Gamma-ray Spectra and Enhancement Factors for Positron Annihilation with Core Electrons

Many-body theory is developed to calculate the γ spectra for positron annihilation in noble-gas atoms. Inclusion of electron-positron correlation effects and core annihilation gives spectra in excellent agreement with experiment [K. Iwata et al., Phys. Rev. Lett. 79, 39 (1997)]. The calculated correlation enhancement factors γnl for individual electron orbitals nl are found to scale with the ionization energy I_nl (in eV), as γnl=1+ √A/Inl+(B/I_nl)β, where A≈40  eV, B≈24  eV, and β≈2.3.

Modulated electrostatic modes in pair plasmas: Modulational stability profile and envelope excitations

A pair plasma consisting of two types of ions, possessing equal masses and opposite charges, is considered. The nonlinear propagation of modulated electrostatic wave packets is studied by employing a two-fluid plasma model. Considering propagation parallel to the external magnetic field, two distinct electrostatic modes are obtained, namely a quasiacoustic lower moddfe and a Langmuir-like, as optic-type upper one, in agreement with experimental observations and theoretical predictions. Considering small yet weakly nonlinear deviations from equilibrium, and adopting a multiple-scale technique, the basic set of model equations is reduced to a nonlinear Schrodinger equation for the slowly varying electric field perturbation amplitude. The analysis reveals that the lower (acoustic) mode is stable and may propagate in the form of a dark-type envelope soliton (a void) modulating a carrier wave packet, while the upper linear mode is intrinsically unstable, and may favor the formation of bright-type envelope soliton (pulse) modulated wave packets. These results are relevant to recent observations of electrostatic waves in pair-ion (fullerene) plasmas, and also with respect to electron-positron plasma emission in pulsar magnetospheres. (c) 2006 American Institute of Physics.

Electrostatic mode envelope excitations in e-p-i plasmas - application in warm pair ion plasmas with a small fraction of stationary ions

The nonlinear propagation of amplitude-modulated electrostatic wavepackets in an electron-positron-ion (e-p-i) plasma is considered, by employing a two-fluid plasma model. Considering propagation parallel to the external magnetic field, two distinct electrostatic modes are obtained, namely a quasi-thermal acoustic-like lower mode and a Langmuir-like optic-type upper one. These results equally apply in warm pair ion ( e. g. fullerene) plasmas contaminated by a small fraction of stationary ions ( or dust), in agreement with experimental observations and theoretical predictions in pair plasmas. Considering small yet weakly nonlinear deviations from equilibrium, and adopting a multiple-scales perturbation technique, the basic set of model equations is reduced to a nonlinear Schrodinger (NLS) equation for the slowly varying electric field perturbation amplitude. The analysis reveals that the lower ( acoustic) mode is mostly stable for large wavelengths, and may propagate in the form of a dark-type envelope soliton ( a void) modulating a carrier wavepacket, while the upper linear mode is intrinsically unstable, and thus favours the formation of bright-type envelope soliton ( pulse) modulated wavepackets. The stability ( instability) range for the acoustic ( Langmuir-like optic) mode shifts to larger wavenumbers as the positive-to-negative ion temperature ( density) ratio increases. These results may be of relevance in astrophysical contexts, where e-p-i plasmas are encountered, and may also serve as prediction of the behaviour of doped ( or dust-contaminated) fullerene plasmas, in the laboratory.

Acoustic solitary waves in dusty and/or multi-ion plasmas with cold, adiabatic, and hot constituents

