The mean lifetime of delocalized parapositronium in alpha-SiO2 has been determined directly for the first time using a newly developed positron lifetime spectrometer based on BaF2 scintillators and a fast digital oscilloscope. The lifetime is found to be 156+/-4 ps, which is much longer than its intrinsic lifetime of 125 ps. This indicates clearly that the primary many-body effect on positronium in alpha-SiO2 is the screening of the Coulomb interaction between the constituent particles by electrons of the medium and that the average distance between the electron and positron in positronium becomes larger than its vacuum value.

We present high-resolution Compton scattering spectra from Li single crystals together with corresponding highly accurate local-density-approximation (LDA) based computations.The data are analyzed to obtain Fermi surface radii along the three principal symmetry directions; the maximum measured anisotropy is found to be (4.6~1.0)%. Comparisons between the measured and computed spectra clearly reveal departures from the conventional one-particle LDA picture of the ground state momentum density of the electron gas. This study establishes the potential of Compton scattering as a tool for investigating Fermiology-related issues in materials. PACS numbers: 61.80.Ed, 41.60.Ap, 71.25.Hc, 71.45.Gm It is well known that in a Compton scattering experiment one measures the momentum distribution [1](2) J(p~) = n(p) dp~dpy,where n(p) is the ground state electron momentum density, 2 n(p) = g W;(r) exp(ip . r) dr in terms of the electron wave functions 'Ir;(r). The summation in (2) extends over all occupied states. Therefore, the Compton profile J(p, ) contains fingerprints of Fermi surface (FS) breaks in the underlying 3D momentum distribution n(p). The size of the FS discontinuity in the momentum density and its possible renormalization due to electron-electron correlations is a fundamental property of the ground state electronic structure, inaccessible to other k-resolved spectroscopies such as angle-resolved photoemission, the de Haasevan Alphen effect (dHvA), and positron annihilation [2]. This unique capability of the Compton technique for exploring Fermiology-related issues has, however, been difficult to exploit to date because the momentum resolution available using y-ray sources is not adequate for this purpose. The advent of high intensity, high energy, well polarized synchrotron sources removes this limitation and offers new opportunities for developing Compton scattering as a tool for investigating spectral properties of the electron gas at and near the Fermi energy in wide classes of materials [3].With this motivation, we present in this Letter a highresolution synchrotron-based Compton study of Li single crystals [4], and identify, for the first time, Fermi surface signatures in the data. The presence of a 2D integral in Eq. (1) implies that the structure associated with FS cross-ings will generally be more apparent in the derivatives of J(p, ), rather than J(p, ) itself. Accordingly, parallel highly accurate computations of the Compton profiles (CP's) within the band theory framework are reported; we are not aware of a previous calculation of CP's in the literature where the line shape in the derivative spectra has been properly computed. Comparisons between the measured and computed Compton spectra clearly reveal departures from the simple one-particle local-densityapproximation (LDA) based picture of the momentum density.It is noteworthy that the Compton experiment essentially measures a ground state property of the electron gas [5] in contrast with techniques such as dHvA and angle-resolved photoemission. Als...

The momentum-transfer cross section for positronium-He scattering has been determined by analysing the thermalization process of orthopositronium in the gas. The momentum distribution of the positronium is observed by using the one-dimensional angular correlation of the annihilation radiation method. Silica aerogel is used to form a sufficient amount of positronium in a restricted region necessary for the high-resolution measurements. The average energies of orthopositronium for mean lifetimes ranging from 3.2 to 86 ns are determined by applying static magnetic fields. A thermalization model which includes the momentum-transfer cross section as an adjustable parameter is fitted to the average energy. The cross section thus obtained is σ m = (11 ± 3) × 10 −16 cm 2 for positronium in the energy range below 0.3 eV.

Angular correlation of annihilation radiation (ACAR) from silica-powder pellets and silica aerogel has been measured in order to investigate the slowing down of free positronium (Ps) atoms by collisions with silica grains and gas molecules. The data for the pellets and the aerogel in vacuum show that the slowing down of parapositronium (p-Ps) in the free space between the silica grains depends on the number of collisions and hence on the mean distance between the grains. The momentum distribution of orthopositronium (o-Ps) shows further slowing down because of its long lifetime. From the ACAR data obtained from specimens of aerogel 611ed with gases (He, Ne, Ar, Kr, Xe, H2, CH4, CO2, and iso-C4Hqo), the momentum-transfer cross sections between Ps and the gas molecules are estimated. It is concluded that the Ps kinetic energy is transferred only to the translational motion of the gas molecules, i.e. , the excitations of vibration and rotation of the molecules are negligible. PACS number(s): 36.10.Dr, 34.50. -s, 78.70.Bj I. INTR. ODU CTIONIt was observed in the mid 1960s that the angular correlation of annihilation radiation (ACAR) and positron lifetime spectra in metal oxide and metal Huoride powders [1,2] showed formation of positronium (Ps). In 1968, Paulin and Ambrosino [3] reported that the Ps component in the positron lifetime spectra for silica powders depends on the grain diameter. It was postulated that the Ps atoms form inside the grains and then disuse out of them [4]. Paulin and Ambrosino also observed the eA'ect of air on the o-Ps annihilation. Following this, silica powders were used for investigating the interactions between Ps and paramagnetic gases [5 -8].

No abstract