Doppler broadening of energy spectra in Compton cameras

Abstract

Compton cameras utilize the Compton effect to achieve directional localization of gamma rays. The accuracy of the localization depends, in part, on the uncertainties in the measurement of energies. These uncertainties have conventionally been assumed to be due to the effects of finite detector energy resolution alone. There is another source of energy uncertainty that none of the Compton cameras proposed or built thus far accounts for the Doppler broadening of energy spectra that arises from the Compton interaction between gamma rays and moving electrons bound to atoms. The authors have used Monte Carlo simulations and direct calculations to demonstrate that the energy uncertainties due to Doppler broadening are non-negligible. For low-energy gamma rays, these uncertainties are generally more significant than those due to the finite energy resolutions of semiconductor detectors (for example, germanium and silicon). Expressions for estimating the angular uncertainties in Compton cameras due to energy resolution and Doppler broadening are derived. The authors conclude that the accuracy of the calculation of the scatter angle can be improved if the electron pre-collision momentum is known. However, even without such knowledge. Compton cameras still have the potential to achieve spatial resolutions that are comparable to those of mechanically collimated systems.