Compton Imaging of MeV Gamma-Rays with the Liquid Xenon Gamma-Ray Imaging Telescope (LXeGRIT)

The Liquid Xenon Gamma-Ray Imaging Telescope (LXeGRIT) is the first realization of a liquid xenon time projection chamber for Compton imaging of MeV γ-ray sources in astrophysics. By measuring the energy deposit and the three spatial coordinates of individual γ-ray scattering points, the location of the source in the sky is inferred with Compton kinematics reconstruction. The angular resolution is determined by the detector’s energy and spatial resolutions, as well as by the separation in space between the first and second scattering. The imaging response of LXeGRIT was established with γ-rays from radioactive sources, during calibration and integration at the Columbia Astrophysics Laboratory, prior to the 2000 balloon flight mission. In this paper we describe in detail the various steps involved in imaging sources with LXeGRIT and present experimental results on angular resolution and other parameters which characterize its performance as a Compton telescope. Introduction The Liquid Xenon Gamma Ray Imaging Telescope (LXeGRIT) is a prototype of Compton telescope (CT) based on a liquid xenon time projection chamber (LXeTPC), which combines good energy resolution with imaging of individual 1 Present address: Yale University, Physics Dept., New Haven, CT, USA 2 Present address: Waseda University,Tokyo, Japan 3 Present address: High Energy Astrophysics Lab, Institute of High Energy Physics, Beijing, China Preprint submitted to Elsevier 2 May 2008 MeV γ-ray interactions with submillimiter position resolution (Sec. 1). A CT images γ-ray sources in the energy range from few 100 keV to more than 10 MeV, reconstructing the direction of individual γ-rays through Compton kinematics. The scatter angle on a free electron (φ̄) is given by the Compton formula cos φ̄ = 1 + 1 W0 − 1 W1 , with: Wi = Ei mec (1) where mec 2 = 0.511 MeV, E0 is the initial energy and E1 is the energy of the scattered γ-ray. Fig. 1 helps illustrating the principle of Compton imaging. A