Systematics of Mössbauer hyperfine parameters in Np intermetallics

Data for intermetallic compounds of neptunium obtained with the 60 keV Mössbauer resonance of²³⁷Np are reviewed. Measurements of temperature, pressure and field dependencies are available. The main questions addressed are: (a) the degree of delocalization of 5f-electrons, (b) the formal charge state of Np, and (c) the influence of the ligand on the neptunium electronic structure. For this purpose, we present an evaluation of systematic behavior concerning mainly the hyperfine field and isomer shift in the cubic Laves phase materials NpX₂, the NaCl-type monochalcogenides and monopnictides, and intermetallics with AuCu₃ and ThCr₂Si₂ structures. Analogies to corresponding rare-earth compounds will be pointed out.     

The Mössbauer effect of ferroelectric transitions

The observed anomalous behavior of the Mössbauer fraction during the ferroelectric transition in PbTiO₃ is explained using a model consistent with Cochran's analysis of ferroelectricity. Our calculations show no anomalous behavior in the second order Doppler shift.     

T-violation experiments using Mössbauer transitions

We analyse the effects of time-reversal (T) violation on the transmission of photons through a system of magnetized foils that have a Mössbauer transition with comparableM1 andE2 strength (e. g.⁹⁹Ru or¹⁹⁷Au). We consider experiments that are true tests of T violation and therefore are not subject to effects from final-state interactions, discussing observables that probe either parity-violating or parity-conserving T-odd interactions. We propose a novel three-foil experiment sensitive to a T-odd, P-odd multipole admixture. This experiment should be insensitive to most systematic problems faced by the earlier experiments.     

Mössbauer study of thermal magnetic transitions in Cu0.5Zn0.5Fe2O4

The dependence of the Mössbauer spectra of the ferrite Cu_0.5Zn_0.5Fe₂O₄ on the temperature is investigated. The profiles of the hyperfine magnetic fields distributionP(H) are obtained from the spectra by a decomposition method. The results are interpreted by assuming a variation of the magnetic correlation radius in the region of the thermal magnetic transition.     

Phase transitions in DyVO4 and DyAsO4

This paper reports on optical, magnetic, and specific heat experiments at low temperatures on DyVO₄ and DyAsO₄, including a summary of previous works on these two substances. The crystallographic phase transition in DyVO₄ is second order, whereas in DyAsO₄ there is a latent heat at the distortion temperature. In contrast to the results of neutron diffraction experiments, no evidence for a canted antiferromagnetic structure in DyAsO₄ below the magnetic transition temperature was found. Magnetically, DyAsO₄ behaves almost identically to DyVO₄.Dedicated to Prof. Dr. W. Buckel on the occasion of his 60th birthdayThis work is a somewhat shortened translation of the Habilitationsschrift submitted to the Fakultät für Physik der Universität Karlsruhe     

An experimental investigation of the onset of theL 2-L 3 M 4 andL 2-L 3 M 5 Coster-Kronig transitions forZ>90

L-M coincidence measurements were made on x rays from the radioactive decays of ₉₆ ²⁴⁴ Cm, ₉₄ ²³⁸ Pu, and ₉₂ ²³² U. The observation ofM x rays in coincidence with theL ₃ N _{4, 5} transition indicates that the onset of theL ₂-L ₃ M ₅ Coster-Kronig transition takes place atZ=91 or 92. The measured ratio of theM x-ray counting rates, coincident with theL ₃ N _{4, 5} andL ₂ M ₄ transitions, establishes that both theL ₂-L ₃ M ₄ and theL ₂-L ₃ M ₅ transitions are energetically possible forZ≧94, but the results do not rule out the possibility that these transitions begin as low asZ=92. The measurement ofL-M x-ray coincidences, together with relative Coster-Kronig electron intensities from theory or experiment, provides a new way to measure the totalL ₂-L ₃ Coster-Kronig transition probability,f ₂₃, at highZ for radioactive sources where the primary vacancies are created primarily in theL ₂ andL ₃ subshells. The present result atZ=94 indicates that the theoretical calculations of McGuire of the relative partial Coster-Kronig transition rates forL ₂-L ₃ M _4,5 are accurate to within 10 percent.     

