Perturbed Angular Correlation

Electronic structure calculations of cubic SrTiO3 and SrHfO3 are presented. The full-potential linear augmented-plane-wave method is used and exchange-correlation effects are treated by the local-density approximation. The tendency to ferroelectricity of both compounds is explored and compared by displacing the transition metal atom (Ti or Hf) towards one of the oxygens (001 direction). The calculations show that ferroelectricity is favored in SrTiO3 with respect to SrHfO3 and that this fact may be correlated with the degree of hybridization between transition metal d-O p bands as has been found for other related systems. Also a detailed discussion of the calculated electric field gradients is presented.

The dependence of the electric field gradients (EFGs) in the Hf-doped LaNis-hydrogen system, as a function of hydrogen composition ratio (X), has been investigated in the range 0<-X-<5.7 using the Time Differential Perturbed Angular Correlation (TDPAC) technique. The TDPAC spectrum of LaNi~ was fitted with a single EFG, while two EFGs were needed to fit the LaNisH x spectra. The first EFG was similar to the one found in metallic LaNi 5 and the second one was interpreted as related to those probe atoms located in the basal plane next to hydrogen interstitial sites located in the same plane. The relative fraction of the probe atoms experiencing these EFGs is dependent on the relative occupation of hydrogen in these sites.

Digital imaging provides an effective means to electronically acquire, archive, distribute, and view medical images. Medical imaging display stations are an integral part of these operations. Therefore, it is vitally important to assure that electronic display devices do not compromise image quality and ultimately patient care. The AAPM Task Group 18 ͑TG18͒ recently published guidelines and acceptance criteria for acceptance testing and quality control of medical display devices. This paper is an executive summary of the TG18 report. TG18 guidelines include visual, quantitative, and advanced testing methodologies for primary and secondary class display devices. The characteristics, tested in conjunction with specially designed test patterns ͑i.e., TG18 patterns͒, include reflection, geometric distortion, luminance, the spatial and angular dependencies of luminance, resolution, noise, glare, chromaticity, and display artifacts. Geometric distortions are evaluated by linear measurements of the TG18-QC test pattern, which should render distortion coefficients less than 2%/5% for primary/secondary displays, respectively. Reflection measurements include specular and diffuse reflection coefficients from which the maximum allowable ambient lighting is determined such that contrast degradation due to display reflection remains below a 20% limit and the level of ambient luminance ͑L amb ͒ does not unduly compromise luminance ratio ͑LR͒ and contrast at low luminance levels. Luminance evaluation relies on visual assessment of low contrast features in the TG18-CT and TG18-MP test patterns, or quantitative measurements at 18 distinct luminance levels of the TG18-LN test patterns. The major acceptable criteria for primary/ secondary displays are maximum luminance of greater than 170/ 100 cd/ m 2 , LR of greater than 250/ 100, and contrast conformance to that of the grayscale standard display function ͑GSDF͒ of better than 10% / 20%, respectively. The angular response is tested to ascertain the viewing cone within which contrast conformance to the GSDF is better than 30% / 60% and LR is greater than 175/ 70 for primary/secondary displays, or alternatively, within which the on-axis contrast thresholds of the TG18-CT test pattern remain discernible. The evaluation of luminance spatial uniformity at two distinct luminance levels across the display faceplate using TG18-UNL test patterns should yield nonuniformity coefficients smaller than 30%. The resolution evaluation includes the visual scoring of the CX test target in the TG18-QC or TG18-CX test patterns, which should yield scores greater than 4 / 6 for primary/secondary displays. Noise evaluation includes visual evaluation of the contrast threshold in the TG18-AFC test pattern, which should yield a minimum of 3 / 2 targets visible for primary/secondary displays. The guidelines also include methodologies for more quantitative resolution and noise measurements based on MTF and NPS analyses. The display glare test, based on the visibility of the low-contrast targets of the TG18-GV test pattern or the measurement of the glare ratio ͑GR͒, is expected to yield scores greater than 3 / 1 and GRs greater than 400/ 150 for primary/secondary displays. Chromaticity, measured across a display faceplate or between two display devices, is expected to render a uЈ,vЈ color separation of less than 0.01 for primary displays. The report offers further descriptions of prior standardization efforts, current display technologies, testing prerequisites, streamlined procedures and timelines, and TG18 test patterns.

