Silicon strip detectors are mounted on the top and the bottom side of a detector tray. A set of detectors on one side of a tray forms a detector layer measuring X or Y of a track. An x-y plane is formed between two trays when one is stacked on the other; a top layer of the lower tray and a bottom layer of the upper tray forms an x-y plane. Of the 14 trays, the top-most tray carries detectors only on its bottom side, and the bottom-most tray only on its top side. Detectors are mounted on the both side on the rest of fifteen trays. In the beam test tracker there are 26 layers which form 13 x-y planes, in total.
A silicon detector layer consists of 3, 4, or 5 detector ladders
depending on the location of the layer, where a detector ladder consists
either of five 4-inch detectors or of three 6-inch detectors glued and
wire-bonded together. A 4-inch detector is a square strip detector of
6.4 cm by 6.4 cm with 320 p 
strips at 194 
m pitch, and a 6-inch
one is rectangle, 6.4 cm wide and 10.68 cm long, with the same number of
strips with the same pitch as a 4-inch detector. This was designed so
that both of five 4-inch detectors and three 6-inch detectors form a
detector ladder 6.4 cm wide and 32 cm long. And five detector ladders
will cover a square region of 32 cm by 32 cm with 1600 strips. In the
tracker 112 ladders are used in total; 32 ladders of them are of 4-inch
detectors (160 detectors) and 80 of 6-inch detectors (240 detectors).
Of the 13 x-y planes 5 planes from the top of the tracker are only
partially covered by detectors; the top 4 planes consist of three
ladders on each of X- and Y-measuring layer (3 
3 plane) and the
5th plane from the top of the tracker consists of four ladders on each
(4 4 plane). The rest of 8 planes are fully covered, namely,
five ladders are on both layers (5 5 plane).
Also, on the upstream side of each x-y plane there is a lead converter. There are three different thicknesses, 3.6 % radiation length (normal thickness), 28 % radiation length (called "super GLAST"), and 0 % radiation length (or no lead sheet). The 3.6 % lead sheets are placed on the top 8 planes and the 28 % lead sheets are on the 9th, 10th, and 11th planes. There is no lead sheet placed in front of the bottom two planes.