[en] Gamma ray astronomy suffers from a sensitivity gap between 0.1 and 100Mev. With high angular resolution for the electrons, it will also be possible to probe the linear polarisation of the photons. An accurate simulation is necessary to correctly design and compare these detectors. We establish baseline distributions of key kinematic variables as simulated by a 5D, exact down to threshold, and polarised event generator. We compare them to simulations with the low energy electromagnetic models available in Geant4 and in EGS5. We show that different generators give a different picture of the optimal angular resolution of pair telescopes. We also show that, of all the simulations we used, only the full 5D generator describes accurately the angular asymmetry in the case of polarised photons.

[en] We estimated the dynamic penetration effect for internal pair conversion assuming a surface-current model. It turns out that the pair conversion coefficient is slightly modified by this additional contribution. (author)

[en] The probability of positronium creation from the nuclear transitions is calculated for arbitrary electric and magnetic multipolarity. It is of the order of α⁴ with respect to the corresponding gamma radiation probability. The ratio of positronium creation probability to the k-shell internal conversion probability is found. (author)

[en] The Large Area Telescope (LAT) on-board the Fermi Gamma-ray Space Telescope is a pair-conversion telescope designed to survey the gamma-ray sky from 20 MeV to several hundreds of GeV. In this energy band there are no astronomical sources with sufficiently well known and sharp spectral features to allow an absolute calibration of the LAT energy scale. However, the geomagnetic cutoff in the cosmic ray electron-plus-positron (CRE) spectrum in low Earth orbit does provide such a spectral feature. The energy and spectral shape of this cutoff can be calculated with the aid of a numerical code tracing charged particles in the Earth's magnetic field. By comparing the cutoff value with that measured by the LAT in different geomagnetic positions, we have obtained several calibration points between ∼6 and ∼13 GeV with an estimated uncertainty of ∼2%. An energy calibration with such high accuracy reduces the systematic uncertainty in LAT measurements of, for example, the spectral cutoff in the emission from gamma ray pulsars.

[en] We describe a new version of the 5D, exact, polarised, Bethe–Heitler event generator of $\gamma$-ray conversions to $e^{+}{e}^{-}$, developed in the context of the HARPO project, that is able to simulate successive events with different photon energies and on different atomic targets without any substantial CPU overhead. The strong correlation between kinematic variables in the region where the five dimensional cross section diverges is mitigated by performing each step of the conversion in the appropriate Lorentz frame. We extend the verification range down to 1 keV above threshold and up to 1 EeV. This work could pave the way to the precise simulation of the high-performance $\gamma$-ray telescopes and polarimeters of the post-Fermi-LAT area.

[en] The polarization effects of the reaction of the electron-positron pair conversion into the muon pair are considered. The invariant form of the helical amplitude of this reaction is identified. The expression for the cross sections are determined in dependence on the helicity signs of the colliding and outgoing particles. The differential and total cross sections of this process are also identified for case with the nonpolarized particles

[en] The conversion coefficient for internal electron-positron pair formation is calculated for arbitrary electric and magnetic multipolarity. Analytical expressions for the differential conversion coefficient dβ/dEp can be derived in the range O<=Z<=118, if one employs relativistic Coulomb wave functions according to pointlike nuclei for the electron. Especially for ML-transitions in the high-Z-region we found large deviations from results obtained within the Born approximation. (orig.)

[en] The internal bremsstrahlung of α-particles emmited by α-decaying nuclei is not responsible for the internal pair production in α-decay. The probability of this process is of the order of 10^-20, in strong disagreement with previous results. It is suggested that the internal pair production in α-decay is due to excited states in nuclei. For the probability of this process a value of the order of 10^-12 has been obtained. (orig.)

[en] Triggered by a recently observed deviation in internal pair conversion at ∼ 9 MeV/c² invariant mass, indicating the possible existence of a new pseudoscalar boson, a previous constraint is reconsidered. (author)

[en] The GLAST Large Area Telescope (LAT) is the next generation satellite experiment for high-energy gamma-ray astronomy. It is a pair conversion telescope built with a plastic anticoincidence shield, a segmented CsI electromagnetic calorimeter, and the largest silicon strip tracker ever built. It will cover the energy range from 30 MeV to 300 GeV, shedding light on many issues left open by its predecessor EGRET. One of the most exciting science topics is the detection and observation of gamma-ray bursts (GRBs). In this paper we present the work done so far by the GRB LAT science group in studying the performance of the LAT detector to observe GRBs.We report on the simulation framework developed by the group as well as on the science tools dedicated to GRBs data analysis. We present the LAT sensitivity to GRBs obtained with such simulations, and, finally, the general scheme of GRBs detection that will be adopted on orbit