Angular Correlation Experiments Research Articles

Lifetime measurements inNb93from photon and inelastic neutron scattering

The low-spin structure of $^{93}\mathrm{Nb}$ has been studied using the $^{93}\mathrm{Nb}$$(\ensuremath{\gamma},{\ensuremath{\gamma}}^{'})$, $^{93}\mathrm{Nb}$$(n,{n}^{'}\ensuremath{\gamma})$ and $^{94}\mathrm{Zr}$$(p,2n\ensuremath{\gamma}\ensuremath{\gamma})$$^{93}\mathrm{Nb}$ reactions. Lifetimes were determined from both $^{93}\mathrm{Nb}$$(\ensuremath{\gamma},{\ensuremath{\gamma}}^{'})$ and $^{93}\mathrm{Nb}$$(n,{n}^{'}\ensuremath{\gamma})$ measurements. Branching ratios were measured, and multipolarities and spin assignments were determined from the $^{94}\mathrm{Zr}$$(p,2n\ensuremath{\gamma}\ensuremath{\gamma})$$^{93}\mathrm{Nb}$ angular correlation experiment. From $M1$ and $E2$ strengths, the ${J}^{\ensuremath{\pi}}=9/{2}^{+}$ and $7/{2}^{+}$ states at 1297.1 keV and 1483.6 keV, respectively, are proposed as members of the quintet of mixed-symmetry states associated with the $\ensuremath{\pi}1{g}_{9/2}\ensuremath{\bigotimes}({2}_{1,\mathrm{MS}}^{+}$,$^{92}\mathrm{Zr}$) configuration. The large $B(M1)$ values determined in $^{93}\mathrm{Nb}$ cannot be explained within the weak coupling limit of the interacting boson fermion model.

Electric field gradients at 181Ta sites in HfO x

In the present work we report preliminary results about the possibility to study properties of the order–disorder transition in HfO x solid solutions via the determination of the electric field gradient (EFG) at 181Ta radioactive probes. Oxygen solution into the metal was achieved by arc melting stoichiometric amounts of metallic Hf and HfO 2 under argon atmosphere. Samples of HfO x with x = 0.1 and 0.2 were prepared. Two types of samples were used for the perturbed angular correlation (PAC) experiments by doping alternatively with 181Ta, by neutron irradiation, the metallic Hf or the hafnium oxide. The PAC results on both samples were identical, with disappearing hyperfine signals of the metal and the oxide, showing a complete diffusion of the probes independent of the way of doping. The PAC signal of the HfO x solid solution consisted in a wide distribution of EFGs due to the oxygen disorder. This scheme held even after long thermal treatments at high temperature (several days at 1273 K). Annealing treatments at moderate temperature (600 K) were also made. In these cases the samples were cooling at a very low rate. These results, together with those obtained by measuring samples below and above the order temperature are reported.

Grain Size Effect on the Temperature Dependence of the Electric Field Gradient in Nanocrystalline In

Nanocrystalline indium (nano-In) was prepared with different particle sizes by electrochemical deposition. The temperature dependence of the local electric field gradient (EFG) of nano-In was investigated in a temperature range of 20–300 K using the probe 111In for perturbed γγ angular correlation (PAC) experiments. The temperature dependence of the EFG of nano-In can be described by a (1 − BT 3/2) dependence as in bulk-In. It is shown that the low temperature value of the EFG and the proportionality constant B vary systematically with particle size.

Lattice distortion around impurity atoms as dopants in CdTe

We have measured the lattice distortion around As (acceptor), Se (isovalent), and Br (donor) in CdTe with fluorescence detected X-ray absorption spectroscopy (XAFS). The experimental challenge lies in the compromise between a concentration high enough for X-ray absorption and low enough to avoid compensation and clustering. We could experimentally verify the lattice relaxation with a bond length reduction of 8% around the As atom as inferred indirectly from ab initio calculations of the electric field gradient in comparison with the measured value in a Perturbed Angular Correlation (PAC) experiment as recently reported. Our calculations of relaxation were performed with the WIEN97 package using the linearised augmented plane wave (LAPW) method and the FHI96md pseudopotential (PP) program. Using a super-cell approach for As on the Te site in CdTe, we find good convergence with increasing cell size so that the result can be taken as representative for the low doping limit. The extension to the neighbouring isovalent element Se again yields perfect agreement between experiment and model calculation with a sizeable lattice mismatch. In the case of Br, we find a sizeable increase of the bond length which may be taken as evidence for the high tendency of Br to form a Br A-center (Br plus metal vacancy) in CdTe.

