Angular Correlation Experiments Research Articles

TDPAC experiments with 140Ba- 140La in YBaCuO compounds

In order to investigate the internal field around the Ba atom in YBaCuO superconducting compounds, the time-differential perturbed angular correlation(TDPAC) experiments have been performed by means of a radioactive ion implantation technique. The significant perturbation due to a magnetic dipole interaction has been observed for a non-superconducting YBa 2Cu 3O 6 compound under a magnetic field of 0.4 T, while no perturbation has been found for a superconducting YBa 2Cu 3O 7 specimen.

Measurement of coherence parameters for the electron impact excitation of the 31D state of helium from an angular correlation experiment

An angular correlation experiment has been performed to measure three target parameters gamma , Pl and rho 00 for the electron impact excitation of the 31D state of helium. This was done by measuring the angular distribution of the 31D to 21P decay radiation ( lambda =667.8 nm) emitted in the scattering plane in coincidence with the inelastically scattered electrons. The photon intensity perpendicular to the scattering plane was also measured in coincidence with the inelastically scattered electrons. Results are presented over an angular range of 10'-40' at incident electron energies of 60 and 80 eV. The results of gamma and Pl are compared with polarisation-correlation experimental data and a variety of theoretical calculations. The measured values of rho 00 are compared with available theoretical data, since no other experimental results for rho 00 are available.

Decay of 132Cs and nuclear structure of 132Xe.

Gamma spectroscopy, coincidence, and angular correlation experiments were performed in an extensive study of $^{132}\mathrm{Cs}$ decay. The population intensities of states in $^{132}\mathrm{Xe}$ and $^{132}\mathrm{Ba}$ were measured and some upper limits determined. The logft value of the transition to the ${4}_{1}^{+}$ state of $^{132}\mathrm{Ba}$ differs significantly from previously published values. A new gamma transition of 688 keV was detected and located in the level scheme of $^{132}\mathrm{Xe}$. Angular correlation measurements were performed with a Ge(Li)-NaI(Tl) spectrometer. Kumar-Baranger-type calculations were performed for the $^{132}\mathrm{Xe}$ and the results are compared to those of other collective models.

Hyperfine-interaction parameters for a Cd probe atom at the Fe/Kr interface.

Polycrystalline iron foils implanted to high krypton doses, in order to produce rare-gas inclusions, were doped with $^{111}\mathrm{In}$ and the magnetic hyperfine field as well as the electric-field gradient was measured. Time-differential perturbed angular correlation experiments were performed in the as-implanted and the annealed sample condition. We observed a substantial fraction of probe atoms in a ``defect site,'' characterized by the hyperfine interaction parameters \ensuremath{\Delta}\ensuremath{\Vert}${\mathit{B}}_{\mathrm{hf}}$\ensuremath{\Vert}=-6.89(4)% and ${\mathit{V}}_{\mathit{z}\mathit{z}}$\ensuremath{\simeq}2.6\ifmmode\times\else\texttimes\fi{}${10}^{17}$ V/${\mathrm{cm}}^{2}$, in close similarity with those expected for the Kr/Fe interface. The impact of the present approach for metal surface studies is stressed.

Dynamical observation of the structure change at the melting point of In metal by a time-differential perturbed angular correlation method

In order to study the structure change between solid and liquid in a metal, the time-differential perturbed angular correlation (TDPAC) experiment has been performed on In metal at the melting point utilizing radioactive 111In probe atoms. The fraction of liquid in the solid state has been determined quantitatively as a function of time. The attenuation factors in the freezing process were found to be different from those in the melting process.

Identification of the C-defect in nickel found by means of PAC

The annihilation of C-defects in nickel by self-interstitials was observed in a γ-γ perturbed angular correlation experiment. The results corroborate the assigned vacancy character of the C-defect and, consequently, prove that recovery stage III in nickel is associated with vacancy migration. This conclusion has far-reaching consequences for the acceptability of either the one or two-interstitial model of defects in metals.

Perturbed angular correlation study of impurity interactions in a cubic host metal: Hf in Nb.

