Angular Correlation Studies Research Articles

Theoretical and Experimental Study of the Angular Correlations in Auger Cascades in Noble Gas Atoms

An overview of recent experimental and theoretical studies of angular correlations in cascades of Auger transitions photoinduced in rare-gas atoms is given. In particular, the resonant Auger transitions from the photoexcited Ne 1s−13p, Ar 2p−14s, Kr 3d−15p, and Xe 4d−16p states to the states of ns−2n'l' and ns−1np−1n'l' configurations with a following Auger (Coster–Kronig) transition have been studied by means of angle-resolved electron spectroscopy including coincidence measurements of two emitted electrons. A theoretical interpretation of experimental results is given in the framework of the MCDF approach. In general the multi-configuration calculations agree very well with experiment.

Atomic “After Effects” Following181Hfβ--Decay

Strong evidence of atomic “after effects” following β−-decay of 181Hf was observed from perturbed angular correlation (PAC) studies in different viscous and non-viscous molecular solutions and in ionic as well as non-ionic solutions. In non-viscous acetone and H2O solutions, the PAC spectra have been found to be completely different to what was expected considering molecular motions in these media. Similarly, in the glycerol-H2O system, PAC spectra remain significantly unchanged with the change of viscosity indicating that molecular motions are not reflected in these media. Rather, perturbations from the interaction of the nucleus-excited atomic state, which was so far considered to be absent for 181Hf β−-decay, have been found to be dominant. The atomic shell recovery times in molecular liquids have been found to be in the range 50–150 nsec. In insulating solid HfCl4 medium, however, the PAC spectrum indicates that the atomic shell following 181Hf β−-decay remains unchanged within the lifetime of the intermediate state. The lifetime for the 615 keV level has been remeasured and a value of T1/2=12.1±0.1 nsec only has been obtained. This value, although in strong disagreement with the earlier reported value (17.83 μsec), helps explain atomic “after effects” in 181Hf β−-decay.

Angular Correlation and Polarization Studies for Radiative Electron Capture into High-Z Ions

Recent photon correlation studies for Radiative Electron Capture into high-Z projectiles are reviewed. Emphasis is given to the investigation of polarization phenomena which are now accessible due to recent developments in position sensitive solid-states detectors. It is shown, that REC may provide a tool for the diagnostics and detection of the spin–polarization of particles involved in atomic collisions. Also the impact of REC studies for atomic structure studies is outlined. Here the strong alignment of excited states induced by REC allowed us to observe an interference between competing decay branches for the case of the Lyman-α1 transition in hydrogen-like ions.

Perturbed angular correlation study of static and dynamic hyperfine interactions at181Tain the Laves-phase hydridesErFe2Hx

The static magnetic hyperfine interaction and the hydrogen-induced dynamic electric quadrupole interaction (QI) at the nuclear probe ${}^{181}\mathrm{Ta}$ on cubic Er sites in magnetically ordered $C15$ Laves-phase hydrides ${\mathrm{ErFe}}_{2}{\mathrm{H}}_{x}$ have been investigated by perturbed angular correlation (PAC) spectroscopy as a function of temperature $(10 \mathrm{K}<~T<~600 \mathrm{K})$ and hydrogen concentration $(0<~x<~3.2).$ At room temperature and concentrations $x<2,$ the PAC spectra, supported by x-ray diffraction measurements, indicate the coexistence of two hydride phases ${\mathrm{ErFe}}_{2}{\mathrm{H}}_{x}$ with $x<~0.1$ and $x\ensuremath{\sim}1.6--1.7,$ respectively, and a linear increase of the fraction of the high-concentration phase with increasing x. At low temperatures the PAC spectra of the high-concentration hydride reflect a broad distribution of strong static hyperfine interactions. The dynamic QI caused by rapidly diffusing H atoms becomes observable at $T>400 \mathrm{K}.$ The resulting ${}^{181}\mathrm{Ta}$ nuclear quadrupole relaxation rates show an Arrhenius behavior with an activation energy of ${E}_{a}=0.39(3) \mathrm{eV}$ for $x=1.5$ and 2.0 and permit an estimate of the effective charge associated with the diffusing H atom of ${Z}^{\ensuremath{'}}e\ensuremath{\approx}0.095e.$ The vanishing time average of the nuclear quadrupole interaction in the fast-fluctuation region allows the separation of the magnetic and electric hyperfine interactions and the determination of the ${}^{181}\mathrm{Ta}$ magnetic hyperfine field ${B}_{\mathrm{hf}}$ as a function of temperature and H concentration. At a given temperature $T>~450 \mathrm{K}$ in the fast-fluctuation region, ${B}_{\mathrm{hf}}$ decreases with increasing x, indicating a decrease of the s conduction electron polarization at the probe site. The temperature dependence of ${B}_{\mathrm{hf}}$ in the fast-fluctuation region is also affected by the hydrogen concentration: at $x=1.1,$ ${B}_{\mathrm{hf}}$ was found to decrease, at $x=1.5$ and 2.0 to increase with increasing T. Irreversible changes of the PAC spectra indicate that the thermal stability of crystalline ${\mathrm{ErFe}}_{2}{\mathrm{H}}_{2}$ encapsulated under vacuum is limited to $T<~550 \mathrm{K}.$ ${\mathrm{ErH}}_{2}$ was identified by x-ray diffraction as one of the dissociation products, but there was no evidence for the precipitation of elemental Fe and a hydrogen-induced amorphization step.

