Calibration of the Egret Gamma Ray Telescope With a Back-Scattered Laser Beam

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One of the three co-aligned gamma ray telescopes to be placed in orbit aboard the Gamma Ray Observatory is EGRET, the energetic gamma ray experiment telescope. EGRET will measure celestial gamma rays in the energy range 20 - 24,000 MeV. The telescope uses conversion foils and a spark chamber to resolve gamma ray direction and is triggered by a directionally sensitive pair of plastic scintillator arrays. An 8" thick N aI(TI) spectrometer below the spark chamber yields information about the gamma ray energy. The sensitive area is 20 times larger than previous instruments of its type. A full sky survey is expected to resolve ~10 extragalactic and ~100 galactic point gamma ray sources. Diffuse gamma ray emission will also be examined.

A calibration of the telescope to determine efficiency, angular resolution and energy resolution as a function of gamma ray energy and arrival direction is necessary to analyze the flight data. EGRET received a calibration gamma ray exposure with a back-scattered laser beam. Gamma rays were produced by the compton scattering of frequency-doubled YAG laser photons from a high energy electron beam. The SLAC beam was constructed originally to produce 20 GeV gamma rays. Modifications were made to operate over the energy range required for calibration. The beam was operated at ten gamma ray energies between 15 MeV (0.65 Ge V electron energy) and 10 GeV (22.4 GeV electron energy). The back-scattered gamma rays were collimated to produce an energy spectrum of ~15% FWHM.

The beam intensity used was ~0.3 gamma rays per machine pulse. The intensity was monitored primarily by placing a 15 cm thick plastic scintillator in the beam to convert and detect a known fraction of the gamma rays . The response of the NaI(TI) spectrometer in EGRET was also used for beam intensity monitoring.

A large sample of photomultiplier tubes was evaluated for use with the energy spectrometer. Absolute measurements of gain indicate that the scintillation efficiency of NaI(TI) is at least 17±2% - significantly higher than has been previously assumed. The gain was monitored during high anode current. A temporary gain increase of ~10% was noted for 52% of the photomultipliers, while 33% showed a temporary gain decrease of the same magnitude.

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