Speaker
Description
High-density and high-Z crystals are a key element of most detectors used to observe High Energy (HE) $\gamma$-rays and cosmic rays from space, such as Fermi-LAT and DAMPE. The lattice structure of these materials is usually ignored in the instrument design, simulation and calibration, but recent studies have shown that this is a rough approximation, since photons and e$^\pm$ with an energy above few GeV impinging along the axis of an oriented crystals interact differently from the ordinary. Specifically, if the angle between the photon (e$^\pm$) trajectory and the crystal axis is smaller than $\sim 0.1^\circ$, a large enhancement of the pair-production (bremsstrahlung) cross-section is observed. Smaller enhancements can be observed even for incidence angles as large as $\sim 1^\circ$. The intensity of these effects grows for energies up to few TeV and then saturates. Notably, for photon energies above a few GeV and incidence angles up to several degrees, the pair-production cross-section exhibits a pronounced dependence on the crystal orientation with respect to the photon polarization vector.
In this contribution we discuss how oriented crystals could be used to develop a novel class of light-weight pointing space-borne $\gamma$-ray telescopes, capable of achieving an improved sensitivity and resolution while coping with the strict requirements in terms of mass and volume of space detectors, thanks to a better shower containment in a smaller volume with respect to non-oriented crystalline detectors. We will also discuss how an oriented tracker-converter system could be used to measure the polarization of a $\gamma$-ray source above few GeV, in a regime that remains unexplorable through any other detection technique. This novel detector concept could open new pathways in the study of the physics of extreme astrophysical environments and potentially improve the detector sensitivity for indirect Dark Matter searches in space.
Finally, in this contribution we will highlight some results obtained by the OREO collaboration, which at present time is the first and only group that was ever able to fully develop and characterize (with beams of high energy hadrons and e$^\pm$) an oriented calorimeter composed of PbWO$_4$ crystals.