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Drift Chamber

The primary functions of the cylindrical drift chamber [13], which subtends a solid angle of sr for kaon decay products of momentum P> 150 MeV/c, are to provide a momentum measurement in the 1-T magnetic field and to achieve good tracking between the target and the range stack. Momentum resolution of (rms) is required to obtain the necessary level of background rejection related to and decays. The chamber is required to have a short memory time in order to operate successfully in the presence of high rates of kaons and pions in the beam.

The chamber wires are arranged in five superlayers of 36, 40, 50, 60, and 70 cells, respectively. The wires of layers 1, 3 and 5 are strung axially, and those of layers 2 and 4 are strung between feedthroughs offset by one cell, corresponding to stereo angles of approximately to enable axial position measurements. The configuration of wires and wire diameters within a unit cell is shown in Fig. 7a.

 
Figure 7: (a) Schematic showing the wire geometry in a drift cell. (b) Trajectories of ionization electrons to the sense wires in a 1.0-T magnetic field.  

Cells within a layer are identical, but cell size varies from layer to layer between 12-mm and 17-mm half width. The cathode-wire plane, consisting of 19 Be-Cu wires strung 2.54-mm apart to a tension of 150 g , is shared by neighboring cells. The sense-wire plane has two Be-Cu end guard wires and eight 20-m-diameter gold-plated tungsten anode wires spaced 5.08 mm apart at a tension of 50 g and staggered 254 m from the midplane to resolve the left-right ambiguity locally within the cell. The end wires on both anode and cathode planes are of larger diameter as shown in Fig. 7a to optimize electric field uniformity and to maintain surface fields <20 kV cm on the cathode wires when those on the sense wires are about 250 kV cm. This arrangement without focussing or potential wires in the anode plane provided a high electric field in the drift region of kV cm, and reduced the Lorentz angle. To provide the correct potentials at the cylindrical boundaries of the chamber, thin copper-plated Mylar foils were attached to the graphite-epoxy support cylinders and set at the appropriate voltages. The trajectories calculated for a typical cell, of ionization electrons in a 1-T magnetic field are shown in Fig. 7b. Only the six middle anode wires are read out in order to avoid the cell-end distortions in the electric field and to reduce the effects of the Lorentz angle which is about 25 for a drift velocity of 5 cm s. The chamber is operated at atmospheric pressure with a gas mixture of Ar : CH = 50 : 50. The argon fraction is bubbled through ethanol at 0 C.

The chamber occupies the radial region between 95 mm and 432 mm with a total length, including preamplifiers, connectors, and cables of 650 mm. The active volume, 508-mm long, is enclosed between 9.5-mm-thick precision-machined Al endplates carrying the slots to receive the feedthroughs which position and support the wires. Each feedthrough is made up of a machined comb of Vespel epoxied into an injection-molded Ryton insert which is epoxied into the slot. The overall mechanical tolerance for the location of each wire was measured to be 20 m. The endplates are supported by inner (80-mg cm thick) and outer (94-mg cm thick) cylinders made of graphite-fiber epoxy, which was built up of four (inner) and five (outer) pre-impregnated layers with fiber orientation alternately axial and . The endplates were prestressed prior to stringing with a system of temporary tie-rods which were adjusted and then removed as wires were installed, taking up the tension load. The total endplate loading due to wire tensions is 880 kg. The outer cylinder was installed after stringing was complete. During stringing, its compression load was taken up by a set of temporary posts. Subsequent replacement of individual broken or faulty wires has been done with the outer cylinder in place. The chamber is supported in the detector by a 3.2-mm-thick aluminum cylinder which extends from the magnet pole and is attached at the circumference of the downstream endplate.

Contact to each wire for high voltage (HV), ground, or onboard preamplifier, is via a printed edge connector card fitted into the feedthrough, to which the wire was soldered after tensioning. For the cathodes, HV is distributed to one third of a layer from one of 15 Bertan 1755N HV power supplies. For each of the 15 sections of HV distribution the voltage is graded in a resistor chain to compensate for the variation in cell width with radius in order to form a uniform drift field throughout the cell. The external regions of both endplates, on which the HV distribution and preamplifiers are mounted, are sealed and dry N is circulated to control temperature and to eliminate HV breakdown due to atmospheric humidity.

Each sense-wire plane has six preamplifiers mounted on a card plugged onto the downstream feedthrough and grounded locally to the endplate. A typical preamplifier gain is 10 mV A with a risetime of 3 ns and power dissipation of 24 mW. The positive-ion-induced crosstalk of signals between adjacent wires of 15% is reduced to by a compensation resistor network which splits the input charge at the front ends of the preamplifiers. Each preamplifier drives 34 m of 50- coaxial cable which degrades the signals to about half amplitude and risetime of about 7 ns when they reach the post-amplifier in the counting house. All cables, including signal, HV, preamplifier power, and test pulse, exit the magnet and go through a bulkhead connection at the downstream end of the detector.

The post-amplifier-discriminator circuits are housed, 24 per module, in a DIN 41494 standard sub-rack system (Euro-crates). The input signals are decoupled from ground by transformers and split for separate analog and discriminator processing. The gains of the amplifiers are 7--10 for the analog output and 100 for the discriminator path. Pole-zero cancellation is employed to minimize fall time and reduce the pulse tail, due to positive ions and to the 34-m cable, to a level of . The discriminator produces a time-over-threshold output with a 10-ns minimum width and feeds an LRS 1879 Fastbus pipeline TDC for drift time (up to 350 ns) measurement.

The drift velocity v, Lorentz angle , and time offset (pedestal) are determined iteratively by fitting tracks in the chamber. For v, zero-magnetic-field data were used. Position resolution obtained from data, shown by the residuals from track fits in Fig. 8,

 
Figure 8: Typical local position resolution in the drift chamber as a function of drift distance for stereo and axial layers.  

varies with drift distance and layer between 130 and 250 m for the axial layers. The z (axial) resolution obtained from the stereo layers is between 2.2 and 4.2 mm. The momentum resolution, after correcting for the energy loss in the target, is shown in Fig. 9.

 
Figure 9: Momentum for events corrected for the energy loss in the target system.  



next up previous
Next: Range stack Up: Detector Previous: Target



Experiment E787
Tue Sep 28 01:41:06 EDT 1999