Angular Correlation Studies Research Articles

Identified two-hadron correlations at STAR using Λ, Λ̄, and K0 Short triggers with charged hadrons in Au + Au collisions at $\sqrt{s_{NN}}$ =200 GeV

Here we present initial studies of two-dimensional angular correlations of Λ, Λ, and K0Short triggers with unidentified charged hadrons in Au+Au collisions at \(\sqrt{s_{NN}}\) =200 GeV measured by STAR. Distributions of pseudo-rapidity difference, Δη, and azimuthal separation, Δφ, are constructed containing structures observed in unidentified hadron correlations, including a jet peak at small Δη-Δφ accompanied by a flow-like ridge extended over Δη. These features are studied as a function of centrality via integrated yields and fitting to projections onto Δη and Δφ axes. Yields are found to be consistent with unidentified correlation analyses, and no clear distinction is observed between the three species.

Atomic scale study of oxidation of hafnium: Formation of hafnium core and oxide shell

Perturbed angular correlation studies on hafnium foil subjected to annealing in oxygen atmosphere show that there exists three distinct Hf sites which are identified as probe atoms associated with hafnium metallic clusters, hafnium oxide, and oxygen deficient oxide zones. Kinetics of oxidation has been studied. Variations of quadrupole parameters with annealing treatments show that there is a formation of hafnium core surrounded by a shell of hafnium oxide. Inner hafnium core and outer oxide shell are identified to be separated by oxygen deficient oxide zones. Results suggest that annealing at elevated temperatures leads to a reduction in the size of hafnium cores to submicron, surrounded predominantly by well grown hafnium oxide. Occurrence of nanometallic hafnium clusters in the hafnium oxide matrix has been identified by low frequency laser Raman scattering studies.

PAC characterization of Gd and Y doped nanostructured zirconia solid solutions

A perturbed angular correlation (PAC) study as a function of temperature has been carried out on spray pyrolysis-derived powders and compacts of 2.5 mol% Y 2O 3–ZrO 2 and 2 mol% Gd 2O 3–ZrO 2 nanostructured tetragonal zirconias. The powders undergo the ordinary thermal transformation between the two known defective t′- and regular t-tetragonal forms and also a partial and irreversible change to an ordered cubic configuration. The dynamical nature of the t′-form leads to an activation energy of about 0.15 eV for the oxygen vacancies movement. The as-obtained compacts do not exhibit any known cubic nanostructure but some additional contributions. In both of them a hyperfine component assigned to the orthorhombic phase is determined. In the smaller cation Y doped ceramic a small amount of monoclinic phase reflects an incomplete stabilization.

Clarification of the low-lying states of 9Be

A newly developed technique for dealing with three-body decays of broad isolated levels is extended to deal with the broad, overlapping levels found at 2–9 MeV excitation energy in 9Be. The levels are populated through beta-decay of 9Li. The method gives firm evidence for the existence of several levels. Angular correlation studies allow spin values to be assigned.

Lattice location and perturbed angular correlation studies of implanted Ag in SrTiO3

Lattice site location and local environment characterization of implanted 111Ag in SrTiO3 by means of the emission channeling (EC) and γ–γ perturbed angular correlation (PAC) techniques are reported. The angular distribution of β− particles emitted from the 111Ag decay was monitored with a position-sensitive detector as a function of annealing temperature up to 900°C. In the as-implanted state Ag occupies several lattice sites in SrTiO3. Upon annealing, near-Sr and near-Ti occupancies increased to 64% and 18%, while an octahedral interstitial fraction vanished. Ag atoms on near-Sr and near-Ti sites are still displaced by ∼0.2Å from ideal cubic positions. Subsequent PAC measurements confirmed that ∼20% of 111Ag atoms are in specific sites of non-cubic environment, characterized by a unique electrical-field-gradient (EFG), while ∼80% were subject to a wide EFG distribution.