Investigation of the Magnetic Hyperfine Field at 140Ce on Gd Sites in GdCo2 Compound

Perturbed angular correlation experiments on 140La–140Ce probes substituting for the Gd positions on GdCo2 demonstrate that the magnetic hyperfine field (MHF) on Ce atoms follows an expected Brillouin function but with a substantially reduced saturation value as compared with the MHF acting on free Ce+3 ions. The results were interpreted with the aid of first principles electronic structure calculations showing that the reduced value of the MHF is a consequence of a small orbital contribution to the MHF which was attributed to the de-localization of the Ce 4f electronic state.

The Electric Field Gradient of 111Ag in Macrocyclic Crown Thioethers

Time differential perturbed angular correlation experiments and ab-initio density functional theory calculation were used to determine the electric field gradients of the metal centres of the macrocyclic crown thioethers Ag(15S5)[BF4], Ag(18S6-CH2OH)[CF3SO3], Ag(18S6)+, Ag(19S6-OH)[Tosylat] and Ag(20S6-OH)[CF3SO3]. The density functional theory calculations have been performed with the Amsterdam Density Functional code ADF. A “fingerprint system” is introduced, which allows to assign electric field gradients to certain Ag coordinations in these crown thioether complexes.

Experimental Verification of Calculated Lattice Relaxations Around Impurities in CdTE

We have measured the lattice distortion around As (acceptor) and Br (donor) in CdTe with fluorescence detected X-ray absorption spectroscopy. We could experimentally verify the lattice relaxation with a bond length reduction of 8% around the As atom as inferred indirectly from ab initio calculations of the electric field gradient performed with the WIEN97 package in comparison with the measured value in a Perturbed Angular Correlation experiment as recently reported. We have complemented our own calculations of relaxation with WIEN97 with calculations using the FHI96md pseudo-potential program, which allows the use of larger super-cell sizes. Encouraged by the good agreement between experiment and model calculation for As in CdTe as well as similarly for the isovalent Se in CdTe, we extended our investigation to Br in CdTe, where the electric field gradient has also been measured, and could not only verify the derived lattice expansion around Br with our EXAFS analysis but additionally observe fractions of Br in the A-center as well as in a DX-center configuration.

Cd-vacancy andCd-interstitial complexes inSiandGe

The electrical field gradient (EFG), measured e.g. in perturbed angular correlation (PAC) experiments, gives particularly useful information about the interaction of probe atoms like 111In / 111Cd with other defects. The interpretation of the EFG is, however, a difficult task. This paper aims at understanding the interaction of Cd impurities with vacancies and interstitials in Si and Ge, which represents a controversial issue. We apply two complementary ab initio methods in the framework of density functional theory (DFT), (i) the all electron Korringa-Kohn-Rostoker (KKR) Greenfunction method and (ii) the Pseudopotential-Plane-Wave (PPW) method, to search for the correct local geometry. Surprisingly we find that both in Si and Ge the substitutional Cd-vacancy complex is unstable and relaxes to a split-vacancy complex with the Cd on the bond-center site. This complex has a very small EFG, allowing a unique assignment of the small measured EFGs of 54MHz in Ge and 28MHz in Si. Also, for the Cd-selfinterstitial complex we obtain a highly symmetrical split configuration with large EFGs, being in reasonable agreement with experiments.

Angular correlation measurements in 155Eu nuclei

A method of analysis for gamma angular correlation experiments was developed for the case of a multi-detector setup, tested with several known sources of radiation, and applied in the decay of the 155Eu nucleus. The method has shown good reliability. This method was applied for five cascades for the transitional nucleus 155Eu and produced the following results: d(65 keV) = 0.09(11), d(203 keV) = -0.44(17), d(246 keV) = 0.281(22), d(664 keV) = -0.231(21), d(1002 keV) = -0.35(6).

Different nature of magnetism at cerium sublattices in CeMn 2Si 2 and CeMn 2Ge 2 compounds

The low-temperature magnetic properties of Ce atoms in the CeMn 2Ge 2 and CeMn 2Si 2 were investigated by means of (1) perturbed angular correlation experiment, and (2) ab initio density-functional calculations. They are found to be fundamentally different, despite the Ce magnetic moment is measured to be zero in both compounds. While the Ce atoms are intrinsically non-magnetic in silicide, they display a latent magnetism in the CeMn 2Ge 2 since their zero total moment is composed of the finite spin and orbital components which cancel each other.

