Barrel modules

There are minor differences between the barrel and the end-cap modules. The barrel resistive plate counters which were constructed in the United States consist of two parallel sheets of 2.4 mm-thick commercially available float glass, the content of which is 73 % silicon dioxide, 14 % sodium oxide, 9 % calcium oxide, and 4 % trace elements. The bulk resistivity of the glass is - $\Omega$ cm at room temperature. The plates are separated by 1.9 mm thick extruded noryl spacers epoxied to both plates using 3M 2216 epoxi. Fig. shows a barrel RPC with the spacers placed every 10 cm so that they channel the gas flow through the RPC to provide uniform gas composition throughout the active volume. A T-shaped noryl spacer was epoxied around the perimeter forming a gas tight unit. The spacers have the cross sections shown in Fig. . They were designed with concave regions for the epoxy joints and were extruded to an accuracy of $\pm$ 0.05 mm. Tilting table tops were used to lift the RCPs into the vertical orientation to avoid flexing the epoxy joints. After assembly, the RCPs were always moved in the vertical orientation or supported by a rigid flat surface. The barrel RPCs are rectangular in shape and vary in size 2.2 x 1.5 m$^2$ to 2.2 x 2.7 m$^2$.

Figure: Schematic diagram of the internal spacer arrangement for barrel RPC.

Figure: Cross section of the internal spacer and edge in KLM.

To distribute the high voltage on the glass, the outer surface was coated with Koh-i-noor 3080F india ink. The ink was mixed 30 % black and 70 % white by weight to achieve a surface resistivity of - $\Omega$/square. This resistivity is chosen so that this surface does not shield the discharge signal from the external pickup pads but is small compared to the resistivity of the glass to provide a uniform potential across the entire surface.
Figure shows the cross section of a super-layer, in which two RPCs are sandwiched between the orthogonal $\theta$ and $\phi$ pickup-strips with the ground planes for signal reference and proper impedance. This unit structure of two RPCs and two readout-planes is enclosed in an aluminum box and is less than 3.7 cm thick. Each RPC is electrically insulated with a double layer of 0.125 mm thick mylar. Signals from both RPCs are picked up by copper strips above and below the pair of RPCs, providing a three-dimensional space point for particle tracking. Multiple scattering of particles as they travel through the iron is typically a few centimeters. This sets the scale for the desired spatial resolution of KLM. The pickup strips in the barrel vary in width from layer to layer but are approximately 50 mm wide with lengths from 1.5 to 2.7 m. The geometry of the pickup strips was chosen so that the pickup strip behaves as a transmission line with a characteristic impedance of $\sim$ 50 $\Omega$ to minimize signal reflections at the junction with the twisted-pair readout cable. The barrel modules have a 100 $\Omega$ resistor connecting the pickup strip to ground at the cable end of the pickup strip to create an effective impedance of 50 $\Omega$ at that point. This reduces the size of the signal which reaches the readout boards for the barrel modules by a factor of two.

Figure: Cross section of a KLM super-layer.

The double-gap design provides redundancy and results in high super-layer efficiency of 98 %, despite the relatively low single-layer RPC efficiency of 90 % to 95 %. In particular, the effects of dead regions near the spacers are minimized by offsetting their locations for the two RPCs that comprise a super-layer. To provide overall operational redundancy, care is taken to supply gas and HV independently for each RPC layer so that the super-layer can continue to operate even if a problem develops with one RPC.
Each barrel module has two rectangular RPCs with 48 $z$ pickup strips perpendicular to the beam direction. The smaller 7 super-layers closest to the interaction point have 36 $\phi$ strips and the outer 8 super-layers have 48 $\phi$ strips orthogonal to the $z$ strips. The backward region of the upper octant has modules that are 63 cm shorter than the modules in the other octants in order to accommodate plumbing for the cooling of the superconducting solenoid. This chimney region can be seen in Fig. . This amounts to less than 2 % of the solid angle of the barrel coverage and has a minimal effect on the acceptance since it is in the backward hemisphere.
The glass RPCs are relatively robust except for overpressure situations which can push the two sheets of glass apart, breaking the glass-spacer epoxy joint. To avoid this hazard, the gas volume was not sealed during shipping. Relief bubblers protect the RPCs during operation. The RPCs were checked for gas leaks prior to installation. The sensitivity of our measurement was about 0.05 cc/min and this was the leak rate limit we set for all installed RPCs.