The intermetallic compounds LaRu 2 and CeRu 2 , as well as several quasibinary systems Ce 1Ϫx La x Ru 2 , were synthesized at a pressure of 8 GPa. The x-ray diffraction data showed the pure cubic C15 structure for all samples. The x-ray absorption measurements gave the value 3.23 for the Ce valence. Time-differential perturbed angular correlation measurements with the 111 In-doped samples have shown ͑i͒ 111 In substituted for Ru; ͑ii͒ in LaRu 2 a single value of the quadrupole frequency Q ϭ234 MHz for the daughter 111 Cd nuclei was observed; ͑iii͒ in CeRu 2 two distinct equally populated sites for 111 Cd were revealed with Q ϭ218 and Q ϭ153 MHz. The low-temperature magnetic measurements showed that magnetic moments about 0.3 B per formula unit are inherent in all samples and indicated at a coexistence of superconductivity and magnetic ordering. In particular, the magnetic and superconducting correlations are developing, most probably, in the same system of ruthenium 4d electrons.

The spatial dependence of the magnetic moment perturbation near the ͑1-10͒-Fe/Co interface was derived from hyperfine fields determined by perturbed angular correlation. The amplitude of the moment perturbation decays exponentially with the distance to the interface. The moment perturbation itself is modulated by a sine function of the distance d to the interface ⌬ϭA*exp(Ϫp*͉d͉)*sin(Ϫk*d). Between 90 K and 570 K, we observe that the amplitude and the penetration depth of the perturbation as well as the wave vector of the modulation are essentially constant within the accuracy of the values. In this temperature region the average values for the parameters determining the exact moment perturbation are Aϭ1.03 B , pϭ2.25ϫ10 9 m and kϭ1.45ϫ10 10 m Ϫ1 when d is defined positive at the Co side of the interface. © 199 7 American Institut e of Physics .

The structure of metallic multilayer systems has been established to be important and of big interest both in basic research as well as in technological processes. The aim of this work is to study the formation, the structure and the evolution of near surface layers and of buried inter- faces, mainly by means of new perturbed gamma-gamma angular correlation spectrometer. The measurements are based on in-situ film preparation in an UHV-system in the Institute of Nuclear Physics, Cracow. The development and putting into operation of the thin film evaporation system was performed. Also the testing of the measurement setup and the data acquisition system for the γ-γ-perturbed angular spectrometer (PAC) was continued. A series of metallic multilayer systems were produced and studied by X-ray diffraction (XRD) and X-ray reflection (XRR) (Münster, MS), scanning force microscopy (SFM) (1,2) and giant magnetoresistivity (GMR) (Kraków, KR), which was recently added to the tools for thin layer...

The electric field gradients in perovskite SrHfO3 and BaHfO3 powder samples were investigated by means of Perturbed Angular Correlation spectroscopy, using implanted In-111 hyperfine probes. The measurements cover the temperature ranges from 22 to 893 K for SrHfO3 and from 293 to 875 K for BaHfO3, respectively. In both compounds, three quadrupole interactions were established. The largest fraction showing a pronounced dynamic interaction is assigned to In-111/Cd-111 probe atoms on substitutional Hf sites in both compounds. For SrHfO3, the temperature dependence of the dynamic interaction was associated to the Pnma <-> Imma phase transition for SrHfO3 at about 700 K. We discuss the results in relation to those obtained for In-111/Cd-111-probes in PbZrO3 and BaTiO3, and for Hf-181/Ta-181-probes in AHfO(3) (A = Ca, Sr, Ba).

Bare and coated TiO2 nanoparticles with particle sizes d < 5 nm have been synthesized in a microwave plasma process. Structural properties of these materials have been investigated by transmission electron microscopy, x-ray diffraction, and perturbed angular correlation (PAC) measurements of the electric quadrupole interaction (QI) at the probe nucleus Ta-181 on the metal site of TiO2 at temperatures 290 <= T <= 1450 K. The electron diffraction of the uncoated nanoparticles in the as-synthesized state reflects long range order in the Ti sublattice. Depending on the particles size, either the anatase or the rutile phase of TiO2 was found. Anatase appears to be the stable form of nanocrystalline TiO2 below d similar to 10 nm. The PAC spectra of these nanocrystalline oxides are characterized by a broad distribution of strong quadrupole interactions, indicating a strongly disordered oxygen environment of the metal sites. Upon annealing, the grain size grows from d < 5 nm after synthesis to d > 100 nm after 1300 K. PAC spectra taken in the same temperature range show that with increasing temperature, the initially disordered state transforms to well-ordered rutile TiO2. The data suggest a critical grain size of d similar to 10 nm for the onset of the ordering process. The spectra of coarse-grained TiO2 are reached at a particle size d >= 30 nm. In n-TiO2 coated with Al2O3 and ZrO2 both the cores and the coatings were found to grow with increasing temperature; the cores of the coated particles, however, grow much less than those of the noncoated particles. The PAC method was used to investigate the QI in both TiO2 cores and in the ZrO2 coating of n-TiO2/ZrO2 at different temperatures. These data suggest that although the coated particles grow with temperature, the ordering process is obstructed, possibly by a solid state reaction between the TiO2 kernels and the coatings. (

