Energy Distribution Of Positrons Research Articles

Angular and energy distributions of positrons created in subcritical and supercritical slow collisions of heavy nuclei

Angular and energy distributions of positrons created in subcritical and supercritical slow collisions of heavy nuclei

Positron energy distribution in a factorized trident process

We estimate the energy distribution of positrons produced in the interaction of ultra-relativistic electrons with a high-intensity laser beam. The underlying trident process is factorized on the probabilistic level. That is, we deploy a two-step mechanism for the formation of electron-positron pairs. In the first step, a high-energy photon is produced as a result of nonlinear Compton scattering. In the second step, an electron-positron pair is created by the nonlinear (multi-photon) Breit-Wheeler process.

Deep-Learning Based Positron Range Correction of PET Images

Positron emission tomography (PET) is a molecular imaging technique that provides a 3D image of functional processes in the body in vivo. Some of the radionuclides proposed for PET imaging emit high-energy positrons, which travel some distance before they annihilate (positron range), creating significant blurring in the reconstructed images. Their large positron range compromises the achievable spatial resolution of the system, which is more significant when using high-resolution scanners designed for the imaging of small animals. In this work, we trained a deep neural network named Deep-PRC to correct PET images for positron range effects. Deep-PRC was trained with modeled cases using a realistic Monte Carlo simulation tool that considers the positron energy distribution and the materials and tissues it propagates into. Quantification of the reconstructed PET images corrected with Deep-PRC showed that it was able to restore the images by up to 95% without any significant noise increase. The proposed method, which is accessible via Github, can provide an accurate positron range correction in a few seconds for a typical PET acquisition.

Simulation of channeling radiation and positron production in thick diamond structures and a tungsten single crystal

The process of de-channeling for electrons of energy 2 GeV channeled in the (110) plane of silicon and tungsten single crystals has been studied. The dynamics of the particle distribution density has been investigated in dependence on both the energy and scattering distribution of the initial electron beam. The influence of de-channeling on the spectral intensity of channeling radiation has been investigated by solving the Fokker-Planck equation. A non-conventional positron source is described based on the generation of radiation by relativistic electrons channeled along different crystallographic planes or axes of W, Si, Ge and C (diamond) single crystals and the subsequent conversion of the radiation into e+e−-pairs in an amorphous tungsten target. The simulation of channeling radiation and its dependence on the incidence angle into the crystal has been carried out by means of our Mathematica codes. The conversion of the radiation into e+e−-pairs and the energy distribution of positrons have been simulated using the GEANT4 package.

Trident pair production in colliding bright x-ray laser beams

The magnificent development of strong X-ray lasers motivates the advancement of pair production process studies into higher laser frequency region. In this paper, a resonant electron-positron pair production process with the absorption of two X-ray photons is considered in the impact of an energetic electron at the overlap region of two colliding X-ray laser beams. Laser-dressed QED method is justified to tackle the complexity of the corresponding multiple Feynman diagrams calculation. The dependence of the production rate as well as the positron energy distribution on the relative angles among the directions of the two laser wave vectors and the incoming electron momentum is revealed. It is shown that the non-plane wave laser field configuration arouses novel features in the pair production process compared to the plane-wave case.

Simulation of positron energy distributions from a hybrid positron source based on channeling radiation

Positron energy distributions of a non-conventional positron source which is based on the generation of channeling radiation by relativistic electrons channeled along different crystallographic planes and axes of Si, C, Ge and W single crystals and the subsequent conversion of radiation into e+e−-pairs in an amorphous tungsten target have been simulated. The photon spectra of channeling radiation were calculated using the Doyle–Turner approximation for the continuum potentials of the crystallographic planes and axes considered. The classical equations of motion for channeled electrons have been solved in order to obtain the particle trajectories, velocities and accelerations. Applying classical electrodynamics, spectral-angular distributions of channeling radiation are calculated and their dependence on the incidence angle of the electrons is investigated. The calculation of channeling radiation was carried out using our developed Mathematica codes whereas the conversion of radiation into e+e−-pairs and the energy distributions of the positrons have been simulated by means of the GEANT4 package.

Positron energy distributions from a hybrid positron source based on channeling radiation

A hybrid positron source which is based on the generation of channeling radiation by relativistic electrons channeled along different crystallographic planes and axes of a tungsten single crystal and subsequent conversion of radiation into e+e−-pairs in an amorphous tungsten target is described. The photon spectra of channeling radiation are calculated using the Doyle–Turner approximation for the continuum potentials and classical equations of motion for channeled particles to obtain their trajectories, velocities and accelerations. The spectral-angular distributions of channeling radiation are found applying classical electrodynamics. Finally, the conversion of radiation into e+e−-pairs and the energy distributions of positrons are simulated using the GEANT4 package.

