New concepts for light mechanical structures of cylindrical drift chambers

A significant reduction in the amount of material at the end plates of a drift chamber can be obtained by the simple consideration of separating, in the mechanical structure, the gas containment function from the wire tension support function. According to this scheme, the wires are anchored to a self-sustaining light structure ("wire cage") surrounded by a very thin skin ("gas envelope") of suitable profile to compensate for the gas differential pressure with respect to the outside. The "wire cage" is schematically made of a set of radial spokes, constrained into a polygonal shape at the inner ends and extended to the outer endplate rim, thus subdividing the chamber in identical sectors. The drift chamber is, then, built by stacking up radially, in each of the sectors and between adjacent spokes, printed circuit boards, where the ends of the wires are soldered, alternated with proper spacers, to define the cell width. A system of adjustable tie-rods steers the wire tension to the outer endplate rim, where a rigid cylindrical carbon fibre support structure, bearing the total wire load, is attached. Two thin carbon fibre domes, free to deform under the gas pressure without affecting the wire tension and conveniently shaped to minimize the stress at the inner rim, contribute to the "gas envelope" and, together with an inner thin cylindrical foil and with the outer structural support, enclose the gas volume.