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, ?Q1=449.0(27)MHz, ?1=0.00(4) and ?Q2=144.7(41)MHz, ?2=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.

Starting from results from He-pycnometry, electron diffraction, Extended X-ray Absorption Fine Structure Spectroscopy and Perturbed Angular Correlation Spectroscopy the phase transformations and structures of zirconia are described. From a comparison of these results with those obtained on other oxide nanoparticles it is concluded that the phases and structure of nanoparticles are different compared to those of coarse-grained material. The difference of the transformation temperature of bare and coated nanoparticles was used to estimate enthalpy and entropy of the tetragonal fi monoclinic transformation for nanoparticulate zirconia. By comparison with results obtained from other nanocrystalline oxides, the following rules were derived: Provided the particles are sufficiently small, particles made of materials showing phase transitions crystallize in the high temperature structure. However, compared to coarse-grained materials of the same structure, the density of nanoparticles is reduced. A first estimation limits this phenomenon to particle sizes well below 10 nm. Those nanoparticles follow the generalized phase diagram postulated by Tammann.

The electronic properties of the HfNi-hydrogen system has been investigated, as a function of the hydrogen composition ratio (x), using combined Time Differential Perturbed Angular Correlation (TDPAC) technique and standard full-potential Linearized-Augmented-Plane-Wave method. The experimental TDPAC data confirm the presence of a two-step hydrogenation process in this system, with the octahedral holes filled first. The major part of the electric field gradient at the hafnium site is from pYp contribution, shifted down in energy by hydrogen s-states contribution.

The Zr 2 Al, Zr 3 Al 2 and Zr 6 NiAl 2 intermetallic compounds were characterized by means of time differential perturbed angular correlation (TDPAC) and X-ray diffraction. Our interest in these Zr(Hf) aluminides comes from crystallization studies of Zr(Hf)-based bulk metallic glasses which have a wide supercooled liquid region.

Four defect complexes with indium tracer atoms in undoped and lightly doped Ce02 have been observed by In time-difFerential perturbed-angular-correlation spectroscopy. One complex, identified as an indium-0-vacancy pair, is always present unless the material is doped with Nb or Ta to eliminate oxygen vacancies. The binding energy of the vacancy to indium is greater than 0.35 eV. In materials doped or annealed to introduce as little as 0.05% oxygen vacancies, two different double-vacancy complexes are formed. One is apparently a complex in which the vacancies are trapped on opposite sides of the indium along (ill) directions. The other is likely a complex with one oxygen vacancy near the indium and one in a more distant trap. In material suKciently doped with Nb, the majority of dilute indium dopants are uncomplexed, but a small fraction is strongly bound to a presently unidentified impurity. At low temperatures, an electronic excitation trapped at the time of the parent In decay to Cd clouds interpretation of some details. We speculate that this "aftereffect" affects only Cd nuclei having no vacancies in their nearest-neighbor oxygen shell. Some complexes involving only more distantly trapped vacancies may be unobserved.

The hyperfine interactions at 181 Ta ions on Fe 3+ sites in -Fe 2 O 3 (hematite) were studied in the temperature range 11Y1100 K by means of the perturbed angular correlation (PAC) technique. The 181 Hf( j ) 181 Ta probe nuclei were introduced chemically into the sample during the preparation. The hyperfine interaction measurements allow to observe the magnetic phase transition and to characterize the supertransferred hyperfine magnetic field B hf and the electric field gradient (EFG) at the impurity sites. The angles between B hf and the principal axes of the EFG were determined. The Morin transition was also observed. The results are compared with those of similar experiments carried out using 111 Cd probe.