Perturbed angular correlation study of radiation-induced defects in Rh metal

Radiation-induced defects are studied in cubic rhodium metal, using the local probe technique ‘Time differential perturbed angular correlation’ (TDPAC) at liquid N2 temperature. Isochronal annealing was done at 300, 1073 and 1473 K temperatures. The irradiated sample showed two quadrupole interaction frequencies at 1150 and 93 MHz. The low frequency disappeared at room-temperature annealing, which was assigned to In trapped at a vacancy, whereas the higher frequency remained up to high temperatures and was attributed to In trapped at Rh–C complexes in the Rh matrix.

The perturbed angular correlation studies of Ag–Zr compounds with 111Cd probes

The electric quadrupole hyperfine interactions for 111Cd probes in polycrystalline ZrAg and Zr 2Ag were measured in the wide temperature range. For both samples the two unique quadrupole frequencies ν Q with asymmetry parameters η = 0 were observed indicating that 111In/ 111Cd probe impurities substitute both—Zr and Ag sites in B11 structure of ZrAg and in C11 b structure of Zr 2Ag phase. The results are compared with our previous experiments on isomorphous Hf–Ag compounds.

Coincidence Velocity Imaging Apparatus for Study of Angular Correlations between Photoelectrons and Photofragments

A new apparatus for coincidence velocity mapping has been developed for studies on the inner-shell photoionization dynamics of molecules. It enables velocity imaging of electrons and ions simultaneously, and maintains time focusing. The characteristic feature of the apparatus is its applicability to a wide range of photoelectron kinetic energy from a few to 100 eV. To demonstrate the performance of the new apparatus, photoelectron angular distributions from Ne atoms and randomly oriented NO molecules, photoion angular distributions from CO molecules, and photoelectron angular distributions from fixed-in-space CO and NO molecules, have been examined. In the data processing procedures, a new peeling method has been developed to construct a three-dimensional photoelectron angular distribution, which is not cylindrically symmetric, from a two-dimensional one.

Local structure reconstruction in hydrogenated amorphous silicon from angular correlation and synchrotron diffraction studies

Hydrogenated amorphous silicon (a-Si:H) is a widely used thin film semiconductor material which is still incompletely understood. It is generally assumed to form a continuous random network, with a high concentration of coordination defects (dangling bonds), which are hydrogen terminated. Neither the exact nature of these sites nor the degree of medium range order has been fully determined. In this paper, we present the first results for the local structure, from a combined study using angular correlation of positron annihilation radiation (ACAR) and synchrotron radiation diffraction. Reciprocal space information is obtained directly, for the mesoscale structure and the local defect structure, from the orientation dependent diffraction and 2D-ACAR patterns, respectively. Furthermore, inversion of both patterns yields a comparison of real space information through maps of the silicon–silicon pair correlation function and the electron–positron autocorrelation function B 2 γ ( r). From this information, it is possible to identify the dominant structural defect as a vacancy-size dangling bond cluster, around which the network strain is fully relaxed.

Nuclear Spin Relaxation in Some Non-viscous Media Using the 181Hf Probe

Perturbed angular correlation (PAC) studies have been performed in liquid mercury, HCl media of different acid strengths, H2O and H3PO4 using a 181Hf probe. It is found that, in case of H2O, interaction between the nucleus and its excited atom following the β− decay of 181Hf is a possible mechanism for the nuclear spin relaxation in this environment. In the liquid mercury and different acid media, however, the perturbations are found to arise due to molecular fluctuations.

Magnetic structure of NdScGe

We have determined the magnetic structure of NdScGe (tetragonal I4∕mmm) by high-resolution neutron powder diffraction, ac-susceptibility and magnetometry. The magnetic ordering temperature is 194(2)K and the magnetic structure is characterized by ferromagnetic order of the Nd sublattice, lying in or close to the tetragonal basal plane. The Nd moment at 4K is 3.2(2)μB and, in contrast to the claims of a recent Perturbed Angular Correlation study, we find no evidence for a lattice softening effect suggested to occur around the Curie temperature.

