In 1998, DØ will upgrade the central tracking region of the detector with an entirely new system consisting of a 2 Tesla superconducting solenoid, a 6000 channel preshower detector made of scintillator with wavelength shifting fiber readout, an outer 80000 channel scintillating fiber tracker and an inner 837,000 channel silicon strip tracker[1]. The optical signal from the fibers will be detected by Visible Light Photon Counters(VLPC's). VLPC's are arsenic doped silicon diodes that operate near a temperature of 6.5 K with a bias of approximately 6.5 V[2]. High quantum efficiency, relatively high gain with low dispersion, combined with high signal forming speed and low noise, make these devices interesting for many high energy physics applications, in particular fiber tracking.
Since no experiment has ever used a fiber tracker read-out with many thousands of VLPC's, we decided to build a scaled-down version of the final fiber tracking system, triggering on cosmic rays. The Cosmic Ray Test Stand is a 3072 channel prototype of the final fiber tracker in the D0 upgrade. The primary purpose for this facility is to confirm that operation of large numbers of VLPC's is feasible and well understood. The detailed examination of the characteristics of the VLPC's is the subject of this report. In particular we focus on understanding the general operating parameters of the VLPC's, on determining the optimal operating points for the devices in the sample, and on issues like homogenity of the sample, yield, and a quantitative investigation of failure modes.