{sup 181}Ta perturbed angular correlation experiments have been performed on Nb-Hf alloys containing 0.5--5.5 at. % Hf. A Maxwellian frequency distribution model is proposed to account for the electric-field-gradient distribution due to impurities in a cubic matrix. The model was used to analyze the Nb-Hf data. If an unperturbed fraction is included with the Maxwellian distribution, then agreement with experiment is excellent. The unperturbed site is apparently required to correct for underestimation of the actual distribution by the Maxwellian at low frequencies. The unperturbed fraction is large only for small impurity concentrations and corresponds approximately to sites more distant than five lattice spacings from the nearest impurity. The peak of the Maxwellian quadrupole frequency distribution is linear in Hf concentration.

Perturbation functions: PAC probe nuclei, [formula omitted

The program, ACOEFF, calculates several theoretical quantities that are useful for analyzing Perturbed Angular Correlation (PAC) experiments. In particular, ACOEFF calculates perturbation functions, G k k ( t), a( k) n, n'-coefficients, and interaction frequencies for nuclear electric quadrupole interactions of PAC probe nuclei with non-axially symmetric electric field gradients. The program calculates these quantities for the specific cases of spin, I = 2, 5 2 , and 3, for polycrystalline sources. The program is written in FORTRAN IV and should run on any mainframe or minicomputer. It was tested using the WATFIV and VS compilers. The input consists of one data card, and the output is straightforward to understand.

Electric quadrupole interactions in non-axially symmetric electric field gradients of spinI=2 and 3 nuclei in polycrystalline sources

Calculations and formulae are presented for the purpose of understanding and analyzing Perturbed Angular Correlation (PAC) experiments that use spin-two and spin-three probe nuclei. For electric quadrupole interactions of probe nuclei in a polycrystalline source with non-axially symmetric electric field gradients, the interaction frequencies and perturbation functions for spin-two and spin-three nuclei differ qualitatively and quantitatively from those for spin-five-halves nuclei. These differences result primarily because them-states of integral-spin nuclei are nondegenerate in an asymmetric electric field gradient. To help the experimenter deal with the added complexity of the integral-spin casesI=2 and 3, closed-form expressions are provided for the energy eigenvalues and the eigenvectors as a function of the asymmetry parameter η. To deal with the problem of calculating the perturbation functionsGkk(t) for η>0, the summation overm-state quantum numbers is formulated in terms ofan′,n(k)-coefficients. These coefficients are analogous to theSkn-coefficients used in the case for η=0. To illustrate the differences between the half-integral-spin caseI=5/2 and the integral-spin casesI=2 and 3, energy difference diagrams andan′,n′(2) diagrams are presented and discussed.

List mode acquisition method for time-differential perturbed angular correlations

A new data acquisition method for the time-differential perturbed angular correlation (TDPAC) experiment has been applied to the measurement of electric field gradient in In metal under melting. Combining a 4-detector goniometer with a multiparameter data acquisition system, the temperature of the sample and the time spectrum for each angle are stored event by event in a list mode. The fraction of liquid phase in solid state of In metal has been quantitatively determined as a function of time by this method and the “hysteresis” has been observed in the course of liquefaction. This technique can be applied to study hyperfine interactions by means of the TDPAC method in high frequency magnetic fields.

Noniterative fitting of deconvoluted positron annihilation lineshapes

A noniterative method for fitting deconvoluted lineshapes from positron annihilation (Doppler-broadening or angular correlation) experiments to the conventional model of a Gaussian and parabola is presented. The method has been tested on synthetic curves and returns the component widths to within 0.05 of the dispersion width and the parabola intensity to within 0.2% of the total intensity. The method has also been used to fit angular correlation curves from positron annihilation in magnetised iron.

Positron-annihilation study of the electronic structure of Cu91Ge

A novel effect of alloying on the electron momentum density in a disordered ..cap alpha..-Cu/sub 91/Ge/sub 9/ alloy is reported. The effect, which is the appearance of a bump in the derivative spectra at low momenta, is predicted by theory employing the coherent-potential approximation and is observed in two-dimensional angular correlation experiments on the alloy. It reflects the presence of an s-p impurity band far below the Fermi level. Accurate experimental values for the Fermi-surface radii k/sub 100/ and k/sub 110/ and for the associated disorder-induced smearings in CuGe are also reported for the first time.

Computation of the G-factor in angular correlation experiments using gas targets and finite transverse size of the beam. II

We discuss a numerical method to evaluate the G-factor for a system of two telescopes in coincidence and a gas target. Both telescopes are determined by two circular apertures lying on parallel planes. The beam is assumed to have uniform density over a circular cross section. A simple program is presented which requires a short computer time.