Ion production by positron impact on H2O

The energy distributions of positrons scattered around 0° from H2O have been measured in coincidence with the remnant ions (H2O+, OH+ and H+) at 100 eV and 153 eV incident energies. The maxima of the distributions (d2σ/dE+dΩ) associated with the production of OH+ and H+ are about 5–10 times smaller than that for H2O+. At both incident energies, a small shoulder has been observed at ∼28 eV in the energy loss spectra associated with H2O+ production possibly arising from a shake-up band associated with the 2a1 orbital. The branching ratios for the H+ and HO+ fragments are also presented.

Fragmentation of positronium in collision with He atoms.

The absolute cross section for the fragmentation of positronium in collision with He atoms has been measured. The results are compared with available theories. The longitudinal energy distributions of positrons resulting from fragmentation have also been determined and are found to display a peak situated just below half the residual energy. This is suggestive of the occurrence of "electron loss to the continuum" in which the two residual charged particles lie in a low relative-velocity Coulomb-continuum state.

Enhancement of positron annihilation on molecules due to vibrational Feshbach resonances

The zero-range potential model is used to investigate positron collisions and annihilation with molecules. The Kr 2 dimer is considered as an example. It is shown that (i) although positrons do not bind to individual Kr atoms, they do form bound states with Kr 2. (ii) A sequence of vibrationally excited states of the positron–molecule complex extends into the positron continuum, where it manifests as vibrational Feshbach resonances. (iii) These resonances give a very large contribution to the positron annihilation rate. Even after averaging over the thermal positron energy distribution, the contribution of the lowest Feshbach resonance exceeds that of the non-resonant background by an order of magnitude.

Method for experimental determination of β-decay end point energies

In this paper we discuss a novel method based on using a β-emitter source, a surface barrier silicon detector for β-particles and two high purity germanium detectors for photons in a compact T geometry. A positron energy distribution was measured using a 58Co source and experimental positron end point energy determined by Fermi plot analyses being found to be in agreement with the literature.

Preliminary results of simulating interaction of 0.01–3.0 MeV positrons with tungsten targets

Reflection and transmission of positrons falling on tungsten targets were simulated by the Monte-Carlo method. The calculations were made for a simplified approximation of a narrow beam of monoenergetic positrons on a grid of 15 points in the 0.01–3.0 MeV energy range and for positrons with the 62 Cu β-decay energy spectrum, for various incidence angles. Histograms by coordinates of moderated positron stop points in the target and by angle–energy distribution of positrons leaving the target, for cases of a semi-infinite target and a single foil 0.02 mm thick, are depicted.

Positron interactions with overlayers of oxygen on GaAs(100)

The positron work-function, re-emission yield, and positronium fraction of an n-doped GaAs(100) surface were measured as a function of oxygen exposure. The energy distribution of positrons observed to be re-emitted indicated that the clean and oxygen exposed n-doped GaAs(100) surfaces had negative positron work-functions. The fraction of incident positrons re-emitted as bare positrons, (Y), was found to increase and the fraction re-emitted as positronium, (f Ps), to decrease with increasing oxygen exposure. This suggests that surface modified GaAs may be useful as a contact material in the fabrication of GaAs based FAMs.

Mechanisms of positron annihilation on molecules

The aim of this work is to identify the mechanisms responsible for very large rates and other peculiarities observed in low-energy positron annihilation on molecules. The two mechanisms considered are the following: (i) Direct annihilation of the incoming positron with one of the molecular electrons. This mechanism dominates for atoms and small molecules. I show that its contribution to the annihilation rate can be related to the positron elastic scattering cross section. This mechanism is characterized by a strong energy dependence of ${Z}_{\mathrm{eff}}$ at small positron energies and high ${Z}_{\mathrm{eff}}$ values (up to ${10}^{3})$ for room-temperature positrons, if a low-lying virtual level or a weakly bound state exists for the positron. (ii) Resonant annihilation, which takes place when the positron undergoes resonant capture into a vibrationally excited quasibound state of the positron-molecule complex. This mechanism dominates for larger molecules capable of forming bound states with the positron. For this mechanism ${Z}_{\mathrm{eff}}$ averaged over some energy interval, e.g., due to thermal positron energy distribution, is proportional to the level density of the positron-molecule complex, which is basically determined by the spectrum of molecular vibrational states populated in the positron capture. The resonant mechanism can produce very large annihilation rates corresponding to ${Z}_{\mathrm{eff}}\ensuremath{\sim}{10}^{8}.$ It is highly sensitive to molecular structure, and shows a characteristic ${\ensuremath{\varepsilon}}^{\ensuremath{-}1/2}$ behavior of ${Z}_{\mathrm{eff}}$ at small positron energies $\ensuremath{\varepsilon}.$ The theory is used to analyze calculated and measured ${Z}_{\mathrm{eff}}$ for a number of atoms and molecules.

