51 layer detector, with a 000+000- axial/stereo layer arrangement. DR2 had only one layer of field wires between each layer of sense wires, allowing many more layers to fit in the allotted space. The axial sense wires had a half-cell stagger to help resolve the left-right ambiguity of the original drift chamber. The inner and outer field layers of the chamber were cathode strips to make measurements of the longitudinal coordinate of tracks. DR2 was also designed to make dE/dx measurements in addition to tracking measurements. The IZ chamber was replaced with a ten-layer drift chamber (VD) in 1984. When the beampipe radius was reduced from 7.5 to 5.0 cm in 1986, a three-layer straw chamber (IV) was built to occupy the newly available space. The IV was replaced during the CLEO II upgrade with a five-layer straw tube with a 3.5 cm inner radius. The CLEO III drift chamber (DR3) was designed to have similar performance as the CLEO II/II.V drift chamber even though it would be smaller to allow space for the RICH detector. The innermost sixteen layers were axial, and the outermost 31 layers were grouped in alternating stereo four-layer superlayers. The outer wall of the drift chamber was instrumented with 1 cm wide cathode pads to provide additional z measurements. The last drift chamber built for CLEO was the inner drift chamber ZD for the CLEO-c upgrade. Its six layer, all stereo layer design would provide longitudinal measurements of low-momentum tracks that would not reach stereo layers of the main drift chamber. With the exception of the larger stereo angle and smaller cell size, the ZD design was very similar to the DR3 design. Silicon detectors CLEO built its first silicon vertex detector for the CLEO II.V upgrade. The silicon detector was a three-layer device, arranged in octants. The innermost layer was at a radius of 2.4 cm and the outermost layer was at a radius of 4.7 cm. A total of 96 silicon wafers were used, with a total of 26208 readout channels. The CLEO III upgrade included a new four layer, double-sided silicon vertex detector. It was made of 447 identical 1 in × 2 in wafers with a 50 micrometre strip pitch on the r-φ side and a 100 micrometre pitch on the z side. The silicon detector achieved 85% efficiency after installation, but soon began to suffer increasingly large inefficiencies. The inefficiencies