrm{Hg}$ as a fifth supermultiplet member to the so-called magical quartet consisting of $^{194,195}\mathrm{Pt}$ and $^{195,196}\mathrm{Au}$.

First principles calculations of hyperfine parameters on the Ca manganite with substitutional Cd-modeling of a PAC experiment

The APW + lo (augmented plane waves + local orbitals) method of density functional theory, as implemented in the Wien2k code, is applied to calculate the electric field gradient of manganites. We report the first principles calculations on CaMnO 3 , and modeling of a perturbed angular correlation experiment with implanted Cd 111 m isotope. To model the experiment we additionally calculate in Ca 1 - x Cd x MnO 3 , where we substitute Cd at the Ca site. Increasing Cd dilution is done with the use of supercells. We find that the experimental CaMnO 3 low-temperature value V zz ≈ 6 × 10 - 21 V / m 2 is reproduced, when optimizing internal parameters. The analysis of the EFG tensor at the Ca atoms, with different electric field gradients at inequivalent positions, reveals that the convergence of the calculations is obtained.

Two-fermion–four-boson description ofHg198within theUν(6/12)⊗Uπ(6/4)extended nuclear structure supersymmetry

Using the U{sub {nu}}(6/12) x U{sub {pi}}(6/4) extended supersymmetry, we constructed the energy spectrum and electromagnetic transition properties of the supermultiplet member {sup 198}Hg with two proton fermions coupled to a neutron boson core. Consistency between the supersymmetric interacting boson fermion fermion approximation (IBFFA) description and the F-spin symmetric interacting boson approximation (IBA-2) description is shown for this two-fermion-N-boson multiplet member. The data of a {gamma}{gamma} angular correlation experiment using the HORUS cube {gamma}-ray spectrometer--determining new multipole mixing ratios, level spins, {gamma} transitions, and energy states--shows quite a good agreement, also for the low-energy part of the spectrum, when comparing theoretical predictions and experimental data. This is contrary to the usual assumption that a two-fermion-N-boson constellation should describe just the excited two-quasiparticle states.

Lattice location study of implanted In in Ge

We report on emission channeling experiments to determine the lattice location and the thermal stability of implanted I111n atoms in Ge. The majority of the In atoms was found on the substitutional site, which is a thermally stable site at least up to 500 °C. We also found strong indication that directly after implantation, a fraction of the implanted I111n atoms occupies the bond-centered (BC) site. This fraction disappears after annealing at 300 °C. From comparison with ab initio calculations, electrical studies, and perturbed angular correlation experiments, the In atoms on the BC site can be related to In-vacancy and In-self-interstitial defect complexes. The activation energy for dissociation of this BC related defect was found to be below 1.6 eV.

Field-cooling effect in biquadratically interlayer-coupledFe∕Cr∕Fetrilayers

Received 19 July 2006; revised manuscript received 20 December 2006; published 5 February 2007We report on the modified magnetic properties of an epitaxial Fe/Cr/Fe trilayer when it is cooled in anapplied magnetic field. Magnetization measurements are carried out to investigate the dependence of thefield-cooling effect on the magnetic field strength and on the temperature. Perturbed angular correlation ex-periments are performed to study the microscopic origin of the field-cooling effect.DOI: 10.1103/PhysRevB.75.060402 PACS number s : 75.70.Cn