Defect interactions of group-I elements in cubic II-VI compounds

By means of perturbed \ensuremath{\gamma}\ensuremath{\gamma} angular correlation (PAC) experiments using the radioactive dopants ${}^{77}\mathrm{Br},$ ${}^{111}\mathrm{Ag},$ and ${}^{111}\mathrm{In},$ the behavior of the group-I elements Li, Cu, Ag, and Au is investigated. Group-I elements are shown to form nearest- (NN) and next-nearest-neighbor donor-acceptor pairs in the II-VI semiconductor CdTe. The simultaneous occurrence of cation vacancies indicates the transition of group-I elements from substitutional to interstitial lattice sites. In case of NN pairs, the electric-field gradients measured in the PAC experiments are compared with those obtained theoretically by density-functional theory calculations. PAC experiments performed in ZnTe and ZnSe show a similar behavior regarding the donor-acceptor pairing with group-I elements in these compounds.

Hyperfine interactions studies on Pt–In/Nb 2O 5 catalysts

Pt–In/Nb 2O 5 catalysts were investigated using temperature-programmed reduction (TPR) and time differential perturbed angular correlation (TDPAC) experiments. The results indicated the presence of In–O surface complexes for low loading In/Nb 2O 5 and Pt–In/Nb 2O 5 catalysts after calcination. These complexes did not form a In 2O 3 crystalline phase. After reduction of the Pt–In/Al 2O 3 catalyst, In is present in different states. A fraction of In atoms is bonded to niobia surface, as a surface complex that does not show crystalline structure similar to bulk In 2O 3. Other fraction of In atoms interacts with platinum, in the form of an alloy, in locations that present trigonal symmetry.

Beta-neutrino angular correlation in the decay of 14O

The positrons and coincident gamma-rays following the pure Fermi 0(+) --> 0(+) branch of beta(+)-decay in 140 were measured. The Doppler shift of the gamma-quantum observed is sensitive to the beta-neutrino correlation coefficient, which is unity (alpha = 1) in the absence of scalar coupling. The interatomic interaction of recoiling daughters was noticed, which has prevented to obtain an improved result for scalar coupling despite of the excellent statistical accuracy of the measurement. At the same time, it was shown that the method described can be used as a very sensitive and flexible tool to probe the interatomic interactions in matter at the hundred eV energy level. Perspectives in this direction as well as the future angular-correlation experiments aimed to search for scalar coupling in beta-decay are discussed.

First-principles study of the orthorhombicCdTiO3perovskite

In this work we perform an ab-initio study of CdTiO3 perovskite in its orthorhombic phase using FLAPW method. Our calculations help to decide between the different cristallographic structures proposed for this perovskite from X-Ray measurements. We compute the electric field gradient tensor (EFG) at Cd site and obtain excellent agreement with available experimental information from a perturbed angular correlation (PAC) experiment. We study EFG under an isotropic change of volume and show that in this case the widely used "point charge model approximation" to determine EFG works quite well.

Nuclear techniques characterization of short-range order in Zr-TM-Cu-Al-Ni ( TM= Hf, Ti, Fe) bulk metallic glasses

In this paper an overview of recent and new perturbed angular correlation (PAC), Mössbauer (MS) and positron annihilation (PAS) experiments on Zr- and Hf-based bulk metallic glasses is presented. PAC results showed that all samples in their as-prepared amorphous state, irrespective of the preparation method used, are well described by a dense random packing of ions (DRP). Hf-based samples showed a greater electric field gradient (EFG) value than Zr-based ones for same composition. A general observation is that the relaxed amorphous and supercooled liquid has the same local arrangements than the initial amorphous state. The crystallization behaviour is not preparation-method dependant but composition and impurities have a major influence on it. Preliminary positron annihilation lifetime measurements on amorphous samples yielded one characteristic lifetime, with a value intermediate between those for the well-annealed and defected metallic constituents of the alloys. Mössbauer spectra of as-prepared amorphous alloys consist of a broad asymmetric doublet, originating from a distribution of electric field gradients characteristic for amorphous samples.