On the [formula omitted]-decay of [formula omitted

In β-decay experiments on 9C at CERN/ISOLDE the β-strength was determined to the ground state, the 12.2 MeV excited state and the Isobaric Analog State (IAS) at 14.655 MeV in 9B. A large β-strength asymmetry is deduced for the mirror transitions of 9C and 9Li to states around 12 MeV excitation energy. A satisfactory description of the three-body decay from a narrow energy region around the 12.2 MeV resonance is obtained within a sequential model involving the ground and first-excited states of 5Li and 8Be. From the study of angular correlations the spin of the 12.2 MeV state is determined as 5/2 − . For the first time the population of the IAS is observed in β-decay and new information on the decay of this state is obtained. The advantages of a closely packed, highly segmented detector setup are demonstrated.

Perturbed angular correlation study of magnetic and electric hyperfine interactions at 111Cd in GdAl 2

The magnetic and electric hyperfine interaction of the probe nucleus 111Cd on the Al site of the C15 Laves phase GdAl 2 has been investigated by perturbed angular correlations. The relative temperature dependence of the magnetic hyperfine field ( B hf=20.6 T at 9.7 K) and the local magnetization direction are sensitive to the probe properties. The ferro- and the para-magnetic phase coexist in an interval of ∼2 K below T c=169.3 K.

Irradiation-induced defect configurations in Ge substrates characterised with perturbed angular correlation

Perturbed angular correlation (PAC) studies with radioactive 1 1 1 In probes in crystalline Ge substrates have previously established the presence of two distinct defective configurations following either electron or ion irradiation. Though such defects have been tentatively identified as In-vacancy (In-V) and In-interstitial (In-I) configurations, an unambiguous assignment is still lacking and conflicting interpretations are apparent. For the present report, a series of experiments have been performed as functions of ion dose, background dopant and dopant concentration to examine both the validity of previous suppositions and produce supplementary evidence to aid in determining the microscopic nature of the two defective configurations. The relative fractions of the defective configurations were generally insensitive to dopant concentration and thus did not exhibit a significant Fermi level dependence. The present results suggest that the formation of an In-V complex is not the result of elastic interaction between an In-acceptor and neutral vacancy. Alternative interpretations are discussed.

Hyperfine Interactions at 181Ta in Hf2Co7

In this paper we report a time differential perturbed angular correlation (TDPAC) study of hyperfine interactions at 181Ta in Hf2Co7 compound. The TDPAC measurements have been performed in the temperature range 77–1023K. The measured spectra showed broad distribution of hyperfine interactions up to 388K. Above this temperature, in a reversible manner, two nuclear electric quadrupole interactions (EQI) have been observed: at 393K, νQ1=449.0(27)MHz, η1=0.00(4) and νQ2=144.7(41)MHz, η2=0.44(2). Since the Curie temperature for this compound is known from the literature to be around 420K, only magnetic order transition could be responsible for such an observation. However, the measurements of magnetization did not exhibit any drastic change in this temperature range.