Mössbauer and time-differential perturbed angular correlation studies of the hydrides and deuterides of vanadium

Hydrides and deuterides of vanadium were studied by source and absorber Mössbauer experiments with the resonances of 99Ru, 193Ir and 197Au and by perturbed angular correlation experiments with the 353 — 90 keV γ-γ cascade in 99Ru. The observed isomer shifts and electric quadrupole interactions are discussed in terms of the local environment of the probes in the different hydride and deuteride phases and compared with similar results for the Nb-H system.

TDPAC study of the crystallisation of amorphous Ni-B alloys produced by ion implantation

The crystallisation of Ni79B21 and Ni62B38 amorphous alloys produced by ion implantation is studied via perturbed angular correlation experiments on radioactive 111In probes implanted in the amorphous layer. The hyperfine interaction sensed by 111Cd (daughter of 111In) provides information about the structure (amorphous or crystalline) of the alloy. The crystallisation temperatures and the nature of the crystallised phases are thus determined; they are known to depend strongly on the alloy composition.

Decay of 83-min 139Ba to levels of 139La and the decay of 18-min 141Ba to levels of 141La.

The level structures of odd proton $^{139}\mathrm{La}$ and $^{141}\mathrm{La}$ have been studied in \ensuremath{\gamma}-ray singles, Compton-suppressed singles, coincidence, and angular correlation experiments. No evidence was observed to support the existence of any levels in $^{139}\mathrm{La}$ in the gap between the 166-keV ${d}_{5/2}$ level and the 1209-keV ${(1/2)}^{+}$ level, even though data were accumulated long enough to observe indirect feeding of the ${(1/2)}^{+}$ level. Additional spin and parity assignments are suggested for a number of levels in $^{141}\mathrm{La}$ based on angular correlation measurements.

Simultaneous energy and timing decisions in time differential perturbed angular correlation experiments using BaF2 scintillators

BaF2 scintillation counters, coupled to constant fraction differential discriminatiors, permit simultaneous energy and timing decisions in recording delayed coincidence spectra. We explore the advantages and limitations of this innovation in time differential perturbed angular correlation experiments, and illustrate it for111CdNi and100RhNi.

Observation of isotropic critical spin fluctuations in Gd.

Perturbed \ensuremath{\gamma}\ensuremath{\gamma} angular-correlation experiments on oriented single crystals of Gd, containing 1 ppb of $^{111}$In, demonstrate that critical spin fluctuations are isotropic to within about 1 K above the Curie temperature. Closer to ${T}_{C}$ we find evidence that the critical spin fluctuations fall increasingly along the c axis. Analysis of nuclear relaxation times in the isotropic region leads to spin autocorrelation times that diverge with an exponent w=0.53(8). This value is not consistent with either spin-nonconserved Heisenberg behavior or Ising behavior, but agrees with M\"ossbauer data for Gd $^{161}$Dy.

Electric field gradients in semiconducting alloys

The results of recent perturbed angular correlation (PAC) experiments in semiconducting alloys are discussed. The observed temperature dependence of the electric field gradients (EFG) differs drastically from that of methals. The variation of the conduction electron density with temperature certainly influences the EFG but can not explain in general the data. A model developed for In2Te5 based on the change of the local electron density is discussed in detail.

Angular correlation experiments in the reactions 2H(α, nα)p and 2H(α, pα)n and comparison with the Faddeev theory and the resonating group method

Abstract At the Hamburg Isochronous Cyclotron angular correlation experiments were carried out in the reactions 2 H(α, nα)p and 2 H(α, pα)n, in order to determine the tensor polarization of the ground-state resonances 5 Li ( 3 2 − ) and 5 He ( 3 2 − ) , respectively. We find that the observables for the two mirror reactions 2 H (α, n ) 5 Li and 2 H (α, p ) 5 He are charge-dependent. This result is interpreted as evidence for a violation of charge symmetry. In the whole phase-space region absolute break-up cross sections are generally well reproduced by the Faddeev theory. On the other hand, we find that the resonating group method which has up to now been mainly applied to two-body reactions can also provide valuable information for break-up reactions. The predictions of Hackenbroich, Schutte and coworkers are in quite good agreement with the measured tensor polarization of 5 He and 5 Li and the corresponding angular correlation functions.