γ-ray telescopes using conversions to e+e− pairs: event generators, angular resolution and polarimetry

Abstract

We benchmark various available event generators in Geant4 and EGS5 in the light of ongoing projects for high angular-resolution pairconversion telescopes at low energy. We compare the distributions of key kinematic variables extracted from the geometry of the three final state particles. We validate and use as reference an exact generator using the full 5D differential cross-section of the conversion process. We focus in particular on the effect of the unmeasured recoiling nucleus on the angular resolution. We show that for high resolution trackers, the choice of the generator affects the estimated resolution of the telescope. We also show that the current available generator are unable to describe accurately a linearly polarised photon source.

Gamma-Ray Polarimetry: A New Window for the Nonthermal Universe

“…7), as shown for example by Yadigaroglu (1997). Recently Gros & Bernard (2017a) has shown that the use of the bisector angle of the lepton pair ((φ + + φ − )/2) leads to a maximization of the obtained experimental value of the polarization asymmetry, and therefore to a better precision of the polarization measurement. The second limitation comes from multiple Coulomb scatterings of the charged tracks, leading to an exponential suppression of the asymmetry factor: A ef f = e −2σ 2 φ × A, with σ φ being the azimuthal angular resolution.…”

“…11. Using this technique Gros & Bernard (2017a) has obtained a value of π/4 for the low energy asymptote for the polarization asymmetry. In the high energy limit A ≈ 1/7, as obtained by Boldyshev & Peresunko (1971).…”

“…The discussion of the weights method follows Bernard (2013b);Gros & Bernard (2017a), and the interested reader is encouraged to consult those references for more details.4 For benchmarks of various available event generators seeGros & Bernard (2017b) …”

HARPO, a gas TPC active target for high-performance γ-ray astronomy; demonstration of the polarimetry of MeV γ-rays converting to Bernard1 2019Nuclear Instruments and Methods in Physics Research Section A: Self Cite10040View full textAdd to dashboardCite

“…We determined the bisector φ +− = (φ + + φ − )/2 of the azimuthal angles of the direction of the electron, φ − , and of the positron, φ + , to be the optimal choice [27]. We demonstrated that with that angle definition, our generator is the only one, to our knowledge, to match the known asymptotic expressions for A at low and at high energies [24].…”

“…With that tool we determined the (68 % containment) contribution to the single-photon angular resolution due to the fact that the momentum of the recoil nucleus cannot be measured, to be ≈ 1.5 rad[E/1 MeV] −5/4 [23,24]. We obtained the contribution to the single-photon angular resolution due to the single-track angular resolution in the case of an optimal trackingà la Kalman to be ∝ p −3/4 [23,25].…”