Electrical Field Gradient Studies on % MathType!Translator!2!1!AMS LaTeX.tdl!TeX -- AMS-LaTeX! % MathType!MTEF!2!1!+- % feaaeaart1ev0aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbbjxAHX % garmWu51MyVXgatuuDJXwAK1uy0HwmaeHbfv3ySLgzG0uy0Hgip5wz % aebbnrfifHhDYfgasaacH8qrps0lbbf9q8WrFfeuY-Hhbbf9v8qqaq % Fr0xc9pk0xbba9q8WqFfea0-yr0RYxir-Jbba9q8aq0-yq-He9q8qq % Q8frFve9Fve9Ff0dmeaabaqaciGacaGaaeqabaWaaeWaeaaakeaaca % qGmbGaaeyyamaaBaaaleaacaqGXaGaaeylaiaabIhaaeqaaOGaae4q % aiaabsgadaWgaaWcbaGaaeiEaaqabaGccaqGnbGaaeOBaiaab+eada % WgaaWcbaGaae4maiaabUcacqaH0oazaeqaaaaa!447E! $${{La}}_{{{{1 - x}}}} {{Cd}}_{{{x}}} {{MnO}}_{{{{3 + }}}}$$ System

A Perturbed Angular Correlation (PAC) study was performed on the \({{La}}_{{{{1 - x}}}} {{Cd}}_{{{x}}} {{MnO}}_{{{{3 + }}}}\) system that presents a rich variety of structural and magnetic phase transitions as a function of the oxygen content (δ) or as a function of temperature (T). The PAC signal at room temperature allowed the determination of the Electrical Field Gradient (EFG) parameters and to correlate them with the lattice average symmetry namely the orthorhombic (O′ or O*) Pbnm and rhombohedral (R) \({\text{R}}\overline{{\text{3}}} {\text{C}}\) phases measured by X-ray diffraction.

111Cd PAC Study of Gd–Ni Intermetallic Compounds

This paper presents a perturbed angular correlation study of the magnetic and electric hyperfine interactions of 111Cd on Gd sites of the Gd-Ni intermetallic compounds GdNi, GdNi2, GdNi3, Gd2Ni7, GdNi5 and Gd2Ni17.

Perturbed Angular Correlation Study of Naturally Occurring Zircon with very Small Impurity Concentrations

We used 181Ta-PAC spectroscopy to characterize the electric field gradient at Zr sites of zircon samples from the Harts Range in Australia. Zircons from this region contain very low levels of radioactive impurities. The PAC spectra between room temperature and 1100°C are axially symmetric and show little damping. The quadrupole frequency decreases linearly with increasing temperature, and the slope of the v Q vs. T graph increases above 900°C. This is attributed to a rearrangement of the Si-O coordination, which affects the Zr-O coordination. The spectra of all zircons show a distinct decrease of the anisotropy between 800 and 600°C. This behavior is also seen with samples that do not exhibit the change in slope in the v Q vs. T data. We believe that the decrease in the anisotropy is due to an aftereffect following the β-decay of 181Hf.

Perturbed angular correlation study of order– disorder transition in HfW2O8

The perturbed angular correlation (PAC) technique has been used to characterize the degree of atomic order in the neighbourhood of radioactive 181Hf isotopes in HfW2O8. PAC measurements were carried out at temperatures between 14 and 723 K. The compound was synthesized starting with the oxides HfO2 and WO3, using a method involving ball milling, high temperature annealing and quenching in liquid nitrogen. Fast cooling allows to have the compound at temperatures below 1050 K. The compound has a high degree of stability below such temperature and around 430 K atomic ordering occurs. This transition order-disorder is reversible.

