Th. Wenzel

A compact TDPAC spectrometer

Abstract A fast-fast TDPAC spectrometer consisting of four BaF2 detectors, a simple constant fraction and routing interface module, a TAC, an ADC and a personal computer for data administration has been developed. Four double-window constant fraction discriminators, allowing simultaneous timing- and energy discrimination, a routing unit and an interface to the personal computer are integrated in the double-NIM module. The fast component in the scintillation light of BaF2 make such an exceptionally simple TDPAC spectrometer possible, which has a time resolution of 200(10) ps FWHM for the 1173–1333 keV γ-cascade of 60Co.

A simple six-input multichannel system for Mössbauer spectroscopy

A multichannel system for Mössbauer spectroscopy was developed and is presented. The system, constructed as an NIM module, is connected to the DMA port of an ATARI ST computer and can be used for six detectors simultaneously, thus allowing the operation of simultaneous triple radiation Mössbauer spectroscopy (STRMS). The multichannel system is explained and its capabilities are demonstrated.

Defects in Fe1-xO and the Fe1-xO to Fe3O4 phase transition studied by the perturbed angular correlation method

The time differential perturbed angular correlation (PAC) method has been applied to study defects in iron monoxides, using iron-implanted 111In probes. Besides a broad distribution of electric field gradients two well-defined quadrupole coupling constants have been detected nu Q=97(4) MHz, eta =0 and nu Q=71(2) MHz, eta =0.32(2). The first one was associated with an indium-iron vacancy complex, while the second one, visible in a high-temperature measurement, was tentatively assigned to indium having trapped two iron vacancies and one interstitial iron atom (the so-called Roth-type cluster). Structural transformations between different complexes and the Fe1-xO to Fe3O4 phase transition are discussed. From the analysis of the vacancy/interstitial ratio, a sizeable fraction of 4:1 defect clusters at low temperatures can be excluded. The proposed defects are compared with those recently observed in the chemically and structurally similar cobalt monoxide CoO.

Dynamic Hyperfine Interaction in Cr2O3 Observed via PAC

Perturbed-angular-correlation (PAC) measurements with 111In(EC)111Cd probes, implanted at 400 keV into poly- or mono-crystalline Cr2O3, have been carried out as a function of temperature (290-1370 K). Two different electric-field gradients (e.f.g.s) with identical orientations in the lattice have been found. Their temperature-dependent variations can be well described by a dynamical model which we propose to be connected with a hole trapping at a next-neighbour Cr ion.

Perturbed angular correlations in metal oxides with radioactive tracer beams

Abstract The perturbed γγ-angular correlation method (PAC) allows an investigation of microscopic environments of probe nuclei in solids through the hyperfine interaction. By implantation of radioactive tracer ions below the ppm level the properties of the host material in most cases remain unchanged. The multiple advantages of using an ion implanter to insert the radioactive tracer ions are illustrated by our systematic PAC investigations in oxides. The examples chosen are the annealing of radiation induced defects (e.g. in NiO, CoO); phase transformations between different crystalline structures (Mn3O4 → Mn2O3), a refinement of ionic positions (bixbyite oxides) and dynamics of electronic defects on a 10−9–10−6 s time scale (Cr2O3). In insulating compounds the probe ion itself introduces excited electronic defect levels (La2O3).

Point defects in NiO observed via perturbed angular correlation spectroscopy

Abstract The perturbed angular correlation method has been applied to study the trapping and release of defects in polycrystalline NiO in the temperature range 300–1400 K. Radioactive 111 In probes were implanted into NiO powder samples and oxidized Ni-foils at 400 keV. During annealing sequences at 600–1100 K, a total of eight non-vanishing electric field gradients (EFGs) was observed. Their annealing behavior allowed a classification of six of them as radiation-induced defects. Two further EFGs found after annealing of the NiO-lattice were attributed to intrinsic defects trapped at the 111 In tracers. Below the Neel temperature the PAC spectra are governed by combined (antiferro)magnetic and electric hyperfine interactions. Some of the defect complexes show smaller supertransferred fields which can be explained by missing next-neighbor ions. Point charge calculations were performed which allowed a modelling of these EFGs and suggest defect configurations.

Hyperfine interaction of 111Cd impurities in Mn2O3, Mn3O4 and β-Fe2O3

Abstract The perturbed γ-ray angular correlation (PAC) technique with implanted radioactive 111 In tracers has been used to measure the electric field gradients (efg) of 111 Cd impurities on various substitutional sites in Mn 2 O 3 , Mn 3 O 4 and β-Fe 2 O 3 . The oxides Mn 2 O 3 and β-Fe 2 O 3 have identical lattice structures and in both cases we find two efgs which can be attributed to the substitutional sites with 8 b and 24 d point symmetry. In the spinel oxide Mn 3 O 4 two sites are identified, one having a highly asymmetric efg (η = 0.89) and the other one an approximately symmetric efg (η = 0.13). A simple model is used to discuss the influence of covalent bonding on the field gradients in these oxides.

The magnetic hyperfine interaction of111Cd in the spinels Fe3O4 and Co3O4

The perturbed angular correlation (PAC) method was applied to study the temperature dependence of the magnetic hyperfine fields in111In-doped polycrystalline Fe3O4 and Co3O4. The critical behaviour near the magnetic phase transitions has been investigated. The changes occuring near the Vervey transition in Fe3O4 and the path of the supertransferred magnetic fied in Co3O4 are discussed. The results are compared with results obtained from Mössbauer spectroscopy.

Defects in CoO Perturbed angular correlation spectroscopy of equilibrium and frozen-in defects

Abstract The perturbed angular correlation (PAC) method was applied to study intrinsic defects in polycrystalline CoO after implantation of radioactive 111In or IIImCd ions. Measurements were performed in the temperature range 300–1450 K and under various oxygen partial pressures adjusted via a CO-CO2 gas mixture. Well defined electric field gradients were observed below 900 K near the Co/CoO phase boundary. In annealing experiments above 1100K, an influence of the oxygen activity on the PAC spectra was detected which allowed identification of the defect complexes. In contrast with the 111m Cd probes implantation, no trapping of defects occurred after implantation of the isovalent 111mCd probes. This clearly demonstrates the importance of the probes charge state in ionic compounds. The conditions are discussed under which point defects in thermal equilibrium can be observed via PAC. Point charge model calculations were used to cross-check the assignments made for the structure of the proposed probe defec...

The electric field gradient at111Cd in vanadium oxides

AbstractThe electric field gradient (efg) of111Cd in polycrystalline V2O5 was studied using perturbed angular correlation (PAC) spectroscopy, with the111In activity ion-implanted at 400 keV. Between the individual steps of an isochronal annealing program, a distinct efg (vQ1=88.1(3) MHz, ν1=0.62(2)) was recorded the contribution of which increased with annealing temperature up to 74% at 870 K. Corresponding X-ray analysis of inactive V2O5 samples, which underwent the same annealing treatment, proved that the sample always stayed as V2O5. Since V2O5 has only one equivalent cation site, it is concluded that this efg belongs to111Cd at this site. Oxidation of a vanadium foil atT=675 and 800 K atn% MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamiCamaaBa% aaleaacaqGpbWaaSbaaWqaaiaabkdaaeqaaaWcbeaaaaa!38CD!nn