Hyperfine Interaction Measurements Research Articles

Nuclear level mixing resonances (LMR)

The recently developed Level Mixing Resonance method for the measurement of hyperfine interactions turns out to be a very useful complement to the other methods. Especially in the “difficult” cases, e.g. When the lifetime of the isomer under consideration is very long or when its spin is very high, this new method can yield valuable, otherwise inaccessible, information. In this paper the concept of this method is briefly presented and some of the recent experiments are described.

Mössbauer Spectroscopy as a Tool for Materials Research

In 1958 Rudolph L. Mössbauer reported his discovery of a simple, practical way of observing nuclear gamma ray resonance. One of the remarkable features of the discovery was the high precision with which energy changes can be measured: energy resolutions of one part to 1011 −1013 are possible. With this high resolution capability it is possible to measure hyperfine interactions between the nucleus of an atom and its electronic environment. These interactions affect the line shape which can be described by several experimental Mössbauer parameters. The three primary parameters are the isomer shift (δ), the quadrupole splitting (Δ), and the magnetic hyperfine interaction.The isomer shift, determined by the position of the centroid of a set of lines in a spectrum, is proportional to the electron density at the nucleus. Since only s electrons have a probability of being at the nucleus, it is possible to obtain electronic structure information such as oxidation state and population of certain molecular orbitals.The quadrupole splitting results when the electron environment surrounding the Mössbauer nucleus is not spherical in its charge distribution. Specifically, Δ is proportional to the imbalance in electron density between the axial and equatorial directions. When this hyperfine interaction is present, there is a quadrupole splitting; i.e., a single spectra line will split into two or more lines.

Electron spin resonance of aluminum-related color centers in α-TeO 2:Al

ESR observations of aluminum-doped paratellurite (α-TeO 2:Al) single crystals after electron irradiation near room temperature indicate four radiation-induced electron-like defects with similarities to the intrinsic V o . center, which is also present. From measurements of angular variations and tellurium hyperfine interactions it is evident that the four centers represent a family of V o . centers perturbed by Al 3+; that is, V oAl Te x centers. The new centers are detected only in Al-doped crystals; however, the aluminum impurity ions appear to be too far from the V o . to yield 27Al hyperfine interaction measurable by ESR.

Diffusion of indium in monocrystalline zinc

From hyperfine interaction measurements (PAC) the diffusion of indium in zinc was identified as a two-stage mechanism. The heat of formation of a substitutional dilute Zn(In) alloy is Delta Hs=+0.37(3) eV below 500K and +0.24(3) eV or +0.18(3) eV above 500K, depending on the diffusion direction.

Dysprosium-161 Moessbauer spectroscopy studies of divalent and trivalent dysprosium halides and oxyhalides

/sup 161/Dy (dysprosium 161) hyperfine interaction measurements are reported for the dysprosium dihalides DyX/sub 2/, trihalides DyX/sub 3/, and oxyhalides DyOX (X = chlorine, bromine, iodine (Cl, Br, I)) using the 26-keV Moessbauer resonance. In the dihalides, the electron configuration of dysprosium is divalent (4f/sup 10/). However, it has been impossible to obtain pure phases of these compounds. The oxyhalides order antiferromagnetically below 10 K. The hyperfine parameters and the magnetic ordering in these compounds are interpreted in terms of electronic structure with use of a Hamiltonian including electric crystal field and magnetic exchange effects. The change of mean square nuclear radius of /sup 161/Dy between the ground and 26-keV levels (..delta..(r/sup 2/) = 8.0 x 10/sup -3/ fm/sup 2/) is deduced by combination of the isomer shift results with free-ion electron density calculations. 8 figures, 6 tables.

Measurement of hyperfine interactions with electron spin-echo spectroscopy. Application to F centers in KCl

The accuracy of spin-Hamiltonian determination via analysis of electron spin-echo nuclear modulation signals is investigated for F centers in KCl. It turns out that the accuracy matches that of ENDOR. The linewidth of the signals converted to the frequency domain also compares well with ENDOR. Parametric spectrum estimation with autoregressive modeling (maximum entropy method) yields significant further reduction of the linewidth. Analysis of the signal strength in the frequency domain appears feasible.

Hyperfine interaction and g-factor measurements in Er isotopes

Energy levels in 160Er and 159Er were excited in the reactions 28Si( 136Xe, 4n) 160Er and 28Si( 136Xe, 5n) 159Er, and the product nuclei, recoiling with a velocity v = 0.07 c were stopped in a plunger instrument after traversing a volume of hydrogen gas. The angular distribution of de-excitation γ-rays was studied as a function of flight time and gas pressure. Appreciable perturbations were detected in 160Er at times as short as 2 ps and in an angular momentum region as high as 12ħ. No perturbation could be detected in the 21 2 + level of 159Er even after 12 ps, signifying a low value for the g-factor of this level.