Remeasurement of the half-life of the 615 keV level in 181Ta

The lifetimes of the 615 and 619 keV levels in 181Ta have been remeasured by using a \(\beta\)-\(\gamma\) delayed coincidence set up with fast plastic (Pilot U) and BaF2 detectors. The presently measured values for both levels have been found to be in strong disagreement with results reported earlier. For the 615 keV level, the half-life has been found to be \(T_{1 / 2} = 12.1 \pm 0.1\) ns in striking contrast to the earlier reported value of 17.6 \(\mu \)s. The half-life of the 619 keV level has been found to be \(T_{1 / 2} \leqslant 230\) ps, whereas values of 0.87 ns and 2.4 ns were reported previously. The nanosecond half-life of the 615 keV level has been confirmed from a separate measurement by employing a NaI(Tl)-Pilot U detector combination. The importance of lifetime measurements for the 615 keV level in perturbed angular correlation studies is discussed.

Interaction between HfC precipitates and vacancies in quenched Cu:Hf as studied byTDPAC and positron lifetime measurements

A Cu:Hf sample with 1 wt% Hf as prepared by arc melting is characterized by TEM andmicrodiffraction analysis to contain HfC precipitates. HfC precipitates in a Cu matrix bindvacancies and divacancies strongly in the quenched Cu:Hf sample as deducedby time differential perturbed angular correlation (TDPAC) studies. Isochronalannealing studies using TDPAC and positron lifetime measurements indicatethe stability of these vacancy complexes in the quenched sample for annealingtreatments up to 1200 K, beyond which the de-trapping of the vacancies fromHfC precipitates is observed to occur. This shows that HfC precipitates presentin Cu inhibit the formation of voids by strongly binding quenched vacancies.

Angular correlation measurements for theα+He6decay ofBe10

The $\ensuremath{\alpha}+^{6}\mathrm{He}$ decay of $^{10}\mathrm{Be}$ has been studied via the $^{7}\mathrm{Li}(^{7}\mathrm{Li},\ensuremath{\alpha}\phantom{\rule{0.2em}{0ex}}^{6}\mathrm{He})\ensuremath{\alpha}$ reaction at $58\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$. The excitation energy of the $^{10}\mathrm{Be}$ nucleus was determined following the coincident detection of the $\ensuremath{\alpha}$ and $^{6}\mathrm{He}$ decay products. A study of the fragment angular correlations has provided a spin assignment consistent with the previously reported value of ${3}^{\ensuremath{-}}$ for the $10.15\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$ state in $^{10}\mathrm{Be}$. A tentative assignment of $({4}^{+})$, ${6}^{+}$ is proposed for the $11.76\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$ state.

The electronic structure of Hf2Co: perturbed angular correlation study and first principle calculations

The results of high resolution TDPAC measurements of electric quadrupole interaction (EQI) at the181Ta probe ion in the polycrystalline intermetallic compoundHf2Co ofthe Ti2Ni structure type, in a temperature range from 77 to 1200 K, are presented. The results showthe presence of two independent EQIs. At room temperature their frequencies areωQ1 = 36(3) Mrad s−1 at the16c, and ωQ2 = 230(3) Mrad s−1 at the 48f position. The low frequency interaction is characterized by an unusual temperaturedependence which shows a pronounced maximum. The temperature dependence of allrelevant physical parameters (the electric field gradient (EFG) principal componentVZZ, the EFG distributionparameter δ, theasymmetry parameter η, and the fractions of ions contributing to the specific interaction) are also presented. Inaddition, the EFG parameters of the same structure are calculated using the full-potentiallinearized augmented plane-wave (FP-LAPW) method as implemented in the WIEN97package, and the results are compared with the measured data and some previouscalculations obtained using the full-potential linear muffin-tin orbital (FP-LMTO) methodin the atomic sphere approximation (ASA).

Local probe studies on LaMnO3+ using the angular correlation technique

Abstract We performed a perturbed angular correlation (PAC) study on the prototypical LaMnO 3+ δ system that presents a rich variety of structural and magnetic phase transitions as a function of the oxygen content ( δ ) or as a function of temperature ( T ). The PAC signal at room temperature allowed the determination of the electrical field gradient parameters in the orthorhombic (O′ or O ∗ ) or rhombohedral (R) phases, which show a drastic changes in the O ∗ –R transition.