Position Calibrations and Preliminary Angular Resolution of the Prototype Nuclear Compton Telescope

Abstract

The Nuclear Compton Telescope (NCT) is a balloon-borne soft gamma ray (0.2-10 MeV) telescope designed to study astrophysical sources of nuclear line emission and gamma ray polarization. A prototype instrument was successfully launched from Ft. Sumner, NM on June 1, 2005. The NCT prototype consists of two 3D position sensitive High-Purity-Germanium (HPGe) strip detectors fabricated with amorphous Ge contacts. The novel ultra-compact design and new technologies allow NCT to achieve high efficiencies with excellent spectral resolution and background reduction. We have completed our preliminary calibrations of both the energy and the 3D position of interactions for the prototype instrument. Determination of both the energy and the position is crucial for Compton imaging, and minimizing the errors in each improves the angular resolution. Because of the compact design of the detectors and the high spectral resolution of germanium, we expect the position uncertainties to dominate over energy uncertainties when determining the angular resolution. Detailed depth calibrations and a preliminary determination of angular resolution as a function of energy are described. We determine how measurement uncertainties and physical limitations (energy uncertainty, position uncertainty, Doppler broadening, and systematics) constrain the ultimate angular resolution.