Many-body theory calculations of positron binding to negative ions

A many-body theory approach developed by the authors [Phys. Rev. A 70, 032720 (2004)] is applied to positron bound states and annihilation rates in atomic systems. Within the formalism, full account of virtual positronium (Ps) formation is made by summing the electron-positron ladder diagram series, thus enabling the theory to include all important many-body correlation effects in the positron problem. Numerical calculations have been performed for positron bound states with the hydrogen and halogen negative ions, also known as Ps hydride and Ps halides. The Ps binding energies of 1.118, 2.718, 2.245, 1.873 and 1.393 eV and annihilation rates of 2.544, 2.482, 1.984, 1.913 and 1.809 ns$^{-1}$, have been obtained for PsH, PsF, PsCl, PsBr and PsI, respectively.Comment: 19 pages, 13 figures, submitted to International Review of Atomic and Molecular Physic

Privatising public prisons: Penality, law and practice

In October 2011, HM Prison Birmingham was transferred from public to private management, under G4S. This was the first time that an existing operational public prison was privatised in the UK. The move marked the third and most far reaching phase of prison privatisation policy, and was intended both to increase quality of life for prisoners, from a low baseline, and to reduce costs. Prior to 2011, private prisons had all been new-builds. Private contractors had thus far avoided the additional challenges of inheriting a pre-existing workforce and operating in old, often unsuitable, buildings. This article reports on a longitudinal evaluation of the complex process of the transition, and some outcomes for both staff and prisoners. As an experiment in the reorganisation of work and life in a ‘traditional’ public sector prison, the exercise was unprecedented, and has set the agenda for future transformations. The example illustrates the intense, distinctive and rapidly changing nature of penality as it makes itself felt in the lived prison experience, and raises important questions about the changing use of State power. The MQPL, G4S, National Offender Management Servic

van der Waals coefficients for positronium interactions with atoms

The random-phase approximation with exchange (RPAE) is used with a $B$-spline basis to compute dynamic dipole polarizabilities of noble-gas atoms and several other closed-shell atoms (Be, Mg, Ca, Zn, Sr, Cd, and Ba). From these, values of the van der Waals $C_6$ constants for positronium interactions with these atoms are determined and compared with existing data. Our best predictions of $C_6$ for Ps--noble-gas pairs are expected to be accurate to within 1%, and to within a few per cent for the alkaline earths. We also used accurate dynamic dipole polarizabilities from the literature to compute the $C_6$ coefficients for the alkali-metal atoms. Implications of increased $C_6$ values for Ps scattering from more polarizable atoms are discussed.Comment: 6 pages, submitted to Physical Review

Determination of the 5d6s 3D1 state lifetime and blackbody radiation clock shift in Yb

The Stark shift of the ytterbium optical clock transition due to room temperature blackbody radiation is dominated by a static Stark effect, which was recently measured to high accuracy [J. A. Sherman et al., Phys. Rev. Lett. 108, 153002 (2012)]. However, room temperature operation of the clock at 10^{-18} inaccuracy requires a dynamic correction to this static approximation. This dynamic correction largely depends on a single electric dipole matrix element for which theoretically and experimentally derived values disagree significantly. We determine this important matrix element by two independent methods, which yield consistent values. Along with precise radiative lifetimes of 6s6p 3P1 and 5d6s 3D1, we report the clock's blackbody radiation shift to 0.05% precision

Positron scattering and annihilation on noble gas atoms

Positron scattering and annihilation on noble gas atoms below the positronium formation threshold is studied ab initio using many-body theory methods. The many-body theory provides a near-complete understanding of the positron-noble-gas-atom system at these energies and yields accurate numerical results. It accounts for positron-atom and electron-positron correlations, e.g., polarization of the atom by the incident positron and the non-perturbative process of virtual positronium formation. These correlations have a large effect on the scattering dynamics and result in a strong enhancement of the annihilation rates compared to the independent-particle mean-field description. Computed elastic scattering cross sections are found to be in good agreement with recent experimental results and Kohn variational and convergent close-coupling calculations. The calculated values of the annihilation rate parameter $Z_{\rm eff}$ (effective number of electrons participating in annihilation) rise steeply along the sequence of noble gas atoms due to the increasing strength of the correlation effects, and agree well with experimental data.Comment: 24 pages, 17 figure

High accuracy measure of atomic polarizability in an optical lattice clock

Despite being a canonical example of quantum mechanical perturbation theory, as well as one of the earliest observed spectroscopic shifts, the Stark effect contributes the largest source of uncertainty in a modern optical atomic clock through blackbody radiation. By employing an ultracold, trapped atomic ensemble and high stability optical clock, we characterize the quadratic Stark effect with unprecedented precision. We report the ytterbium optical clock's sensitivity to electric fields (such as blackbody radiation) as the differential static polarizability of the ground and excited clock levels: 36.2612(7) kHz (kV/cm)^{-2}. The clock's fractional uncertainty due to room temperature blackbody radiation is reduced an order of magnitude to 3 \times 10^{-17}.Comment: 5 pages, 3 figures, 2 table