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*This work is supported in part by the U.S. Department of Energy, Contract W-7405, Eng-36.

Melynda Brooks
MS-H846, Group-P25
Los Alamos National Laboratory
Los Alamos, NM 87545

phone: (505)667-6909
FAX: (505)665-7920
email: mbrooks@lanl.gov

The PHENIX muon tracking system is comprised of three cathode strip chambers at each of three stations, all of which sit in a magnetic volume whose B.dl varies from 2.6-7.7 gauss-meters. One of the primary measurements for the heavy ion program will be to reconstruct vector mesons which decay into muons. We need to reconstruct the highest mass mesons, upsilons, with a mass resolution of approximately 200 MeV/c, which can be done if we have a spectrometer resolution of slightly better than 2% at momenta=10-30 GeV/c. To achieve this momentum resolution, we require a chamber resolution of approximately 100 m per chamber plane. We have studied the resolution we can achieve with various detector geometries, noise levels, and as a function of Lorentz angle and various incident angles so that we can optimize the design of the muon chambers, to achieve our desired resolution of 100 m or better. Some of the results of these studies are shown in Figure 1.

The noise levels of our cathode strip readout channels directly affect the ability to accurately fit the charge distribution in the cathode plane, which affects our chamber resolution. We have specified a noise level of 3000 electrons or better to limit the affect of the noise on resolution to approximately 50 m.

The strip width of the chambers has been chosen to be 5 mm, with 10 mm readout to achieve the desired resolution, while keeping the number of readout channels required to a minimum. Since the anode wires will also be read out to give a position measurement in the r direction, the anode wire readout spacing (1 cm) has been chosen to obtain the radial resolution required for good momentum reconstruction.

The geometry of the muon spectrometer is such that the incident angles of tracks on the chambers can vary in the angle with respect to the strips as well as the angle with respect to the anode wires. Both of these angles affect the resolution. We have studied the affect on the resolution and designed the anode wires to minimize the resolution degradation while still making the anode wire placement as simple as possible.

The angle of the strips with respect to the anode wires very strongly affects the chamber resolution if the position of the particle in the gap is not well known. We have studied the comparison of the performance if the position in the gap is determined with pattern recognition and the resolution is corrected for the angle of the strips with respect to the wires to the performance if the angle of the strips to the wires is kept small.

Finally, the affect of the Lorentz angle on our resolution has been studied for the CO-CF gas mixture which we expect to use, and with our maximum expected B-field values. We then combined all the above affects and simulated what the resolution should be over the entire chambers, and found that we should be able to achieve a resolution of approximately 85 m.

Figure 1: The CSC resolution, with various starting conditions, versus the angle the strips are from perpendicular to the anode wires, the Lorentz angle, the noise levels on the strips, the phi incidence angle, the strip width, and the theta incidence angle. The dotted lines show the baseline values for our system.