THE QUADRUPOLE INTERATION IN ZINC METAL

Mossbauer experiments have been performed with the 93.3 keV 1/2-5/2 transition in 67Zn metal. The electric field gradient was found to be axially symmetric with a value of eq = +(3.02 ± 0.05) 10J7 V/cm2. The 93.3 keV resonance in G7Zn possesses an extreme sensivity for the measurement of hyperfine interactions. The nuclear Zeeman splitting can be observed in magnetic fields of -100 Oe /l/. The cen­ ter shifts of the resonance patterns of various zinc chalcogenides /2,3/ which all contain the zinc atom in the +2 oxidation state are very large compared to the line width. The quadrupole interaction of the spin _-?. groundstate of 67Zn (the excited state has I = -L and thus shows no quadrupole splitting) has been thoroughly studied in ZnO /4/. Only incomplete data were available for zinc metal /5/. In this note we report on measurements to determine the electric field gradient in zinc metal. For a source we used 67Ga in Cu. The 6 7Ga

Electron paramagnetic resonance spectra of the Group 4 hexafluoride anion radicals

Anisotropic EPR spectra detected at 30/sup 0/K in ..gamma..-irradiated powdered BaGeF/sub 6/, K/sub 2/SnF/sub 6/, and BaPbF/sub 6/ have been assigned to the hexafluoride anion radicals GeF/sub 6//sup 3 -/, SnF/sub 6//sup 3 -/, and PbG/sub 6//sup 3 -/, respectively. These species are members of a series of paramagnetic hexafluorides of the elements of groups 4 to 7 which constitutes one of the largest known groups of isoelectronic free radicals. The nature of the semioccupied orbital in this type of radical is discussed in the light of unpaired electron spin densities computed from the measured hyperfine interactions for the hexafluorides.

Hyperfine interactions of neutron-activated ? emitters in insulators and metals

The general features of hyperfine interaction measurements using neutron-activated β emitters are described and the applications of this technique are reviewed: nuclear magnetic resonance and transients of the nuclear polarization give informations on nuclear moments, interactions in undisturbed lattices, radiation induced and thermal defects in insulators and metals.

Hyperfine interactions in intermetallic compounds between Gd and 3 d transition metals

Measurements of hyperfine interactions at 155Gd nuclei in metallic compounds between Gd and 3 d transition metals and at 61Ni in GdNi compounds by Mössbauer spectroscopy are reported. The results are discussed in terms of various models proposed for the electronic structure of these compounds. The Gd isomer shifts with respect to metallic Gd are at variance with the model of a strong d electron transfer from rare earths to transition metal ions. From the observation of a linear relation between magnetic hyperfine fields at Gd and at Dy nuclei in corresponding compounds it is inferred that crystal-field induced variations of Dy moments are neglible and that the conduction electron polarization induced by 4 f moments is directly related to that caused by 3 d moments.

An analysis of recoil distance measurements in the coincidence mode

A large number of corrections for recoil distance measurements either for lifetime measurements or for hyperfine interaction measurements have been calculated. The results are expressed in terms of the intensities of the shifted and the unshifted gamma ray peaks and in terms of the fractional change of energy of the shifted gamma ray. The accuracy of the results should be limited only by counting statistics as the corrections are carried to order v/ c (of the recoiling nucleus). Suggestions are made for reducing uncertainties in the hyperfine interaction correction to lifetime measurements and for improving the accuracy in hyperfine interaction measurements.

Study of the rare-earth-oxygen interaction in iron by lattice location and perturbed-angular- correlation experiments

In a study of the rare-earth-oxygen interaction in iron, lattice location and integral perturbed-angular-correlation experiments were performed after implantation of Yb and $^{16}\mathrm{O}$ in iron single crystals and foils. The ion-implantation energies were chosen so that the depth profiles overlapped almost completely. A strong Yb-O interaction is evidenced in both types of experiments. However, its effects on channeling results and on hyperfine-interaction measurements are found to be entirely different: In the present case, the usually assumed simple corespondence between impurity lattice location and hyperfine-field values certainly does not hold. The results are discussed in the light of a simple statistical model for the interaction probability between rare-earth and oxygen atoms.

Paramagnetic hexafluoride anions of Group VI

Certain isotropic ESR spectra observed in γ−irradiated SF6 and in γ−irradiated solid solutions of SeF6 and TeF6 in SF6 are attributed to the hexafluoride anions of the Group VI elements. Hyperfine interaction measurements for the species 33SF6−, 77SeF6−, and 125TeF6−, observed in natural abundance, show that the central atom outer s orbital makes a very large contribution to the (totally symmetric) semioccupied orbital in these radicals. INDO MO calculations for SF6− gratifyingly predict a half−filled orbital which has the representation A1g in Oh symmetry and which is primarily composed of S 3s and F 2pσ atomic orbitals. An additional isotropic spectrum present in γ−irradiated 1% SeF6 in SF6 is believed to be that of SeF5 in which hyperfine interaction with the unique (apical) F nucleus is not detectable.