Site-fractions of indium impurity probe atoms occupying up to three inequivalent Al-sites in Al 3 Ti, Al 3 V and Al 3 Zr phases were measured using perturbed angular correlation of gamma rays (PAC). Sites were identified via characteristic nuclear quadrupole interactions. Ratios of site-fractions were measured in thermal equilibrium in the range 600 to 1,210 K. Arrhenius plots of the ratios were fitted with thermally activated expressions, yielding differences in vibrational entropies and siteenthalpies. Enthalpy differences were greatest for Al 3 Zr, ∼0.22 eV, and smaller for Al 3 Ti and Al 3 V, which is correlated with the excess volume of the transition-metal atom over the Al-atom. Vibrational entropy differences were small, in the range 0 to −0.25 k B .

Bare and coated TiO 2 nanoparticles with particle sizes d Ͻ 5 nm have been synthesized in a microwave plasma process. Structural properties of these materials have been investigated by transmission electron microscopy, x-ray diffraction, and perturbed angular correlation ͑PAC͒ measurements of the electric quadrupole interaction ͑QI͒ at the probe nucleus 181 Ta on the metal site of TiO 2 at temperatures 290ഛ T ഛ 1450 K. The electron diffraction of the uncoated nanoparticles in the as-synthesized state reflects long range order in the Ti sublattice. Depending on the particles size, either the anatase or the rutile phase of TiO 2 was found. Anatase appears to be the stable form of nanocrystalline TiO 2 below d ϳ 10 nm. The PAC spectra of these nanocrystalline oxides are characterized by a broad distribution of strong quadrupole interactions, indicating a strongly disordered oxygen environment of the metal sites. Upon annealing, the grain size grows from d Ͻ 5 nm after synthesis to d Ͼ 100 nm after 1300 K. PAC spectra taken in the same temperature range show that with increasing temperature, the initially disordered state transforms to well-ordered rutile TiO 2 . The data suggest a critical grain size of d ϳ 10 nm for the onset of the ordering process. The spectra of coarse-grained TiO 2 are reached at a particle size d ജ 30 nm. In n-TiO 2 coated with Al 2 O 3 and ZrO 2 both the cores and the coatings were found to grow with increasing temperature; the cores of the coated particles, however, grow much less than those of the noncoated particles. The PAC method was used to investigate the QI in both TiO 2 cores and in the ZrO 2 coating of n-TiO 2 / ZrO 2 at different temperatures. These data suggest that although the coated particles grow with temperature, the ordering process is obstructed, possibly by a solid state reaction between the TiO 2 kernels and the coatings.

The intermetallic compounds LaRu 2 and CeRu 2 , as well as several quasibinary systems Ce 1Ϫx La x Ru 2 , were synthesized at a pressure of 8 GPa. The x-ray diffraction data showed the pure cubic C15 structure for all samples. The x-ray absorption measurements gave the value 3.23 for the Ce valence. Time-differential perturbed angular correlation measurements with the 111 In-doped samples have shown ͑i͒ 111 In substituted for Ru; ͑ii͒ in LaRu 2 a single value of the quadrupole frequency Q ϭ234 MHz for the daughter 111 Cd nuclei was observed; ͑iii͒ in CeRu 2 two distinct equally populated sites for 111 Cd were revealed with Q ϭ218 and Q ϭ153 MHz. The low-temperature magnetic measurements showed that magnetic moments about 0.3 B per formula unit are inherent in all samples and indicated at a coexistence of superconductivity and magnetic ordering. In particular, the magnetic and superconducting correlations are developing, most probably, in the same system of ruthenium 4d electrons.

The local magnetic interaction at the transition metal sites in RMO 3 (R = La, Nd; M = Cr, Fe) compounds has been investigated by perturbed angular correlation (PAC) technique using 181 Hf→ 181 Ta probe nuclei. The present measurements cover a temperature range from 10 K to 1000 K. Above the respective Néel temperature, each compound shows a unique quadrupolar frequency that decreases linearly with temperature. These interactions were assigned to the radioactive probe substituting Cr or Fe sites. Below T N , a combined electric plus magnetic hyperfine interaction was observed. The magnetic interaction revealed that the super transferred hyperfine fields on 181 Ta at the Cr sites in (La,Nd)CrO 3 extrapolated to 0 K, are much smaller than the corresponding values at Fe sites in (La,Nd)FeO 3 . This difference was attributed to different distribution of d electrons in Cr 3+ (3d 3 ) and Fe 3+ (3d 5 ) ions in each compound. As the fields for Nd compounds are smaller than those for La compounds, the role of rare-earth ions in the magnetism of these oxides is also discussed.

