Optical-model potential for electron and positron elastic scattering by atoms

An optical-model potential for systematic calculations of elastic scattering of electrons and positrons by atoms and positive ions is proposed. The electrostatic interaction is determined from the Dirac-Hartree-Fock self-consistent atomic electron density. In the case of electron projectiles, the exchange interaction is described by means of the local-approximation of Furness and McCarthy. The correlation-polarization potential is obtained by combining the correlation potential derived from the local density approximation with a long-range polarization interaction, which is represented by means of a Buckingham potential with an empirical energy-dependent cutoff parameter. The absorption potential is obtained from the local-density approximation, using the Born-Ochkur approximation and the Lindhard dielectric function to describe the binary collisions with a free-electron gas. The strength of the absorption potential is adjusted by means of an empirical parameter, which has been determined by fitting available absolute elastic differential cross-section data for noble gases and mercury. The Dirac partial-wave analysis with this optical-model potential provides a realistic description of elastic scattering of electrons and positrons with energies in the range from ~100 eV up to ~5 keV. At higher energies, correlation-polarization and absorption corrections are small and the usual static-exchange approximation is sufficiently accurate for most practical purposes

HIV Risk Among Men Who Have Sex With Men, Women Who Have Sex With Women, Lesbian, Gay, Bisexual and Transgender Populations in South Africa: A Mini-Review

BACKGROUND: The HIV epidemic in South Africa is characterized mainly by heterosexual transmission. Recently, the importance of targeting key populations and marginalized groups, including men who have sex with men (MSM) and transgender people, has been added to the national agenda. OBJECTIVES: This mini-review explores the current state of empirical research on HIV risk and MSM, women who have sex with women (WSW), lesbian, gay, bisexual and transgender (LGBT) populations in South Africa in order to assess the current state of research and identify gaps in the literature. METHOD: Peer-reviewed empirical social and behavioral articles on HIV prevalence and risk focusing on MSM, WSW, and LGBT populations published since 2006 were included in this mini-review. RESULTS: In total 35 articles were included: 30 on MSM, gay, and/or bisexual male-identified populations, three on WSW, lesbian, and/or bisexual female-identified populations, two on LGB youth, and none on transgender populations. CONCLUSION: Despite South Africa being the country with the largest number of people living with HIV in the world, there is a limited amount of research in South Africa on HIV and non-normative gender identities and sexualities, especially WSW, lesbian, and/or bisexual female-identified populations, transgender populations, and LGB youth. Research with MSM, WSW, and LGBT populations should be prioritized in South Africa in order to appropriately inform HIV prevention strategies that meet the specific needs of these marginalized groups