An investigation of positrons interacting with solid argon, krypton and xenon

With this article we intend to shed some light on all the important characteristics of the up-to-date most efficient positron moderators, the rare gas solids. We stress on the importance of the impurities in the performance of the solid rare gas moderators. The impurity factor is linked with the crystalline changes to explain the effect of annealing, and we demonstrate the role of impurities in the endurance. Significant increase in the low energy positron yield is observed after repeated anneals. The positron energy distributions from Ar, Kr, and Xe moderators are measured to be about 2 eV (FWHM).

Positron dynamics in surface-charged solid argon.

Studies have been made of the reemission of positrons incident at low energies upon solid argon to which electric fields were applied by charging an overlayer of molecular oxygen. An enhancement in positron reemission was observed which reached a maximum for an applied field of around 7 kV mm{sup {minus}1}. At this field strength the same yield was observed for implantation energies ranging from 1 to 10 keV, consistent with enhancement due to field-induced positron drift to the exit surface. At higher electric fields, the observed gradual decrease in enhancement was attributed to the heating of the positron energy distribution above the positronium formation threshold. Quantitative agreement with our experimental results has been obtained using a Monte Carlo simulation from which estimates for the positron diffusion length and mobility of 1.7(+2.0,{minus}0.4) {mu}m and 4.7(+2.9,{minus}0.4){times}10{sup {minus}3} m{sup 2} V{sup {minus}1} s{sup {minus}1}, respectively, have been derived. This model was also able to successfully reproduce previous results obtained using surface-charged argon {beta}{sup +} moderators. An abrupt and almost complete reduction in positron reemission was observed for applied surface potentials above a value which showed a weak dependence on film thickness.

Positron Facilities in Europe

Positron annihilation is a well established method in solid state physics and material science. The positron being a very sensitive probe, can give very precise information on the momentum distribution of electrons in metals and alloys as well as on lattice defects in crystals. Starting with the energy distribution of positrons from a radioactive decay, the current development is directed more to monoenergetic positrons of variable energy and of high intensity. Pulsed and continuous beams of moderated positrons have been designed and developed to study very efficiently surface and near surface regions in metals and semiconductors. Especially a pulsed positron beam with narrow beam pulses (∼ 150 ps) enables positron lifetime experiments as a function of the positron energy. The impact of intense positron beams is straightforward: a decrease of the counting time. There are various possibilities and approaches to realize intense positron beams. Developments at several institutes in Europe are under way. The aim is to obtain a beam intensity in the order of ∼ 10 10 positrons/sec. Parallel to the installment of intense positron beams, the development of positron microscopes is pursued. Two types of positron microscopes are being set up: a scanning positron microscope with a pulsed beam of 100 ps and a beam diameter of 1 μm and a positron reemission microscope with about the same beam diameter

Intense sources for positron research

Positron annihilation is a well-established method in solid state physics and material science. The positron being a very sensitive probe, can give very precise information on the momentum distribution of electrons in metals and alloys as well as on lattice defects in crystals. Starting with the energy distribution of positrons from a radioactive decay, the current development is directed more to monoenergetic positrons of variable energy and of high intensity. The impact of intense positron beams is straightforward: a decrease of the counting time. There are various possibilities and approaches to realize intense positron beams. The aim is to obtain a beam intensity in the order of ∼ 1010 positrons/s. Parallel to the instalment of intense positron beams, the development of positron microscopes is pursued.

Transport model of thermal and epithermal positrons in solids. II.

Epithermal positron transport in solids is treated by employing a one-dimensional Boltzmann equation, which, in combination with the diffusion equation for the thermalized positrons, gives rise to a more realistic model for transport of positrons implanted into solids with variable energies. Thermalization is accounted for by the averaged inelastic scattering of the epithermal positrons, although the epithermal positron energy distribution cannot be predicted from this model.

Absence of energy loss in positron emission from metal surfaces.

We have measured the energy distribution of positrons reemitted after implantation into W(110) and Ni(100) surfaces at room temperature. The previously reported broad distribution of the perpendicular component of the energy is now found to be due to a wide angular distribution of essentially monoenergetic positrons. The implied elasticity of the positron-emission process (e.g., less than 25 meV energy loss for 97% of the 1-eV positrons emitted from Ni) is in serious disagreement with present theories that require the emission of energetic electron-hole pairs for positron reemission from metals.