LEGRI operations. Detectors and detector stability

Two years after launch (04.21.97), LEGRI is operating on Minisat-01 in a LE O orbit. The LEGRI detector plane is formed by two type of gamma-ray solid state detectors: HgI2 and CdZnTe. Detectors are embedded in a box containin g the FEE and DFE electronics. This box provides an effective detector pass ive shielding. Detector plane is multiplexed by a Coded Aperture System loc ated at 54 cm and a Ta Collimator with a FCFOV of 22 degrees and 2 degrees angular resolution. The aim of this paper is to summarize the detector beha viour in three different time scales: before launch, during the in-orbit ch eck-out period (IOC), and after two years of routine operation in space. Ma in results can be summarized as follows: A large fraction of the HgI2 detectors presented during LEGRI IOC very high count ratios from their first switch-on (May 1997). Therefore, they induce d saturation in the on-board mass memory. After some unsuccessful attempts to reduce the count ratios by setting up different thresholds during LEGRI IOC, all of them were switched off except nine detectors in column 4, with a higher degree of stability. Oppositely, the 17 CdZnTe detectors present a remarkable stability in both their count-ratios and spectral shapes. Details about CdZnTe ground energy calibration, in-flight calibration (using the Crab) and detector stability are discussed hereafter. Detector efficiency function has been computed wit h the fixed flight threshold used within the calibrated energy range (20-80 KeV). It presents a maximum at 60 KeV, and decreasing efficiencies in the lower and upper energy range ends. Both, non-linear threshold cutting and t he drop in the detector efficiency explain the CdZnTe computed operational efficiency response.