Lattice site location of implanted argon in iron and nickel crystals

Abstract The lattice sites of Ar implanted into Fe and Ni crystals are determined using the channeling technique in combination with proton induced characteristic X-ray emission. The results show that at least 90% of the implanted Ar atoms occupy random sites, which is consistent with hyperfine interaction measurements of Devare and De Waard.

Lattice relaxation in α-iron containing small xenon-vacancy clusters

Binding energies and atomic configurations for small xenon-vacancy clusters in α-iron have been computed using a model based on a two-body potential between the interacting atoms. Various clusters were placed near the center of a sphere containing about 150 iron atoms initially at regular positions in bcc configuration. The results of these calculations are of importance for the interpretation of hyperfine interaction measurements on implanted radioactive sources.

Stroboscopic Observation of Nuclear Larmor Precession

A new resonance method is described which is suited for the measurement of hyperfine interactions at nuclear isomers with mean lives longer than 1 \ensuremath{\mu}sec. The method is characterized by pulsed-beam production of excited nuclei, the Larmor frequency of which is determined by a coherent superposition of the perturbed angular correlation originating from different beam pulses. An experiment was performed on a 4-\ensuremath{\mu}sec state of $^{69}\mathrm{Ge}$. The $g$ factor was determined as $g=\ensuremath{-}0.2224\ifmmode\pm\else\textpm\fi{}0.0007$.

The overlap of interest between hyperfine interaction measurements and ion implantation

Ion implantation involves the theory and experimental techniques of accelerating ion beams, the passage of accelerated ions in solids, their slowing down and stopping, and their final positions in the lattice. Interest is centred on the modifications the implanted ions produce in lattice properties and the variation of these modifications with parameters such as the total number and incident energy of the incident ions and the heat treatment of the lattice. Hyperfine interaction measurements involve the product of a nuclear electro-magnetic moment, electric monopole (charge), magnetic dipole, electric quadrupole, etc., with the appropriate property of the environment of the nucleus, namely the electron charge density, the magnetic field, or the electric field gradient. In this introductory article the areas in which the fields of current interest of these two branches of physics overlap are briefly outlined. We first consider the requirements and capabilities of hyperfine interaction measurements, concentrating on the magnetic dipole case, and show how these may be fulfilled and extended with the aid of ion implantation techniques. We then consider how hyperfine interaction studies could be applied to the problems facing those who use ion implantation for other purposes.

Lifetime and hyperfine structure of the level (6s26d)2D3/2of thallium I

The level-crossing technique in resonance fluorescence has been applied to determine the lifetime and hyperfine structure of the level (6s26d)2D3/2 in Tl 1. No distinction was made between the two stable isotopes 203 and 205 since their nuclear moments are identical within the accuracy of these experiments. The results (5 2 ± 0.8) × 10-9 sec for the lifetime, and 42 2 ± 1 8 Mc/s for the hyperfine interaction constant, are in good agreement with the measurements of Gallagher and Lurio. The measured lifetime agrees with a value calculated from measurements of the spontaneous transition probabilities from the excited state, and the measured hyperfine interaction constant agrees with the value calculated from the Goudsmit-Fermi-Segrè theory. This latter agreement is probably fortuitous because the theory does not take into account the effects of configuration interaction, which might in this case be considerable.

Mössbauer Effect Measurements of Hyperfine Interactions in Laves Phases Containing Fe Combined with Ti, Zr, Y, and the Lanthanide Metals

57Fe Mössbauer absorption spectra have been obtained for the Laves-phase compounds AFe2 in which A=Ti, Zr, Y, Ce, Sm, Gd, Tb, Dy, Ho, Er, Tm, and Lu. TiFe2 and CeFe2 are paramagnets at room temperature and hence only show quadrupole interactions (0.21 and 0.40 mm/sec, respectively). The others exhibit a magnetic hyperfine interaction at room temperature and all show this at the liquid-nitrogen point. The pattern obtained for TiFe2 at 78°K is complex and has been interpreted as indicating two types of Fe, one of which is magnetically ordered and for which the hyperfine field is 71 kOe, whereas the other type is either not magnetically aligned, or if it is, the field is less than 10 kOe. Fe moments for the other compounds are obtained either directly from experiment or estimated from the experimental results using Bleaney's exchange induction effect. These moments are compared with the measured hyperfine fields. Due to the limited range of variability of the moments and fields their functional interrelationship cannot be unambiguously established. A linear relationship seems to be indicated. This is interpreted to imply that only conduction and core electron polarizations contribute significantly to the hyperfine field in the (cubic) AFe2 compounds, and the latter varies linearly with the localized Fe moment.