Satellite-borne $\gamma$-ray astrophysics from coherent interactions in oriented crystals

Abstract

High-density and high-Z crystals are key elements of most space-borne $\gamma$-ray telescopes operating at gigaelectronvolt energies (such as Fermi-LAT). The lattice structure is usually neglected in the development of a crystalline detector, although its effects on the energy deposit development should be taken into account, since the interactions of a high-energy ($\sim$ 10~GeV) photon or e$^\pm$ impinging along the axis of an oriented crystal are different than those observed in a fully isotropic medium. Specifically, if the angle between a 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. Consequently, a photon-induced shower inside an oriented crystal develops within a much more compact region than in an amorphous medium. Moreover, for photon energies above a few gigaelectronvolt and incidence angles up to several degrees, the pair production cross section exhibits a pronounced dependence on the angle between the crystal axis and the photon polarization vector. In this work we show that these effects could be exploited to develop a novel class of light-weight pointing space-borne $\gamma$-ray telescopes, capable of achieving an improved sensitivity and resolution, thanks to a better shower containment in a smaller volume, with respect to non-oriented crystalline detectors. We also show that an oriented tracker-converter system could be used to measure the polarization of a $\gamma$-ray source above few gigaelectronvolts, 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