The hyperfine quadrupole interaction at probes in sites with cubic point group symmetry was measured in many perovskite-type compounds. This interaction is commonly associated to the presence of oxygen vacancies close to probes. The effect of this point defect on Ta-181 probe in BaTi x Hf1-xO3 for x = 0.70, 0.50, 0.30, 0.10, 0.05, and 0.01, is studied. The lattice constant of these oxides at room temperature were measured using XRD technique. The quadrupole parameters corresponding to probe-defect interaction were obtained by means of Perturbed Angular Correlation (PAC) spectroscopy as functions of both, composition and temperature. A static, asymmetric and distributed quadrupole interaction was fitted to all PAC data. This interaction was observed to be strongly composition dependent. These results and those corresponding to compositions 0.75 less than or equal to x < 1 were compared to the ones obtained using the point charge method for the calculation of the electric field gradient. The proposed model took into account polarized oxygen vacancies, different covalence of Ti-O and Hf-O bonds and using computational simulation for cations and oxygen vacancies lattice positions.

The electronic properties of the HfNi-hydrogen system has been investigated, as a function of the hydrogen composition ratio (x), using combined Time Differential Perturbed Angular Correlation (TDPAC) technique and standard full-potential Linearized-Augmented-Plane-Wave method. The experimental TDPAC data confirm the presence of a two-step hydrogenation process in this system, with the octahedral holes filled first. The major part of the electric field gradient at the hafnium site is from pYp contribution, shifted down in energy by hydrogen s-states contribution.

PolyPacFit is an advanced fitting program for time-differential perturbed angular correlation (PAC) spectroscopy. It incorporates stochastic models and provides robust options for customization of fits. Notable features of the program include platform independence and support for (1) fits to stochastic models of hyperfine interactions, (2) user-defined constraints among model parameters, (3) fits to multiple spectra simultaneously, and (4) any spin nuclear probe.

Using perturbed-angular-correlation ͑PAC͒ spectroscopy, via the 181 Hf→ 181 Ta probe, we have measured Mn-site electric-field gradients ͑EFG's͒ at Ta nuclei in ceramic samples of LaMnO 3 . Two crystallographic phases coexist over a temperature interval of Ϸ16 K near the orthorhombic-to-rhombohedral transition at Ϸ724 K, which shows a thermal hysteresis of Ϸ1.7Ϯ0.2 K. Concurrently, in the two phases, we determined the temperature dependence of the EFG parameters, V zz , , and ␦, and the ratio of the probe concentrations A 1 /A 2 . To explain the apparent coexistence of two phases in this weakly first-order transition, we present a model that assumes a spatial distribution of T c values. This distribution could arise from a spatially nonuniform distribution of Mn 4ϩ ions. We show the PAC technique to be a uniquely powerful probe of local symmetries that reflect the effects of a local distribution of valences, which drive the phase transition.

The time-differential perturbed-angular-correlation (TDPAC) technique was applied to the study of the internal electric-field gradient (EFG) in Eu- and Ho-sesquioxides in their cubic bixbyite phases. The results, as well as previous characterizations of the EFG at $^{181}$Ta sites in oxides with the bixbyite structure, were compared to those obtained in experiments using $^{111}$Cd as probe, and to point-charge model and

Samples of PbZr0.52Ti0.48O3 (PZT) were fabricated by the Sol-Gel method and characterized by X-ray diffraction analysis. The Rietveld method was used to obtain lattice parameters and grain sizes as a function of the annealing temperature. A powder sample previously annealed at 873 K was irradiated to activate the Ta-181 probe. Then, Perturbed Angular Correlation (PAC) spectra were measured at RT and temperatures above 873 K to study the thermal behaviour on samples capsulated in SiO2 tubes. The hyperfine parameters observed as a function of temperature show changes in this PZT compound that would correspond to thermal evolution of defects. Furthermore, the reaction of PZT with the substrate was observed at 1173 K, obtaining at this temperature the hyperfine parameters corresponding to SiZrO4. At this temperature no trace of the PZT was left.

Phase transitions in the ferroelectric perovskite BaTiO3 were studied for In-111-implanted polycrystalline samples by measuring the electric field gradients by means of Perturbed Angular Correlation spectroscopy. The phase transitions between the orthorhombic double left right arrow rhombohedral double left right arrow tetragonal double left right arrow cubic lattices were investigated in 2 - 10 K steps, for increasing and decreasing temperatures, in order to determine their hysteresis. The transition parameters are compared with results from measurements of the spontaneous polarization, electric susceptibility and neutron scattering

The magnetic hyperfine field B-hf of the closed-shell probe nucleus Cd-111 in the C15 Laves phase RNi2 (R = Gd, Sm) has been investigated as a function of temperature by perturbed angular correlation (PAC) spectroscopy. The saturation magnetic hyperfine fields at 10 K are B-hf = 7.7(2) and 3.9(1) T for GdNi2 and SmNi2, respectively. Although the probe nucleus resides on the cubic rare-earth site, a strong axially symmetric electric quadrupole interaction (QI) is observed in the paramagnetic phase at T less than or equal to 300 K. The possible relation of this unexpected QI to the structural instability of RNi2 is discussed.

