The purpose of the PHENIX Muon Arms is to enable the study of vector mesons decaying into dimuon pairs, to allow the study of the Drell-Yan process, and to provide muon detection in , all as part of both the relativistic heavy ion and spin physics programs of PHENIX. Each muon arm must both track and identify muons, as well as provide good rejection of pions and kaons; therefore, both a Muon Tracker and a Muon Identifier are needed.

Each arm of the Muon Tracker (muTr) comprises three stations of tracking chambers, with three cathode strip chambers each, mounted inside the end-cap muon magnet. The muon tracking station 1 and 3 chambers are kinematically mounted to the magnet and electrically isolated from it. The station 2 octants are attached to a special support spider that is also electrically isolated from the magnet. A survey system for monitoring chamber positions in real time is an integral part of the tracking system. Two different construction techniques are being used for the muTr chambers: stations 1 and 3 are constructed of honeycomb panels with cathode strips on the inside surfaces and the station 2 are constructed of stacks of wires and etched foils attached to aluminum frames.

The Muon Identifier (muID) system comprises six walls of steel absorber interleaved with 5 layers of plastic proportional tubes of the Iarocci type in each of the two muon arms. The first absorber wall is the steel end-plate flux return of the muon magnet. The tubes have resistive graphite coating on the inner surface that serves as the cathode. The muID provides the L1 trigger for the Muon Arm.

A significant part of the muon tracking system, namely the muon magnet system, is discussed in section 2.8.2 and the reader is referred to that section for a further description of the magnet systems. Section 2.9.7 will focus on the particle absorbers, muon tracking chambers, the muon ID system and system electronics. There are separate north and south muon tracking and identification systems. Section 2.9.7.1 will focus on the North Muon Arm and its suite of detectors. Section 2.9.7.2 will focus its discussion on the South Muon Arm and highlight the differences between the north and south arms. Section 2.10.6 will contain a safety assessment for both arms given the common design elements of both systems.

The North Muon Arm starts with a series of absorbers that filter hadronic and electromagnetic particles from entering the tracking and ID systems. The first absorber is a copper nose cone mounted on the face of the pole tip of the central magnet. The second is a copper plug located between the muon magnet piston and the central magnet pole piece. The copper plug has a recessed cavity to accommodate the Beam-Beam counter. The third absorber is a stainless steel plate used for mounting the electronics of station 1.

Three muon tracking chambers will be installed at stations labeled 1-3 in the MMN in the region between the muon magnet piston and the octagonal "lampshade" which serves as the flux return element. The chambers are in the radial magnetic field confined between the piston and lampshade. The first chamber is mounted in front of the muon magnet and the other two are inside the magnet enclosure. Station 1 is kinematically mounted to the front face of the magnet, station 2 is mounted to the bottom three lampshade panels and to the tea cup flange. Station 3 is kinematically mounted to the back plate of the magnet.

Stations 1 and 3 are constructed using NOMEX honeycomb panels with FR4 copper clad boards laminated to them. Those panels sandwich a stretched wire plane. This construction provides a strong structure and a certain degree of protection against accidental punctures and resulting gas leakage.

Physics constraints dictate that station 2 be built with minimal material to reduce multiple scattering. Consequently, that chamber is constructed using metalized copper mylar foils and wires stretched on a thin aluminum frame. The thin frames are sandwiched between two large aluminum frames that take the load from both the foil planes as well as wire signal planes in between. A stainless steel spider support system holds the eight octants together and attaches to the bottom lampshade panels and the tea cup. The 5-mil mylar windows present a concern regarding the potential for damage during maintenance.

The muTr gas mixture is Isobutane-CF₄ that will flow at a rate of 2.6 liters/min. The gas is recirculated and not purged to the atmosphere.

The Muon Tracker Front-End Electronics is comprised of about 100 chassis containing the approximately 200 Front-End Modules (FEM’s) needed to read out the detectors for one muon arm. Each chassis has an integrated closed-loop circulating-water system to provide cooling for the enclosed electronics. There are also ports for circulating dry-nitrogen gas through the chassis to prevent moisture buildup on the enclosed electronics and potential current leakage in high humidity. The chassis are mounted inside the MMN by means of an octant-based framework that is mechanically and electrically independent of the detector mounts. This support also provides distribution boxes, manifolds and channels for all services runs to both the detectors (gas, high-voltage power) and their front end electronics (low-voltage power, cooling water, insulating dry-nitrogen gas), while keeping all non-detector elements outside the acceptance. The detector high-voltage distribution system includes redundant bleeder elements to guarantee discharge of the detectors in case of a break in the normal return path.

The muon identifier consists of six interleaved layers of steel and detectors respectively. Each detector consists of two layers of Iarocci tubes made from PVC plastic with a resistive coating. A cell consists of 8 tubes with a 9 x 9 mm cross section and a thin 100 micron wire stretched at the center. The plastic tubes are housed inside an aluminum enclosure or panel. The walls of the panel are 100 mils thick and the two layers are separated by an aluminum sheet 125 mils thick. There are six panels of two layers of tubes each per gap, four of which measure 5 x 5 meters and the remaining two are 2.2 x 3.8 meters. These provide overlapping tiles to cover the 12.5 x 9.8 meter area.

The muon Id steel overlaps the DX magnet and provisions in the design allow for access to the vacuum pumps for maintenance and removal if necessary.

The gas used in the detector is CO₂-Isobutane 91-9%, and it is vented into the atmosphere through the 24-inch exhaust ducts at a rate of 50 cubic meters per day. Each muon identifier arm has a gas volume of 25 cubic meters. Normal operation will require a flow rate of 17.5 liters per minute at atmospheric pressure to assure one volume gas exchange per 24 hours. The tubes are rated to withstand 6 times normal overpressure as per purchase agreement (i.e., 60 mbar above atmospheric). Leaks from the tubes to the aluminum panels will be monitored by sensors, and the interior of the panels exterior to the tubes is flushed continuously with nitrogen.

The operating voltage of the tubes with the above gas mixture is 4300 volts and the expected current is 1 microampere per tube. A LeCroy HV supply will be used that provides a maximum of 5000 Volts per channel at a maximum current of 200 microamperes. The tubes are electrically isolated from the aluminum panels and supports. The HV and signal cables go through isolated feeds. The high voltage is fed to individual tubes via a special connector. The connector was certified by the manufacturer, Pol.Hi.Tech, to 10 kV with less than 10 nanoamperes of leakage current. The connector box be filled with araldite to mitigate potential current leakage in high humidity.

The Muon Identifier Front-End Electronics has components that are mounted both inside of the panels and external to the detector. Inside the panels there are pre-amplifer boards. Those boards amplify the raw signals from the tubes and send them out of the panel to the Front-End Modules (FEM’s) in VME-style crates external to the detector. There will also be low-voltage power supplies and gas monitoring systems mounted very near to the FEM’s on raised platforms near the edge of the Muon ID steel. Consequently, there is routing of detector gas, nitrogen gas, low- and high-voltage and signal cables to the muID panels.

The detector technology for the South Muon Arm’s muon tracker and muon ID system are virtually identical to the North Muon Arm’s. The principle difference between the two systems is the size given the need to be able to move the MMS 1.5 m south of its operating position in order to move the central magnet (CM) along the north-south direction. This means that the MMS is shorter than the MMN and has to be mounted on a track system like the CM. The current conceptual design (see Fig. 2.8.2-5 in section 2.8.2) shows that the magnet piston and lampshade of the MMS are smaller than those on the North Arm and a Muon Identifier system identical to the North Arm.