Ab initio QED Treatment of the Two-Photon Annihilation of Positrons with Bound Electrons

The process of a positron-bound-electron annihilation with simultaneous emission of two photons is investigated theoretically. A fully relativistic formalism based on an ab initio QED description of the process is worked out. The developed approach is applied to evaluate the annihilation of a positron with K-shell electrons of a silver atom, for which a strong contradiction between theory and experiment was previously stated. The results obtained here resolve this longstanding disagreement and, moreover, demonstrate a sizable difference with approaches so far used for calculations of the positron-bound-electron annihilation process, namely, Lee's and the impulse approximations.

Big picture of relativistic molecular quantum mechanics

Any quantum mechanical calculation on electronic structure ought to choose first an appropriate Hamiltonian H and then an Ansatz for parameterizing the wave function Ψ, from which the desired energy/property E(λ) can finally be calculated. Therefore, the very first question is: what is the most accurate many-electron Hamiltonian H? It is shown that such a Hamiltonian i.e. effective quantum electrodynamics (eQED) Hamiltonian, can be obtained naturally by incorporating properly the charge conjugation symmetry when normal ordering the second quantized fermion operators. Taking this eQED Hamiltonian as the basis, various approximate relativistic many-electron Hamiltonians can be obtained based entirely on physical arguments. All these Hamiltonians together form a complete and continuous ‘Hamiltonian ladder’, from which one can pick up the right one according to the target physics and accuracy. As for the many-electron wave function Ψ, the most intriguing questions are as follows. (i) How to do relativistic explicit correlation? (ii) How to handle strong correlation? Both general principles and practical strategies are outlined here to handle these issues. Among the electronic properties E(λ) that sample the electronic wave function nearby the nuclear region, nuclear magnetic resonance (NMR) shielding and nuclear spin-rotation (NSR) coupling constant are especially challenging: they require body-fixed molecular Hamiltonians that treat both the electrons and nuclei as relativistic quantum particles. Nevertheless, they have been formulated rigorously. In particular, a very robust ‘relativistic mapping’ between the two properties has been established, which can translate experimentally measured NSR coupling constants to very accurate absolute NMR shielding scales that otherwise cannot be obtained experimentally. Since the most general and fundamental issues pertinent to all the three components of the quantum mechanical equation HΨ = EΨ (i.e. Hamiltonian H, wave function Ψ, and energy/property E(λ)) have fully been understood, the big picture of relativistic molecular quantum mechanics can now be regarded as established.

Perspectives of relativistic quantum chemistry: the negative energy cat smiles

Given the remarkable advances in relativistic quantum chemistry, some conceptual aspects still remain to be addressed. Among others, the role of negative energy states (NES) in electron correlation and other properties requires most attention. Based on critical assessments of the configuration space (CS), no-photon (and no-time) Fock space (FS) and quantum electrodynamics (QED) approaches, it is concluded that only QED provides the correct prescription for the contributions of NES to correlation, while both CS and FS give rise to wrong results. This essentially means that one should work either with the no-pair approximation (which has an intrinsic error of order (Zα)(3)) or with QED. Whether a consistent relativistic many-electron theory does exist in between remains an open question. Even under the no-pair approximation, there still exists an issue arising from that the no-pair Hamiltonian is incompatible with explicitly correlated methods. It turns out that this can nicely be resolved by introducing the concept of extended no-pair projection. Apart from these take-home messages, other immediate prospects of relativistic quantum chemistry are also highlighted for guiding future developments and applications.

The physics of solid-state neutron detector materials and geometries

Detection of neutrons, at high total efficiency, with greater resolution in kinetic energy, time and/or real-space position, is fundamental to the advance of subfields within nuclear medicine, high-energy physics, non-proliferation of special nuclear materials, astrophysics, structural biology and chemistry, magnetism and nuclear energy. Clever indirect-conversion geometries, interaction/transport calculations and modern processing methods for silicon and gallium arsenide allow for the realization of moderate- to high-efficiency neutron detectors as a result of low defect concentrations, tuned reaction product ranges, enhanced effective omnidirectional cross sections and reduced electron-hole pair recombination from more physically abrupt and electronically engineered interfaces. Conversely, semiconductors with high neutron cross sections and unique transduction mechanisms capable of achieving very high total efficiency are gaining greater recognition despite the relative immaturity of their growth, lithographic processing and electronic structure understanding. This review focuses on advances and challenges in charged-particle-based device geometries, materials and associated mechanisms for direct and indirect transduction of thermal to fast neutrons within the context of application. Calorimetry- and radioluminescence-based intermediate processes in the solid state are not included.

What does positron emission tomography offer oncology?

The origins of positron emission tomography (PET) date back 70 years. Since the 1970s, however, its use has increased exponentially in the fields of neurology, cardiology and oncology. [18F]-Fluorodeoxyglucose (FDG) whole-body scanning is by far the most widely utilised and recognised application of PET in oncology. However, PET is a very versatile and powerful imaging modality capable of helping bridge the gap between the laboratory and the clinic. This article reviews the history and current applications of PET in oncology and then explores some of the newer applications and potential future uses of this versatile technology particularly in the area of cancer research.

A brief history of positron emission tomography (PET)

The development of positron emission tomography (PET) illustrates how advances in basic science translates into benefits for human beings. In 1930 Ernest Lawrence and co-workers conceived of the cyclotron. By 1938 Lawrence, Livingston, et al had designed a "medical cyclotron." The subsequent production of C-11, N-13, O-15, and F-18 found many uses in medical and physiologic research. The introduction of F-18 deoxyglucose represents another major step toward practical clinical use of positron-emitting tracers. We have now achieved the transition from the postulation of the existence of positrons to their use in a wide variety of diseases.