Large nonlinear acoustic waves are discussed in a four-component plasma, made up of two superhot isothermal species, and two species with lower thermal velocities, being, respectively, adiabatic and cold. First a model is considered in which the isothermal species are electrons and ions, while the cooler species are positive and/or negative dust. Using a Sagdeev pseudopotential formalism, large dust-acoustic structures have been studied in a systematic way, to delimit the compositional parameter space in which they can be found, without restrictions on the charges and masses of the dust species and their charge signs. Solitary waves can only occur for nonlinear structure velocities smaller than the adiabatic dust thermal velocity, leading to a novel dust-acoustic-like mode based on the interplay between the two dust species. If the cold and adiabatic dust are oppositely charged, only solitary waves exist, having the polarity of the cold dust, their parameter range being limited by infinite compression of the cold dust. However, when the charges of the cold and adiabatic species have the same sign, solitary structures are limited for increasing Mach numbers successively by infinite cold dust compression, by encountering the adiabatic dust sonic point, and by the occurrence of double layers. The latter have, for smaller Mach numbers, the same polarity as the charged dust, but switch at the high Mach number end to the opposite polarity. Typical Sagdeev pseudopotentials and solitary wave profiles have been presented. Finally, the analysis has nowhere used the assumption that the dust would be much more massive than the ions and hence, one or both dust species can easily be replaced by positive and/or negative ions and the conclusions will apply to that plasma model equally well. This would cover a number of different scenarios, such as, for example, very hot electrons and ions, together with a mix of adiabatic ions and dust (of either polarity) or a very hot electron-positron mix, together with a two-ion mix or together with adiabatic ions and cold dust (both of either charge sign), to name but some of the possible plasma compositions.

Low-frequency electrostatic defect mode in doped pair-ion plasmas

The nonlinear amplitude modulation dynamics of electrostatic oscillations of massive charged defects in a three-component pair plasma is investigated; i.e. doped pair-ion plasmas (anticipating the injection of a massive charged component in the background; e.g. in fullerene experiments). Ton-acoustic oscillations in electron-positron-ion (e-p-i) plasmas are also covered, in the appropriate limit. Linear and nonlinear effects (MI, envelope modes) are discussed. The role of the temperature and density ratio between the pair species is stressed.

Nonlinear Dynamics of Rotating Multi-Component Pair Plasmas and e-p-i Plasmas

The propagation of small amplitude stationary profile nonlinear electrostatic excitations in a pair plasma is investigated, mainly drawing inspiration from experiments on fullerene pair-ion plasmas. Two distinct pair ion species are considered of opposite polarity and same mass, in addition to a massive charged background species, which is assumed to be stationary, given the frequency scale of interest. In the pair-ion context, the third species is thought of as a background defect (e.g. charged dust) component. On the other hand, the model also applies formally to electron-positron-ion (e-p-i) plasmas, if one neglects electron-positron annihilation. A two-fluid plasma model is employed, incorporating both Lorentz and Coriolis forces, thus taking into account the interplay between the gyroscopic (Larmor) frequency ?c and the (intrinsic) plasma rotation frequency O0. By employing a multi-dimensional reductive perturbation technique, a Zakharov-Kuznetsov (ZK) type equation is derived for the evolution of the electric potential perturbation. Assuming an arbitrary direction of propagation, with respect to the magnetic field, we derive the exact form of nonlinear solutions, and study their characteristics. A parametric analysis is carried out, as regards the effect of the dusty plasma composition (background number density), species temperature(s) and the relative strength of rotation to Larmor frequencies. It is shown that the Larmor and mechanical rotation affect the pulse dynamics via a parallel-to-transverse mode coupling diffusion term, which in fact diverges at ?c ? ±2O0. Pulses collapse at this limit, as nonlinearity fails to balance dispersion. The analysis is complemented by investigating critical plasma compositions, in fact near-symmetric (T- ˜ T+) “pure” (n- ˜ n+) pair plasmas, i.e. when the concentration of the 3rd background species is negligible, case in which the (quadratic) nonlinearity vanishes, so one needs to resort to higher order nonlinear theory. A modified ZK equation is derived and analyzed. Our results are of relevance in pair-ion (fullerene) experiments and also potentially in astrophysical environments, e.g. in pulsars.

