Design and Performance of 1.5 m2 Wire Spark Chambers

A group of six wire spark chambers is described, the largest of which is 1.5 m2. These chambers are presently being used at the Argonne National Laboratory in an experiment to study the interference between KL and KS 2¿ decay amplitudes. The chambers have an air equivalent scattering length of approximately 30 cm of air. They are constructed of crossed 6-mil duraluminum wires, with spacing of 24/inch. Both Aclar and Mylar films are used to cover the chambers, The frames are 1/4¿ glass epoxy G-10, pre-stressed and reinforced by four pieces of rectangular aluminum tubing. The wires are brought out on both ends of the frame, where the memory planes and resistive terminations are clamped on. Banks of three chambers are mounted as portable units; for each bank, the overall wire positioning error is less than 0.2 mm. In order to improve the pulsing properties, each wire is pulsed from both ends, i.e. through the readout core on one end and a 500-ohm termination on the other. This resistance is large enough to prevent current robbing from the core once a spark is formed, yet small enough to pass the main high voltage pulse. The shape of the pulse as applied to the chambers is roughly triangular, ranging from 120 to 200 nsec in width (depending on the size of the chamber). Readout from both the X and Y planes is obtained by means of shielded magnetic cores that operate in fields up to 400 Gauss.