RCo2The spectrum of the magnetic hyperfine fields at the closed-shell probe nucleus Cd-111 on the rare earth (R) site of the pseudobinary Laves-phase compounds R1-xYxCo2 has been investigated by perturbed angular correlation (PAC) spectroscopy at 10 K for the rare earth R=Tb and Ho at various Y concentrations x <= 0.8 and for R=Gd, Dy, Er at the concentration x=0.3. Up to four components with different magnetic interaction frequencies nu(M)(i) could be resolved from the PAC spectra. The relative intensities of these components are in fair agreement with those of a binomial distribution of Y atoms on the four nearest neighbor (NN) R sites of the probe nucleus. For all R constituents, one finds a strictly linear relation between the number n(R) of NN R atoms and the magnetic hyperfine frequencies: nu(M)(i)=nu(M)(4Y)+Delta nu(M)xn(R).The frequency nu(M)(4Y)=35(2) MHz is independent of the R constituent and of the Y concentration up to x <= 0.6. These properties identify nu(M)(4Y) as the contribution of the Co 3d moments to the hyperfine interaction at the Cd-111 site. The frequency steps Delta nu(M)[<= 0.1 nu(M)(4Y)] reflect the spin polarization directly induced by the 4f spins at the probe nucleus. From Gd to Er, the spin polarization decreases much stronger than expected from the linear variation of the 4f spin in the heavy R series. An indirect 4f contribution caused by a dependence of the Co 3d moment on the number of R neighbors can be excluded. The relation nu(M)(i)=nu(M)(4Y)+Delta nu(M)xn(R) then implies that the contributions of the 3d and 4f spins to the magnetic hyperfine field in RCo2 have the same relative sign.

Using the perturbed angular correlations (PAC) technique, the formation of hafnium-ovotransferrin complexes has been studied. Two binding configurations at each of the two specific binding-sites of the protein have been observed. They are characterized by well-defined electric quadrupole frequencies. Information about the dynamics of the protein was derived from temperature dependent measurements of the relaxation constant. The well-resolved spectra taken with fast BaFz-detectors allow a precise determination of the relaxation behaviour of the protein. The results are compared with the predictions from a hydrodynamic model for the reorientation of macromolecules. Thus the hydrodynamic volume of ovotransferrin and its N-terminal half-molecule were determined, The ovotransferrin volume is in agreement with a value derived for human serum transferrin from small angle neutron scattering. From experiments with immobilized protein material there is evidence for internal protein dynamics which is probed by the Hf-ion bound to the specific metal-sites.

Hyperfine fields in HfFe 2 were measured at 181 Ta probe using the time-differential perturbed angular correlation method (TDPAC) in the temperature range 78-1200 K. Analysis of the spectra revealed two interactions with hyperfine fields of 13.82(7) T and 8.0(2) T, at 293 K. First is ascribed to the interaction at the 8a position in the cubic C15 structure. The second can be assigned to a minor amount of hexagonal C14 phase, or to an irregular position of the probe in the C15 lattice. Results of calculations using LAPW-WIEN97 are in a good agreement with experiment. r

In the previous Comment, Forker and co-workers claim that perturbed angular correlation ͑PAC͒ data leave no alternative to the conclusion that the spontaneous magnetization of PrCo 2 and NdCo 2 undergoes a discontinuous, first-order phase transition at T C . We show here that their claim is in clear contradiction with a wealth of experimental evidence, including our own. Finally, we propose a possible origin for the disagreement between their interpretation of the PAC results and the literature on this subject.

Electronic structure calculations of cubic SrTiO 3 and SrHfO 3 are presented. The full-potential linear augmented-plane-wave method is used and exchange-correlation effects are treated by the local-density approximation. The tendency to ferroelectricity of both compounds is explored and compared by displacing the transition metal atom ͑Ti or Hf͒ towards one of the oxygens ͑001 direction͒. The calculations show that ferroelectricity is favored in SrTiO 3 with respect to SrHfO 3 and that this fact may be correlated with the degree of hybridization between transition metal d-O p bands as has been found for other related systems. Also a detailed discussion of the calculated electric field gradients is presented.

