Pair Creation Threshold Research Articles

Measurement of the Isolated Nuclear Two-Photon Decay in ^{72}Ge.

The nuclear two-photon or double-gamma (2γ) decay is a second-order electromagnetic process whereby a nucleus in an excited state emits two gamma rays simultaneously. To be able to directly measure the 2γ decay rate in the low-energy regime below the electron-positron pair-creation threshold, we combined the isochronous mode of a storage ring with Schottky resonant cavities. The newly developed technique can be applied to isomers with excitation energies down to ∼100 keV and half-lives as short as ∼10 ms. The half-life for the 2γ decay of the first-excited 0^{+} state in bare ^{72}Ge ions was determined to be 23.9(6)ms, which strongly deviates from expectations.

All-Order Coulomb Corrections to Delbrück Scattering above the Pair-Production Threshold.

We report calculations of Delbrück scattering that include all-order Coulomb corrections for photon energies above the threshold of electron-positron pair creation. Our approach is based on the application of the Dirac-Coulomb Green's function and accounts for the interaction between the virtual electron-positron pair and the nucleus to all orders in the nuclear binding strength parameter αZ. Practical calculations are performed for the scattering of 2.754MeV photons off plutonium atoms. We find that including the Coulomb corrections enhances the scattering cross section by up to 50% in this case. The obtained results resolve the long-standing discrepancy between experimental data and theoretical predictions and demonstrate that an accurate treatment of the Coulomb corrections is crucial for the interpretation of existing and guidance of future Delbrück scattering experiments on heavy atoms.

Electron-positron pair creation in the electric fields generated by micro-bubble implosions

We show that electron-positron pair production from the vacuum is possible via the strong Coulomb fields generated by micro-bubble implosions induced by ultra-high intensity lasers. Even in the case where the Coulomb fields are lower than the pair creation threshold, externally injected high energy electrons or photons could be used to generate pairs.

Vacuum Birefringence in a Supercritical Magnetic Field

The birefringence effect in vacuum in strong magnetic fields has been considered. The polarization tensor in a constant external magnetic field is analyzed in the framework of quantum field theory and on the basis of the electron Green’s function calculated as the sum over the Landau levels. The case of the lowest Landau levels for photons with the energies below the electron-positron pair creation threshold is considered, and the corresponding refractive indices of the physical vacuum for anomalous and normal waves are determined.

Polarization operator of a photon in a magnetic field

In the first order of the fine structer constant, the polarization operator of a photon is investigated in a constant and homogeneous magnetic field at arbitrary photon energies. For weak and strong fields H, compared with the Schwinger critical field, approximate expressions have been found. We consider the pure quantum region of photon energy near the threshold of pair creation, as well as the region of high energy levels where the quasiclassical approximation is valid. The general formula has been obtained for the effective mass of photon with given polarization. It is useful for an analysis of the problem under consideration on the whole and at a numerical work

TE resonances in graphene-dielectric structures

We investigate the dispersion relations of TE resonances in different graphene-dielectric structures. Previous work has shown that when a graphene layer is brought into contact with a dielectric material, a gap can appear in its electric band structure. This allows for the formation of TE-plasmons with unusual dispersion relations. In addition, if the dielectric has a finite thickness, graphene strongly modifies the behavior of the waveguiding modes by introducing dissipation above a well-defined cutoff frequency, thus providing the possibility of mode filtering. This cutoff and the properties of TE-plasmons are closely related to the pair-creation threshold of graphene, thus representing quantum mechanical effects that manifest themselves in the electromagnetic response.

THE EXTRAGALACTIC BACKGROUND LIGHT, THE HUBBLE CONSTANT, AND ANOMALIES: CONCLUSIONS FROM 20 YEARS OF TeV GAMMA-RAY OBSERVATIONS

Ground-based observatories have been collecting 0.2 - 20 TeV gamma rays from blazars for about twenty years. These gamma rays can experience absorption along the line of sight due to interactions with the extragalactic background light (EBL). In this paper, we show that the gamma-ray optical depth can be reduced to the convolution product of an EBL kernel with the EBL intensity, assuming a particular form for the EBL evolution. We extract the absorption signal from the most extensive set of TeV spectra from blazars collected so far and unveil a broad-band EBL spectrum from mid-ultraviolet to far infrared. This spectrum is in good agreement with the accumulated emission of galaxies, constraining unresolved populations of sources. We propose a data-driven estimate of the Hubble constant based on the comparison of local and gamma-ray measurements of the EBL. After setting stringent upper-limits on the redshift of four TeV blazars, we investigate the 106 gamma-ray spectra in our sample and find no significant evidence for anomalies. The intrinsic TeV spectra are not harder than their GeV counterpart, and no spectral upturn is visible at the highest optical depths. Finally, we investigate a modification of the pair-creation threshold due to Lorentz invariance violation. A mild excess prevents us from ruling out an effect at the Planck energy, and we constrain for the first time the energy scale of the modification to values larger than sixty percent of the Planck energy.

