Gamma ray polarimetry

Abstract

Spectroscopic instruments currently being proposed may possess polarimetric capabilities which sould be nurtured and enhanced to permit characterization of basic emission mechanisms which are impossible using other techniques. Compton scattering and pair production detected the polarization of high energy (E is greater than 50 keV) protons in laboratory experiments. The polarization properties of a detection system consisting of 19 germanium crystals in a closed packed array are examined and the advantages of such a detector over Thompson scattering are discussed. The possiblity of using pair production to detect polarization of high energy gamma rays, and the associated modulation factors are discussed. The central difficulty involved in using pair production polarimeters in astrophysical applications is that the typical opening of the electron or positron direction with respect to the incident photon aircitron is small, of order E/sq mc. Multiple scattering in the material used to convert the photons to an electron positron pair causes deviations in the direction of the electron and positron.