Home  About us  Contact
 

Annihilation Radiation (annihilation + radiation)

Distribution by Scientific Domains
Physics and Astronomy100%

Selected Abstracts

Vacancy-type defects and electronic structure of perovskite-oxide SrTiO3 from positron annihilation

PHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 2 2006
A. S. Hamid
Abstract The vacancy-type defects in Nb-doped SrTiO3 and in undoped SrTiO3, annealed in H2 flow, were investigated by means of positron lifetime and 2D angular correlation of annihilation radiation (ACAR) experiments. The calculations of the lifetime of positron were performed by using atomic superposition (AT-SUP) method. The results showed that positrons annihilate from a free state in the Nb-doped SrTiO3. The trapping centers in the annealed sample were found to be oxygen vacancies VO associated with relaxation of the surrounding ions. Moreover, the momentum distributions of the samples studied were correlated to the variation of their electronic structure. It was proposed from the drastic change in the momentum distribution upon introduction of VO, that 2D-ACAR technique is a sensitive tool for acquiring information on the electronic and bond structure of the perovskite-oxides. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Spin dependent distribution and Fermi surface of the perovskite manganite compound La0.7Sr0.3MnO3 via 2D-ACAR measurements,

PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 2 2004
A. S. Hamid
Abstract Using 2D angular correlation of positron annihilation radiation (ACAR) experiment, we have performed a systematic study of the spin dependent and Fermi surface of the colossal magneto-resistance CMR La0.7Sr0.3MnO3. The measurements have been carried out using a re-versal magnetic field direction (parallel and anti-parallel to the direction of motion of the polarized posi-trons). The measured spectra have been investigated in the momentum space as well as in the wave vector space. They revealed information about the hybridization effect of Mn(3d eg1) and O(2p) like states. Further, the results showed that the majority spin electrons predominated at the Fermi level. From another perspective, the Fermi surface of La0.7Sr0.3MnO3 revealed a cuboids hole surface centered on R point and a spheroid electron surface centered on , point. A comparison with the earlier results showed qualitative agreement. However, the current results could reveal the dimension of the electron surface centered on , point that was predicted in the earlier 2D-ACAR measurements. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Positron annihilation studies in plastically deformed Fe-Mn-Si-Cr-Ni-C

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 10 2007
J. De Baerdemaeker
Abstract Fe-Mn-Si-Cr-Ni-C samples with deformations from 2 to 20% were investigated using positron annihilation spectroscopy, optical microscopy (OM) and X-ray diffraction (XRD). Both Doppler broadening of the annihilation radiation (DBAR) and positron annihilation lifetime spectroscopy (PALS) measurements were performed. The DBAR experiments, carried out using a slow positron beam, indicate a sudden increase in the defect concentration in between 4 and 6% deformation. This is confirmed by the lifetime measurements. In between 6% and 8% deformation the lineshape parameter significantly changes which coincides with the drastic increase of the concentration of the martensite phase as confirmed by XRD. Positron annihilation spectroscopy indirectly probe the ,/, martensitic phase transformation by probing correlated dislocations. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Irradiated polymethylmethacrylate studied by positron annihilation spectroscopy

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 10 2007
Hamdy F. M. Mohamed
Abstract The effect of gamma irradiation dose on microscopic structure of polymethylmethacrylate (PMMA) has been studied using positron annihilation lifetime (PAL) and Doppler broadening of annihilation radiation (DBAR) techniques. The measurements were performed at room temperature as a function of the ,-irradiation doses from 60-1200 kGy. The observed lifetime spectra were resolved into three components. The size and the fraction of the o-Ps hole volume were estimated from the positron annihilation parameters. The effect of ,-irradiation was identified at lower doses to be cross-linking while at higher doses the chemical degradation of the polymer was observed. Moreover, the distribution of the free volume shifts from a large to small size as the irradiation increases and has very similar Gaussian-like distribution. The PAL results were confirmed with X-ray measurement. A correlation between the macroscopic mechanical properties Hv and positron annihilation parameters has been demonstrated. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Many body effects seen in the positron annihilation experiment

PHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 10 2007
G. Kontrym-Sznajd
Abstract The electron-positron (e-p) momentum density in p -space for copper, magnesium, cadmium and yttrium, calculated by using various theoretical approaches, are compared with corresponding densities reconstructed from two-dimensional angular correlation of annihilation radiation (2D ACAR) experimental spectra. We demonstrate that (at least for positrons in metals) strong electron-electron (e-e) correlations are clearly visible not only in the Compton scattering but also in positron annihilation experiments. Moreover, a proper description of e-p correlations needs to include the lattice-periodical crystal potential. In case of strong lattice effects, the increase of the e-p enhancement with increasing momentum - the so-called Kahana-like enhancement, is vanishing. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]