Measurement of the FD camera.pdfVIP

GAP - 2000 - 010 Roma, 1 March 2000 Measurement of the FD camera light collection efficiency and uniformity P. Facal San Luis Sezione INFN di Roma II, Roma, Italy and Universidad de Santiago de Compostela, Santiago de Compostela, Spain P. Privitera Universita? di Roma II, Tor Vergata, and Sezione INFN di Roma II, Roma, Italy Abstract. Detailed measurements of the light collection efficiency and unifor- mity of the FD camera with a set up which closely simulate the optics of the FD telescope are presented. We observed that light collection at the pixel edges is very much improved when mercedes are used. The light collection efficiency averaged over the FD focal surface is 93%. 1 Introduction The Fluorescence Detector (FD) camera is composed of a matrix of 20x22 hexagonal pixels, positioned on the focal surface of a spherical mirror. Details of the geom- etry and the mechanical characteristics of the camera are given in [1]. Hexagonal phototubes are used to instrument the focal surface as fluorescence light detectors. Even if their hexagonal shape represents the best approximation to the pixels geom- etry, a significant amount of insensitive area is nevertheless present. In fact, some space between PMTs is needed for a safe mechanical packaging on the focal surface; moreover, the effective cathode area is smaller than the area delimited by the PMT glass envelope. A crucial property of the FD camera is its efficiency of collection and uniformity. In fact, large corrections on the energy estimated from the PMT signals would be needed in case of significant losses of light in the pixel borders. 1 The systematic uncertainties on the measurement of the cosmic rays energy and direction due to this effect could spoil the performance of the FD. For this reason, the hexagonal phototubes of the AUGER FD camera are complemented by a simpli- fied version of “Winston cones” [2], which maximize light collection and guarantee a sharp transition between adjacent pixels, The mechanica