In this work an electric-field gradient and magnetic hyperfine field study of the multiferroic AgCrO 2 triangular spin lattice is presented. Perturbed angular correlation measurements at different temperatures, revealed the coexistence of two electric-field gradients, i.e., two distinct local environments at temperatures below 100 K. The emerging second local environment appears as a distortion of the Cr surrounding resulting in a local symmetry lowering. This local distortion emerges much above T N and concomitantly with the onset of short-range magnetic correlations. We claim that, through a magnetoelastic instability, distinct Cr-Cr exchange interaction pathways appear providing a channel for magnetic frustration release.

The order and other properties of the magnetic phase transitions in the rare-earth (R)-cobalt Laves phases RCo2 have been studied for R=Gd, Tb, Dy, Ho, Er, Sm, Nd, and Pr by measuring the temperature dependence of the magnetic hyperfine interaction of the nuclear probe 111Cd on the cubic R sites using the perturbed angular correlation technique. Both for heavy and light R constituents the transitions change from second order (Gd, Tb, Sm) to first order (Dy, Ho, Er, Nd, Pr) at order temperatures of 150–200 K. For heavy R constituents, the order deduced from the hyperfine interaction is in agreement with previous investigations. The observation of first order transitions in NdCo2 and PrCo2, however, is unexpected. In earlier studies the transitions in these compounds are usually classified as second order transitions. Both in the heavy and the light RCo2 the discontinuous jump of the hyperfine interaction at the first order transitions increases with decreasing order temperature. This trend implies that the Co magnetization at the transition increases with decreasing TC which can be related to the temperature dependence of the coefficient of the M4 term of the free energy in the Wohlfarth-Rhodes-Shimizu theory of itinerant electron magnetism. All compounds investigated presented a spread of the order temperature of ∼1–2K which results in a coexistence of the paramagnetic and the magnetically ordered phase near the transition and causes a critical increase of the relative linewidth of the hyperfine frequency diverging as δ∝(1-T/TC)ɛ with ɛ=-1.0(1).

A detailed theoretical study of the structure, electronic properties and the electric field gradients of the Hf2Fe intermetallic compound is presented. Using all-electron full-potential linearized augmented plane wave (FP-LAPW) formalism the equilibrium volume, bulk modulus and electric field gradients are calculated. The obtained results are compared with EFG values inferred from measurements performed using Mössbauer spectroscopy and the earlier reported time differential perturbed angular correlation (TDPAC) measurements. The lattice relaxation and the supercell calculations are found to be essential for the correct interpretation of the experimental results.

We report a study of the impurity-radiation damage interaction for In in silver. Timedifferential perturbed angular correlation experiments were carried out on 111Cd, the radioactive daughter of 111 In produced and recoiled in Ag following the l~ 2n) 111 In nuclear reaction. All irradiations were performed at 80 K. The dose-rate and annealing temperature dependences of the defect-induced quadrupole interaction on 111Cd were investigated. Clear differences are observed between the effects of correlated damage (the impurity interacting with the damage created by its own recoil) and uncorrelated damage (where the impurity is submitted to the influence of other damage cascades). In the former case (low dose-rate experiments), about 6% of the 111 In impurities are subject to a well-defined quadrupole interaction which is ascribed to a near-neighbor vacancy. This proportion is temperature-independent up to 500 K, but the amplitude of the quadrupole frequency is reduced by a factor of three between 300 and 400 K. Assuming vacancy release by the impurity, this would lead to a binding energy of 0.34(15) eV. The remainder of the 11 t In nuclei are subject to a distribution of quadrupole interactions presumably due to a statistical repartition of lattice defects at some distance from the impurities. The width of the distribution changes upon annealing: a clear transition takes place at 200 K, which is the divacancy mobility threshold in Ag deduced from resistivity experiments. When cascade overlap occurs (high dose-rate experiments), no impurity-defect trapping is evidenced but a distribution of quadrupole interactions due to surrounding damage is still found. From these experiments, an order of magnitude of the damage cascade "size" is deduced. Our results are discussed in terms of available information on the impurity-damage interaction in various hosts; a compilation of all results on the impurity-vacancy interaction known to us is also presented in the paper.