Compton scattering in strong magnetic fields: Spin-dependent influences at the cyclotron resonance

The quantum electrodynamical (QED) process of Compton scattering in strong magnetic fields is commonly invoked in atmospheric and inner magnetospheric models of x-ray and soft gamma-ray emission in high-field pulsars and magnetars. A major influence of the field is to introduce resonances at the cyclotron frequency and its harmonics, where the incoming photon accesses thresholds for the creation of virtual electrons or positrons in intermediate states with excited Landau levels. At these resonances, the effective cross section typically exceeds the classical Thomson value by over 2 orders of magnitude. Near and above the quantum critical magnetic field of 44.13 TeraGauss, relativistic corrections must be incorporated when computing this cross section. This paper presents formalism for the QED magnetic Compton differential cross section valid for both subcritical and supercritical fields, yet restricted to scattered photons that are below pair creation threshold. Calculations are developed for the particular case of photons initially propagating along the field, mathematically simple specializations that are germane to interactions involving relativistic electrons frequently found in neutron star magnetospheres. This exposition of relativistic, quantum, magnetic Compton cross sections treats electron spin dependence fully, since this is a critical feature for describing the finite decay lifetimes of the intermediate states. The formalism employs both the Johnson and Lippmann (JL) wave functions and the Sokolov and Ternov (ST) electron eigenfunctions of the magnetic Dirac equation. The ST states are formally correct for self-consistently treating spin-dependent effects that are so important in the resonances. Relatively compact analytic forms for the cross sections are presented that will prove useful for astrophysical modelers.

Resonant Compton scattering associated with pair creation

Studies of Compton scattering by relativistic electrons in a strong magnetic field have been restricted to either incident photon angles $\theta'$ aligned along the magnetic field $B$ or incident photon energies $\omega'$ below the first pair creation threshold $\omega'_{PC}$. When these restrictions are relaxed there is a resonance in Compton scattering associated with pair creation (PC), that is analogous to but independent of known resonances associated with gyromagnetic absorption (GA). As with the GA resonances, that may be labeled by the Landau quantum numbers of the relevant states, there is a sequence of PC resonances where the scattering cross section diverges. In this paper, the lowest divergence is studied for incident photon energies satisfying ${\omega'}^2\sin^2\theta'/(2eB)\ll1$, assuming that the scattering electron is in its ground (Landau) state. This lowest resonance affects only parallel-polarized photons.

Quantum mechanical tunneling in multifield-induced pair creation from vacuum

We study the pair-creation process due to spatially localized but nonoverlapping electric fields and examine the role of quantum mechanical tunneling for fermionic and bosonic systems. While for small distances between the pulses the rates are significantly different, in the tunneling regime where the separations are large the fermionic and bosonic pair-creation rates scale similarly. As long as the distance between two adjacent electric fields is less than a characteristic tunneling length, each subcritical pulse can generate particle pairs. We design a multifield configuration to make use of quantum tunneling to lower the threshold of pair creation.

Coherent quantum transport of charge density waves

Recent experiments show oscillations of dominant period $h/2e$ in conductance vs magnetic flux of charge density wave (CDW) rings above 77 K, revealing macroscopically observable quantum behavior. The time-correlated soliton tunneling model discussed here is based on coherent, Josephson-like tunneling of microscopic quantum solitons of charge $2e$. The model interprets the CDW threshold electric field as a Coulomb blockade threshold for soliton pair creation, often much smaller than the classical depinning field but with the same impurity dependence (e.g., $\ensuremath{\sim}{n}_{i}^{2}$ for weak pinning). This picture draws upon the theory of time-correlated single-electron tunneling to interpret CDW dynamics above threshold. Similar to Feynman's derivation of the Josephson current-phase relation for a superconducting tunnel junction, the picture treats the Schr\"odinger equation as an emergent classical equation to describe the time-evolution of Josephson-coupled order parameters related to soliton dislocation droplets. Vector or time-varying scalar potentials can affect the order parameter phases to enable magnetic quantum interference in CDW rings or lead to interesting behavior in response to oscillatory electric fields. The ability to vary both magnitudes and phases is an aspect important to future applications in quantum computing.

TPC in [formula omitted] astronomy above pair-creation threshold

We examine the performance of a TPC as a γ-ray telescope above the pair-creation threshold. The contributions to the photon angular resolution are studied and their dependence on energy is obtained. The effective area per detector unit mass for such a thin detector is the conversion mass attenuation coefficient. The differential sensitivity for the detection of a point-like source is then derived. Finally, the measurement of track momentum from deflections due to multiple scattering is optimized.These analytical results are exemplified numerically for a few sets of detector parameters. TPCs show an impressive improvement in sensitivity with respect to existing pair-creation-based telescopes in the [MeV–GeV] energy range, even with the modest detector parameters of this study. In addition, gas TPCs allow an improvement in angular resolution of about one order of magnitude.