Extended X-ray absorption fine structure and time differential perturbed angular correlation study of hafnium(IV) sorbed onto amorphous silica

Sorption of Hf(IV) onto amorphous silica at pH values between 1 and 7 is investigated by extended X-ray absorption fine structure (EXAFS) and time differential perturbed angular correlation (TDPAC) spectroscopies. Hafnium(IV) is studied as a chemical analogue for understanding the speciation of tetravalent actinides. In addition, 181Hf is a frequently investigated TDPAC probe nuclide. Spectra obtained from both techniques show that there is a change in Hf-species with sample preparation pH. Results from TDPAC measurements show that two similar surface binding sites exist for Hf(IV) on the silica surface at low pH values. At a pH above 3, a third, unspecific surface binding site for Hf(IV) begins to dominate. EXAFS investigations do not detect the presence of polynuclear surface species. Both TDPAC and Hf L3 edge EXAFS spectra of the sorption samples differ from that of a Hf(IV) solution or a Hf(IV) hydrolysis product. This indicates that the sorbed species are different from solution or simple hydrolysis species. Short Hf-O distances are obtained from theoretical fits to the EXAFS oscillations for all samples. This suggests that the primary interaction of Hf(IV) with the silica surface occurs via silanol groups. Differences in the electric field gradient surrounding sorbed Hf(IV) cations as a function of pH, observed in the TDPAC spectra, must result from changes in overall species geometry rather than a change in the primary Hf-silica surface interaction.

A spectrometer for the study of angular correlations in polarized-orthopositronium decay

We describe a setup for experimental testing of the CPT-invariance by observing the angular correlations in the decay of polarized orthopositronium atoms. Annihilation events are detected by five NaI(T1) sensors of 150-mm diameter and 100-mm thickness, located at the vertices of a regular pentagon. The sensor signals are detected in the double-coincidence circuits gated by signals with different time delays from the positron escape. The setup has a time resolution of 8.0 ns. In order to reduce the effect of the long-term instability and the instrumental asymmetry on the experimental results, the direction of positron incidence on a SiO2 target was reversed with a program-driven period during data acquisition. The value of the asymmetry, measured with due account of statistical and systematic errors, is 0.0008±0.00091, which is twice better as compared with the previous experiment.

Velocity imaging photoionization coincidence apparatus for the study of angular correlations between electrons and fragment ions

A new apparatus has been developed to detect and measure angular correlations between energy-selected photoelectrons and coincident mass-analyzed fragment ions from photoionization at selected wavelengths. It achieves velocity imaging for electrons and ions simultaneously and has high collection efficiency for both particles, with moderate mass and energy resolution. Angular and energy correlations between the two particles are measured, as are the angular distributions of each particle independently relative to the light polarization direction. Fixed-molecule electron angular distributions are deduced in cases of pure axial recoil. Examples of angular distributions from photoionization of diatomic molecules are reported.

Electron-gamma perturbed angular correlation studies on high-T C superconductors

Recent results on the study of high-TC superconductors using the e−–γperturbed angular correlation technique are presented. The basic features of the experimental equipment and its installation at the ISOLDE facility are briefly described. Results obtained from 197mHg implanted into high quality Y1Ba2Cu3O6+δ epitaxy thin films are presented and discussed.

Angular correlation study of H(2p) excitation by electron impact

Excitation of the 2p state of hydrogen by 54.4 eV electrons is studied experimentally using the scattered electron-decay photon angular correlation technique. At small scattering angles previous experimental results and a range of theoretical calculations are confirmed. At large scattering angles, the present data consistently support these theoretical models but do not agree with previous angular correlation data at this energy. This fundamental discrepancy between experiment and theory for the simplest electron excitation process in the simplest atom is therefore resolved in favour of theory.

Simultaneous excitation and ionization

Experimental studies of simultaneous excitation-ionization by electron impact are discussed. Emphasis is placed on very recent (e, 2e) and electron-photon angular correlation studies of the n = 2 states of He + .

On-line perturbed angular correlation studies with the short lived 127Cs probe

On-line Perturbed Angular Correlation (PAC) experiments were performed on the 66.0 keV excited state of 127Cs, using the γ (114.7 keV)–e−L (66.0 keV) cascade from the decay of the short-lived 127Ba (T1/2=13 min) isotope produced at the ISOLDE facility at CERN. The PAC experiments were performed with an optimized γ–e− spectrometer coupled to the ISOLDE beam line, which allowed simultaneous implantation and measurement. The optimization of the experiment is described and the first results on metallic foils and single crystals of Al, Be, Ga, Zn, and Ni are presented and discussed. The derived nuclear moments of the 66.0 keV excited state of 127Cs are |μ|=2.9(2)μN and |Q|=0.58(12)b. Applications of this new PAC isotope are outlined.