Slowly fading super-luminous supernovae that are not pair-instability explosions

Super-luminous supernovae that radiate more than 10 44 ergs per second at their peak luminosity have recently been discovered in faint galaxies at redshifts of 0.1-4. Some evolve slowly, resembling models of 'pair-instability' supernovae. Such models involve stars with original masses 140-260 times that of the Sun that now have carbon-oxygen cores of 65-130 solar masses. In these stars, the photons that prevent gravitational collapse are converted to electron-positron pairs, causing rapid contraction and thermonuclear explosions. Many solar masses of 56 Ni are synthesized; this isotope decays to 56 Fe via 56 Co, powering bright light curves. Such massive progenitors are expected to have formed from metal-poor gas in the early Universe. Recently, supernova 2007bi in a galaxy at redshift 0.127 (about 12 billion years after the Big Bang) with a metallicity one-third that of the Sun was observed to look like a fading pair-instability supernova. Here we report observations of two slow-to-fade super-luminous supernovae that show relatively fast rise times and blue colours, which are incompatible with pair-instability models. Their late-time light-curve and spectral similarities to supernova 2007bi call the nature of that event into question. Our early spectra closely resemble typical fast-declining super-luminous supernovae, which are not powered by radioactivity. Modelling our observations with 10-16 solar masses of magnetar-energized ejecta demonstrates the possibility of a common explosion mechanism. The lack of unambiguous nearby pair-instability events suggests that their local rate of occurrence is less than 6 × 10 -6 times that of the core-collapse rate. © 2013 Macmillan Publishers Limited. All rights reserved.

Electron shell contributions to gamma-ray spectra of positron annihilation in noble gases

Gamma-ray positron annihilation spectra of the noble gases are simulated using computational chemistry tools for the bound electron wavefunctions and plane-wave approximation for the low-energy positron. The present annihilation line shapes, i.e. the full width at half maximum, Delta epsilon, of the gamma-ray annihilation spectra for He and Ar (valence) agree well with available independent atomic calculations using a different algorithm. For other noble gases they achieve moderate agreement with the experimental measurements. It is found that the contributions of various atomic electron shells to the spectra depend significantly on their principal quantum number n and orbital angular momentum quantum number l. The present study further reveals that the outermost ns electrons of the noble gases exhibit spectral line shapes in close agreement with those measured, indicating (as expected) that the measurements are not due to a simple sum over the momentum densities for all atomic electrons. The robust nature of the present approach makes it possible for us to proceed to more complex molecular systems using the tools of modern computational chemistry.

Guiding of Relativistic Electron Beams in Solid Targets by Resistively Controlled Magnetic Fields

Guided transport of a relativistic electron beam in solid is achieved experimentally by exploiting the strong magnetic fields created at the interface of two metals of different electrical resistivities. This is of substantial relevance to the Fast Ignitor approach to fusion energy production [M. Tabak et al., Phys. Plasmas 12, 057305 (2005)], since it allows the electron deposition to be spatially tailored-thus adding substantial design flexibility and preventing inefficiencies due to electron beam spreading. In the experiment, optical transition radiation and thermal emission from the target rear surface provide a clear signature of the electron confinement within a high resistivity tin layer sandwiched transversely between two low resistivity aluminum slabs. The experimental data are found to agree well with numerical simulations.

Observations of the filamentation of high-intensity laser-produced electron beams

Filamented electron beams have been observed to be emitted from the rear of thin solid targets irradiated by a high-intensity short-pulse laser when there is low-density plasma present at the back of the target. These. observations are consistent with a laser-generated beam of relativistic electrons propagating through the, target. which is subsequently fragmented by a Weibel-like instability in the low-density plasma at the. rear. These, measurements are in agreement with particle-in-cell simulations and theory, since the filamentation instability is predicted to be dramatically enhanced when the electron beam density approaches that of the background plasma.

Laser generation of proton beams for the production of short-lived positron emitting radioisotopes

Protons of energies up to 37 MeV have been generated when ultra-intense lasers (up to 10(20) W cm(-2)) interact with hydrogen containing solid targets. These protons can be used to induce nuclear reactions in secondary targets to produce P-emitting nuclei of relevance to the nuclear medicine community, namely C-11 and N-13 via (p, n) and (p, alpha) reactions. Activities of the order of 200 kBq have been measured from a single laser pulse interacting with a thin solid target. The possibility of using ultra-intense lasers to produce commercial amounts of short-lived positron emitting sources for positron emission tomography (PET) is discussed. (C) 2001 Elsevier Science B.V. All rights reserved.