A simple experimental setup to demonstrate the basics of positron emission tomography

We present a setup of a simplified positron emission tomograph (PET) which can be used in a third-year laboratory course for physics students. The equipment of a standard γ–γ angular correlation experiment was modified using bismuth-germanate crystals instead of NaI. Using this setup it is possible to locate several radioactive sources hidden in a closed box. The problems of two-dimensional imaging tomography using one-dimensional projections are illustrated as well as the problems encountered in medical PET examinations.

The Nuclear Quadrupole Interaction of 204mPb in Cadmium Monitored by γ–γ –Perturbed Angular Correlations

For the first time the nuclear probe 204mPb was produced at the on-line isotope separator ISOLDE at CERN and used for time differential perturbed angular correlation experiments. The electric field gradient of 204mPb at room temperature in Cd metal was determined to be = 19(1) 1021 V/m2. Ab initio-calculations of the electric field gradient for the impurities Pt to Bi in cadmium were performed with the full-potential linearized augmented plane waves code WIEN97 to interpret this result. For Au, Hg and Pb, where experimental results are now available, these agree with the calculations within 10 %.

Evolution of nuclear spectroscopy at Saha Institute of Nuclear Physics

Experimental studies of nuclear excitations have been an important subject from the earliest days when the institute was established. The construction of 4 MeV proton cyclotron was mainly aimed to achieve this goal. Early experiments in nuclear spectroscopy were done with radioactive nuclei with the help of beta and gamma ray spectrometers. Small NaI(Tl) detectors were used for gamma-gamma coincidence, angular correlation and life time measurements. The excited states nuclear magnetic moments were measured in perturbed gamma-gamma angular correlation experiments. A high transmission magnetic beta ray spectrometer was used to measure internal conversion coefficients and beta-gamma coincidence studies. A large number of significant contributions were made during 1950–59 using these facilities. Proton beam in the cyclotron was made available in the late 1950’s and together with 14 MeV neutrons obtained from a C-W generator a large number of short-lived nuclei were investigated during 1960’s and 1970’s. The introduction of high resolution Ge gamma detectors and the improved electronics helped to extend the spectroscopic work which include on-line (p 7 p′γ) and (p 7 nγ) reaction studies. Nuclear spectroscopic studies entered a new phase in the 1980’s with the availability of 40–80 MeV alpha beam from the variable energy cyclotron at VECC, Calcutta. A number of experimental groups were formed in the institute to study nuclear level schemes with (α 7 xnγ) reactions. Initially only two unsuppressed Ge detectors were used for coincidence studies. Later in 1989 five Ge detectors with a large six segmented NaI(Tl) multiplicitysum detector system were successfully used to select various channels in (α 7 xnγ) reactions. From 1990 to date a variety of medium energy heavy ions were made available from the BARC-TIFR Pelletron and the Nuclear Science Centre Pelletron. The state of the art gamma detector arrays in these centres enabled the Saha Institute groups to undertake more sophisticated experiments. Front line nuclear spectroscopy works are now being done and new informations are obtained for a large number of nuclei over a wide mass range. Currently Saha Institute is building a multi-element gamma heavy ion neutron array detector (MEGHNAD), which will have six high efficiency clover Ge detector together with charged particle ball and other accessories. The system is expected to be usable in 2002 and will be used in experiments using high energy heavy ions from VECC.

Identification of defects in semiconductors via their electric field gradients

Recent theoretical calculations show for defect complexes in semiconductors, characterized by electric field gradients (EFG), that their chemical compositions, electronic charge states and the induced lattice relaxations can be obtained by comparing the experimental and calculated EFG. The experimental data are obtained by perturbed γγ angular correlation experiments and the calculations are performed using ab initio full potential methods in the framework of density functional theory.

Perturbed angular correlation experiments on the pressure-induced structural modification of bovine serum albumin

The hydrodynamic behaviour of the bovine serum albumin (BSA) was studied by means of the Perturbed Angular Correlation (PAC) technique as a function of the hydrostatic pressure (up to 4.1 kbar) applied to the sample. The results have clearly shown that at moderated pressures (around 1.5 kbar) the BSA molecule suffers structural modifications which produces an increase of the molecular volume and the rotational correlation time of the molecule. About the reversibility of the process, our results indicate that the changes are fully irreversible. Our experiments are the first devoted to the study of the high-pressure behaviour of biological molecules using the PAC technique.