Perturbed angular correlation study of the ion exchange of indium into silicalite zeolites

Two indium-containing silicalite zeolites (In/H–ZSM5) catalysts prepared by wet impregnation and ionic exchange were characterized by the Perturbed Angular Correlation (PAC) technique using 111In as probe to determine the nature of the indium species. Some of these species take part in the catalytic reaction of the selective reduction (SCR) of NOx with methane. PAC experiments were performed at 500oC in air before and after reduction–reoxidation treatments on the catalysts in order to determine the origin of the different hyperfine interactions and then the degree of ionic exchange. Complementary catalytic activity characterizations were also performed. PAC experiments performed on the catalyst obtained by wet impregnation showed that all In-atoms form In2O3 crystallites while almost 70% of In-atoms form In2O3 in the catalyst obtained by ionic exchange. The PAC experiments of both catalysts performed after the reduction–reoxidation treatment revealed the presence of two hyperfine interactions, different from those corresponding to indium in In2O3. These hyperfine interactions should be associated to disperse In species responsible of the catalytic activity located in the ionic exchange-sites of the zeolites.

Perturbed angular correlation study of 3d transition metal multilayers

The present paper aims to show that a perturbed angular correlation (PAC) investigation gives access to unique information on the (interface) magnetism in MBE-grown very thin magnetic multilayers. For the Fe/Cr(1 0 0) multilayer system we observe, below a critical Cr-thickness of 5.0 nm, a collapse of the spin-density-wave (SDW) ordening in chromium. While the magnetization in the Fe-layer is in-plane, the Cr-magnetisation in thicker layers is out-of-plane. In the Fe/Co (1–10) system, the observation of transferred magnetic hyperfine field satellites and their temperature dependence leads to the determination of an oscillating magnetic moment profile at the Fe/Co interface as well as a structure model for the superlattice.

Improved spin basis for angular correlation studies in single top quark production at the Fermilab Tevatron

We show in single top quark production that the spin of the top quark is correlated with the direction of the d-type quark in the event. For single top production in the W* channel, the d-type quark comes dominantly from the antiproton at the Tevatron, whereas for the W-gluon fusion channel the spectator jet is the d-type quark the majority of the time at this machine. Our results are that 98% of the top quarks from the W* process have their spins in the antiproton direction, and 96% of the top quarks in the W-gluon fusion process have their spins in the spectator jet direction. We also compare with the more traditional, but less effective, helicity basis. The direction of the top quark spin is reflected in angular correlations in its decay products.

Laser Raman spectroscopy (LRS) and time differential perturbed angular correlation (TDPAC) study of surface species on Mo/SiO2 and Mo,Na/SiO2. Their role in the partial oxidation of methane

Silica-supported molybdenum (1.6 and 5.0 wt%) and molybdenum (5 wt%)-sodium (0.4 wt%) catalysts have been characterized by laser Raman spectroscopy (LRS), time differential perturbed angular correlation (TDPAC), temperature-programmed reduction (TPR) and X-ray photoelectron spectroscopy (XPS). The presence of different molybdenum species was correlated with activity and selectivity to formaldehyde during the methane partial oxidation reaction. The main species identified on the Mo(5.0 wt%) /SiO2 surface were MoO3 and monomeric species with a single Mo=O terminal bond. The pre-impregnation of the silica support with sodium strongly diminishes the Mo=O concentration due to the formation of Na2Mo2O7 species and tetrahedral monomers with a high degree of symmetry. As a result of these modifications, both methane conversion and formaldehyde formation are strongly inhibited. The combination of LRS and TDPAC techniques resulted in a powerful tool for the identification and quantification of the molybdenum species present on the surface of a silica support.

Perturbed Angular Correlation Study of Fe/Cr (100) and Fe/Co (1-10) Multilayers

Recently we completed the very first and detailed perturbed angular correlation investigation on MBE-grown very thin film multilayers. The present paper aims to show that otherwise rather unaccessible but unique information on the (interface) magnetism could be obtained. For the Fe/Cr(100) multilayer system we observe, below a critical Cr thickness of 5.0 nm, a collapse of the spin-density-wave ordering in chromium. While the magnetization in the Fe-layer is in-plane, the Cr magnetization in thicker layers is out-of-plane. In the Fe/Co (1 —1 0) system, the observation of transferred magnetic hyperfine field satellites and their temperature dependence leads to the determination of an oscillating magnetic moment profile at the Fe/Co interface as well as a structure model for the superlattice.