HARPO—A gaseous TPC for high angular resolution [formula omitted]-ray astronomy and polarimetry from the MeV to the TeV

We propose a “thin” detector as a high-angular-precision telescope and polarimeter for cosmic γ-rays above the pair-creation threshold. We have built a demonstrator based on a gaseous TPC. We are presently characterizing the detector with charged cosmic rays in the laboratory. Here we present some of its properties.

High-angular-precision gamma-ray astronomy and polarimetry above the pair-creation threshold

We are developing a concept of a ``thin'' detector as ahigh-angular-precision telescope and polarimeter for cosmic γ-raysabove the pair-creation threshold.

High-angular-precision [formula omitted] astronomy and polarimetry

We are developing a concept of a “thin” detector as a high-angular-precision telescope and polarimeter for cosmic γ-rays above the pair-creation threshold.

Is the photon paramagnetic?

A photon exhibits a tiny anomalous magnetic moment {mu}{sub {gamma}} due to its interaction with an external constant magnetic field in vacuum through the virtual electron-positron background. It is paramagnetic ({mu}{sub {gamma}}>0) in the whole region of transparency, i.e., below the first threshold energy for pair creation, and has a maximum near this threshold. The photon magnetic moment is different for eigenmodes polarized along and perpendicular to the magnetic field. Explicit expressions are given for {mu}{sub {gamma}} for the cases of photon energies smaller than and closer to the first pair creation threshold. The region beyond the first threshold is briefly discussed.

VUV and cathodoluminescence spectroscopy of 12CaO · 7Al2O3

Reflection, photo- and cathodoluminescence of 12CaO · 7Al2O3 (C12A7) were investigated. Smooth features observed in VUV reflection spectrum of monocrystalline C12A7 evidence its complicated crystalline structure and lattice disorder. Two luminescence bands peaking at 4.9 and 3.35 eV were revealed in C12A7 ceramics having encaged electrons and O2− ions. The 4.9 eV emission has an excitation onset at 6.8 eV, i.e. near the threshold of free electron-hole pair creation, and is assigned to the recombination of encaged electrons with valence band holes. The 3.35 eV emission is ascribed to the recombination of electrons with the holes localized at the encaged O2− ions.

Hypernuclear studies at FAIR with P̄ANDA

The construction of the recently approved Facility for Antiproton and Ion Research (FAIR) at GSI will allow to produce a large number of different double Λ-hypernucleus species by exploiting p ¯ -nucleus annihilations at energies just above the Ξ ¯ + Ξ − pair creation threshold. Among other physics goals, the P̄ANDA (antiProton ANnihilation at DArmstadt) Collaboration is aiming to carry on an extensive program of high resolution γ -spectroscopy of such systems. The double Λ-hypernucleus production method and the proposed experimental setup are briefly discussed.

Vacuum polarization in supercritical fields and at frequencies above2m

The $S$-matrix form for the polarization tensor of the magnetized vacuum involves an integral over the parallel momentum, ${p}_{z}$, and a sum over the Landau levels of the virtual electrons and positrons. When the integrals over ${p}_{z}$ are evaluated, they exhibit resonant-type behavior at each of the pair-creation thresholds. Emphasis is initially placed on the regime of soft photons, where simpler integrals are solved. When the sum is taken using the Euler-Maclaurin summation formula and the result is regularized, the known dispersive properties are reproduced for $\ensuremath{\omega}\ensuremath{\ll}m$. We evaluate the corrections of order ${\ensuremath{\omega}}^{2}$ to these refractive indices and argue that they do not affect the known kinematic selection rules for photon splitting. With a simple modification to the low-frequency results, refractive indices are evaluated at frequencies up to just above the first pair-creation threshold, for supercritical magnetic fields.

High‐Energy Emission from Magnetars

The recently discovered soft gamma-ray emission from the anomalous X-ray pulsar 1E 1841-045 has a luminosity Lγ ~ 1036 ergs s-1. This luminosity exceeds the spin-down power by 3 orders of magnitude and must be fed by an alternative source of energy such as an ultrastrong magnetic field. A gradual release of energy in the stellar magnetosphere is expected if it is twisted and a strong electric current is induced on the closed field lines. We examine two mechanisms of gamma-ray emission associated with the gradual dissipation of this current. (1) A thin surface layer of the star is heated by the downward beam of current-carrying charges, which excite Langmuir turbulence in this layer. As a result, it can reach a temperature kBT ~ 100 keV and emit bremsstrahlung photons up to this characteristic energy. (2) The magnetosphere is also a source of soft gamma rays at a distance of ~100 km from the star, where the electron cyclotron energy is in the kilo-electron volt range. A large electric field develops in this region in response to the outward drag force felt by the current-carrying electrons from the flux of kilo-electron volt photons leaving the star. A seed positron injected in this region undergoes runaway acceleration and upscatters X-ray photons above the threshold for pair creation. The created pairs emit a synchrotron spectrum consistent with the observed 20-100 keV emission. This spectrum is predicted to extend to higher energies and reach a peak at ~1 MeV.