Scintillating fiber calorimeter as a detector for the observation of GeV cosmic gamma rays

A scintillating fiber sampling calorimeter telescope (SSCT) is proposed, which is to be operated on board a satellite for the observation of cosmic γ rays ranging from 200 MeV to 200 GeV. The merit of using scintillating fibers is that the area can be enlarged more than 2 m² without any particular technical difficulty within a reasonable payload limit of a satellite. Detectors with an effective area which is one order of magnitude larger than EGRET can be made with a lower cost. SSCT consists of a target part and a main part which are separated by 60 - 70 cm. A γ ray which enters the target is converted into a pair of electrons which in turn develop into an electron-photon cascade. The cascade is observed in the main part being a calorimeter with imaging capability. Direction of the γ ray is measured by pointing the energy-weighted centers observed in the two parts. We use 2 mm square fibers which are bundled into a 2 mm thick flat belt to detect the shower particles. A pair of such belts crossing with a right angle is inserted between lead plates. Simulation study and test experiments with accelerator beams show the energy resolution of the calorimeter is 20%/√E(GeV) below a few GeV and becomes worse in a higher energy region to reach 24% at 100 GeV. The angular resolution is 1.6° at 1 GeV and 0.34° at 10 GeV.