Use of a tunable quasimonoenergetic gamma-ray beam for the calibration of the EGERT gamma-ray telescope in the range 20–24000 MeV

Abstract

One of the three co-aligned γ-ray telescopes to be carried into space aboard NASA's Gamma Ray Observatory is the high-energy EGRET telescope. The sensitive range of this instrument extends from 20 to 24000 MeV. A calibration of the EGRET response over its full energy range — including efficiency, angular resolution and energy resolution as a function of γ-ray arrival direction and energy — is an essential precursor to launch. A source of calibration γ-rays was provided by a backscattered laser beam that had been developed earlier at SLAC to provide quasi-monoenergetic 20 GeV γ-rays. In principle, the γ-ray energy in this beam can be reduced by lowering the electron beam energy so that, for example, a 760 MeV electron beam produces a 20 MeV γ-ray beam. The practical problems encountered in extending the operating range of the SLAC beam to low energies are described. The techniques used to monitor the γ-ray beam intensity at ten operating energies are indicated, and an overview of the EGRET calibration procedure is provided.