Satellite-borne γ-ray astrophysics from coherent interactions in oriented crystals

TL;DR

This work proposes a novel space-borne gamma-ray telescope concept based on oriented crystals to overcome the limitations of current MeV–GeV instruments. By leveraging strong-field coherence in axial crystal fields, on-axis photons can convert earlier and deposit energy more compactly, enabling improved angular resolution and extended energy coverage. The authors present concrete designs for an oriented silicon-tungsten tracker and an oriented PbWO$_4$ calorimeter, supported by Geant4 simulations and a dedicated oriented-crystal physics library, and discuss potential gains in PSF, energy containment, and polarization sensitivity via Coherent Pair Production. If realized, this approach could enhance sensitivity, allow GeV-scale gamma-ray polarization measurements, and open new avenues for studying extreme astrophysical environments and indirect dark matter signals, while potentially enabling lighter, cost-effective missions.

Abstract

High-density and high-Z crystals are a key element of most space-borne $γ$-ray telescopes operating at GeV 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 ($\sim$~GeV) photon or e$^\pm$ impinging along the axis of an oriented crystal are different than the ones 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 GeV 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 $γ$-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 $γ$-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.

Figures (8)

• Figure 1: Average depth at which high energy photons convert in an e$^\pm$ pair, inside an infinitely thick and infinitely large tungsten crystal aligned along the $\langle 111 \rangle$ axis. The blue dots (red dots) correspond to the case of photons incident on the non-aligned (perfectly aligned on-axis) crystal. Each point shows the average value over 10,000 simulations. (A color version is available on the online journal)
• Figure 2: Average depth at which a 50 GeV photon converts in an e$^\pm$ pair, inside an infinitely thick and infinitely large W crystal, aligned along the $\langle 111 \rangle$ axis. Different beam-axis angles are shown. The upper horizontal axis measures the angle in units of the Strong Field critical value $\Theta_0$, which for W $\langle 111 \rangle$ is $\sim$ 1.74 mrad ($\sim 0.1^\circ$) soldani2023phd. Each point shows the average value over 10,000 simulations. (A color version is available in the online journal)
• Figure 3: Average energy deposited by high energy photons per unit thickness inside an infinitely thick and large PbWO$_4$ crystal aligned along the $\langle 100\rangle$ axis. The solid (dashed) lines represent the case of photons incident on a non-aligned (axially aligned) crystal. Each line is the average value over 10,000 simulations. (A color version is available on the online journal)
• Figure 4: Variation of the longitudinal depth of the maximum energy deposit ($t_{\max}$), as a function of the $\gamma$-ray energy. The depths were computed by fitting the shower longitudinal profiles with the standard gamma function defined in fabjan2003: $dE/dt = c \cdot ( t^{a-1} e^{-bt} )$. The depth of the maximum energy deposit is then $t_{\max} = (a-1)/b$. The blue dots (red dots) represent the case of photons incident on the non-aligned (axially aligned) crystal. The logarithmic fit on the $t_{\max}$ curves were done to show the acceleration of the e.m. shower occurring in an oriented crystal, with respect to an isotropic material. While the logarithmic dependence of the isotropic case is well known fabjan2003, the functional form shown for the oriented case is not reported in any previous study. Each point shows the average value over 10,000 simulations. (A color version is available on the online journal)
• Figure 5: Average energy deposited by high energy photons inside an AGILE/MCAL-like calorimeter (1.5 X$_0$, tavani2009) composed of oriented PbWO$_4$ crystals, aligned along the $\langle 100\rangle$ axis. The solid (dashed) lines represent the case of photons incident on a non-aligned (axially aligned) crystal. Each point shows the average value over 10,000 simulations. (A color version is available on the online journal)