Intensity ratios ofL,M, N,O subshell conversion electrons inM3 andE4 transitions

Conversion electron spectra from threeE3 and fourM4 transitions in¹⁰³Rh,¹⁰⁹Ag,¹¹⁵In,¹²⁵Te,¹³⁴Cs,¹³⁵Ba,¹⁹³Ir have been measured with the (π/2)√13 iron-free spectrometer with momentum resolution 0.5×10^?3 to 1×10^?3. Intensity ratios ofL, M, N, O conversion electrons show no significant departures from theory, but there is some evidence of error in the absolute values of outer-shell conversion coefficients calculated by the approximate method of Dragounet al.     

The Mössbauer effect and magnetic phase transitions

The Mössbauer effect provides a direct method for identifying the spin axis in magnetic crystals and observing magnetic phase transitions. The order of the transition may be inferred from the Mössbauer spectrum. Phase changes can occur as a function of temperature (e.g. when the anisotropy fieldB _A changes sign) or as a function of applied magnetic field. In an antiferromagnet a field ?(2B _E B _A)^1/2 along the spin axis whereB _E is the exchange field causes the spin-flop transition which is normally first order (sharp) whereas the transition to the paramagnetic phase which occurs at higher fields?2B _E is second order (continuous). In quasi-one-dimensional crystals Mössbauer spectra show that the spin-flop transition is first order locally but occurs over a range of fields throughout the crystal, so that the first order character is masked in a conventional magnetization measurement. In fields applied at a finite angle>B _A/2B _E to the spin axis the transition becomes second order, i.e. a continuous rotation of the spins occurs. In canted antiferromagnets (or weak ferromagnets) the spin-flop transition is also continuous; in addition a “screw” re-orientation may be induced by fields applied perpendicular to the spin axis and arises from antisymmetric exchange. For crystals with lowT _N the hyperfine field changes when a magnetic field is applied and has a minimum at a phase transition; this may be used to map out the magnetic phase diagram.     

Radiofrequency stimulated quantum interference on Mössbauer transitions

Redistribution of the elastic and Raman channel intensities in the Mössbauer resonant scattering spectrum can be controlled by the radio frequency (rf) stimulated quantum interference of gamma-transition amplitudes. More general expressions for each channel intensity are written out exactly taking into account the influence of the rf field. Results of calculations for the simple three-level scheme can be easily generalized to the case of ⁵⁷Fe isotope in magnetic materials.     

Shape change and fast M1 transitions in Kr

A shape transition from a probably asymmetric shape at low excitation to a more axial-symmetric shape above 21/2⁺ has been found in ⁸¹Kr. This shape change and the drastic increase of the M1 transition probabilities above spin 21/2 are attributed to the alignment of two g_9/2 protons.     

Study of the angular distribution of Auger electrons for the N 3 O 1O4, 5 and L 3 M 1 M 4, 5 transitions in the Hg atom

The angular distribution of Auger electrons is considered. The results of numerical calculations of the anisotropy parameter of the angular distribution α₂ for the N ₃ O ₁ O _{4, 5} and L ₃ M ₁ M _{4, 5} transitions in the Hg atom are reported. The matrix elements were calculated by the Dirac-Fock method in the relativistic approximation using the intermediate-coupling scheme.     

M1 transitions in the (sdg) boson model

Using the $\text{1}\text{N}$ expansion technique we derive expressions for β→g, γ→g and γ→γ M1 transitions in a general boson model. The M1 matrix elements in the sdg-boson model are similar in form to those in the neutron-proton IBM. Comparisons are made to some selected M1 data exhibiting collective character.     

Anomalous saggins of Mössbauer effect probability at phase transitions

Anomalous saggings of the Mössbauer effect probability observed in various materials near phase transition points, as a function of temperature, are discussed. We show that in all the cases the saggings can be explained by a common reason — the appearance of the so-called heterophase fluctuations near the phase transitions.     

M1 Character of unresolved continuum transitions from X-ray multiplicity measurements

We have measured the X-ray multiplicities of the transitions deexciting the quasi-continuum region as a function of the input spin for deformed and transitional erbium isotopes obtained in (⁴⁰Ar,xn) reactions. Combined withγ-ray angular distribution studies, these measurements clearly show the occurrence of a magnetic dipole component in the energy region 300–800keV. The spin dependence of this “bump” is quite different in deformed and transitional nuclei. In the latter case, a sudden increase in the quasi-continuumK electron multiplicity arises for spins higher than 35? and is strongly correlated to the occurrence of collective modes in these nuclei at the same spins. It is suggested to correspond to collective behavior of bands built on yrast states of single particle nature.