Digital imaging provides an effective means to electronically acquire, archive, distribute, and view medical images. Medical imaging display stations are an integral part of these operations. Therefore, it is vitally important to assure that electronic display devices do not compromise image quality and ultimately patient care. The AAPM Task Group 18 ͑TG18͒ recently published guidelines and acceptance criteria for acceptance testing and quality control of medical display devices. This paper is an executive summary of the TG18 report. TG18 guidelines include visual, quantitative, and advanced testing methodologies for primary and secondary class display devices. The characteristics, tested in conjunction with specially designed test patterns ͑i.e., TG18 patterns͒, include reflection, geometric distortion, luminance, the spatial and angular dependencies of luminance, resolution, noise, glare, chromaticity, and display artifacts. Geometric distortions are evaluated by linear measurements of the TG18-QC test pattern, which should render distortion coefficients less than 2%/5% for primary/secondary displays, respectively. Reflection measurements include specular and diffuse reflection coefficients from which the maximum allowable ambient lighting is determined such that contrast degradation due to display reflection remains below a 20% limit and the level of ambient luminance ͑L amb ͒ does not unduly compromise luminance ratio ͑LR͒ and contrast at low luminance levels. Luminance evaluation relies on visual assessment of low contrast features in the TG18-CT and TG18-MP test patterns, or quantitative measurements at 18 distinct luminance levels of the TG18-LN test patterns. The major acceptable criteria for primary/ secondary displays are maximum luminance of greater than 170/ 100 cd/ m 2 , LR of greater than 250/ 100, and contrast conformance to that of the grayscale standard display function ͑GSDF͒ of better than 10% / 20%, respectively. The angular response is tested to ascertain the viewing cone within which contrast conformance to the GSDF is better than 30% / 60% and LR is greater than 175/ 70 for primary/secondary displays, or alternatively, within which the on-axis contrast thresholds of the TG18-CT test pattern remain discernible. The evaluation of luminance spatial uniformity at two distinct luminance levels across the display faceplate using TG18-UNL test patterns should yield nonuniformity coefficients smaller than 30%. The resolution evaluation includes the visual scoring of the CX test target in the TG18-QC or TG18-CX test patterns, which should yield scores greater than 4 / 6 for primary/secondary displays. Noise evaluation includes visual evaluation of the contrast threshold in the TG18-AFC test pattern, which should yield a minimum of 3 / 2 targets visible for primary/secondary displays. The guidelines also include methodologies for more quantitative resolution and noise measurements based on MTF and NPS analyses. The display glare test, based on the visibility of the low-contrast targets of the TG18-GV test pattern or the measurement of the glare ratio ͑GR͒, is expected to yield scores greater than 3 / 1 and GRs greater than 400/ 150 for primary/secondary displays. Chromaticity, measured across a display faceplate or between two display devices, is expected to render a uЈ,vЈ color separation of less than 0.01 for primary displays. The report offers further descriptions of prior standardization efforts, current display technologies, testing prerequisites, streamlined procedures and timelines, and TG18 test patterns.

We have measured the melting temperature of nanoscale indium islands on a WSe 2 substrate using perturbed angular correlations combined with scanning tunneling microscopy. The indium islands are selfassembled nanostructures whose diameter can vary between about 5 and 100 nm, depending on deposition conditions. The melting point decreases due to surface energies as the islands get smaller. This decrease depends on the faceting of the crystalline nanostructures and interactions between the islands and the substrate.

Phase transition from anatase to rutile for the 70nm TiO 2 crystallite has been investigated by annealing at different temperatures followed by TDPAC measurement of these TiO 2 crystallites adsorbed with 181 Hf tracer. The width of the peaks in XRD spectra for TiO 2 crystallites annealed at different temperatures indicates the growth of the crystallites with temperature. The samples annealed upto 823K for 4hrs showed no phase transition, except the growth of the crystallites. However, it showed phase transition at the same temperature (823K), when annealed for longer duration. Annealing for shorter duration at 1123K showed phase transition. 181 Hf tracer adsorbed on 70 nm anatase TiO 2 is found to be in different polymorphs (anatase & rutile) formed during annealing process. Surface to bulk mass-transfer is found to play a significant role in the phase transition process.

Samples of BaHfO3 were irradiated with fast and thermal neutrons to produce 181Hf. Attenuations of the γ-γ perturbed angular correlations in 181Ta were observed, and they may have been caused by the interaction between defects (produced during or after neutron irradiation) and oxygen vacancies. Several defect structures were detected. For one of the structures, an excitation energy of 12 meV was measured, which was attributed to an electron state below the conduction band. The values of the dielectric constant and of the effective electron mass were also deduced.