Cell Design for Helium Mixtures

Since we have greatly reduced the amount of material in the gas by using a helium mixture, the total mass of wire material is now significant and must also be reduced. One would like to choose a cell design with the smallest wire volume in the chamber, and a wire material that will result in the smallest fraction of a radiation length. 

During the 1991-92 workshops at SLAC [2], we made detailed comparisons between a small-cell and a jet-cell drift chamber for u.se at a i factory. Both cases considered 

40 layers of sense wires, grouped into 8 or 10 superlayers, with the superlayers alternating between axial wires (along the cylindrical axis) and stereo wires (at about 3° to the axis) directions. The half cell sizes used were 7.5 mm for the small cell and 25 mm for the jet cell. The most obvious differences between the two cases are the smaller number of sense and field wires in the jet cell, by about a factor of two, and the linear electric field in the jet cell. The jet cell has the advantage of fewer electronic channels and a simpler time-to-distance relation, but the small cell also has its advantages. 

We studied the amount of wire material, averaged over all cells, for the two designs and found the small cells had less total material. The jet cell, perhaps surprisingly, has more material, because thicker field wires are required to sustain the forces, particularly at the ends of the cell, where the the electric field is high. 

For the case of the hexagonal cell structure, we can calculate the total number of radiation lengths in the BaBar chamber as well as the expected resolution. Table 3.3 shows the fraction of a radiation length for gas and wire in a chamber with 40 cylindrical layers of 

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