Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment
Optical alignment system for the PHENIX muon tracking chambers
Introduction
The PHENIX experiment at RHIC has two muon arms (north and south arms) placed at the forward rapidity regions. The role of the PHENIX muon arms is to track and identify muons, providing good rejection of pions and kaons. To accomplish this, we employ a radial field magnetic spectrometer with precision muon tracking chambers followed by a stack of absorber/low-resolution tracking layers (muon identifier), as shown in Fig. 1. The muon tracking chambers consist of three sets of cathode strip readout tracking chambers spanning 4.5(3.5)m for the north(south) arm, mounted inside conical magnets, with multiple cathode orientations and readout planes in each station [1], [2]. The main goals of the muon arms are to study vector meson production, the Drell-Yan process, heavy quark production, and Z0, W± production at the forward rapidities. Clean separation of J/Ψ from Ψ′ requires a muon-detection position resolution of at each station. To maintain the momentum resolution, an optical alignment system is installed to calibrate the initial placement of the chambers, and to monitor thermal expansion of the chambers.
Section snippets
Optical Alignment System (OASys)
The muon momentum is determined by measuring the displacement of a muon hit position at the station two chamber with respect to a straight line between those at stations one and three; therefore, only the relative straightness must be known to high accuracy. The absolute placement of the chambers is surveyed with respect to a PHENIX hall monument system and has accuracy of 1–. The absolute positions of the chambers need to be known only to a few mm, but the relative alignment of the chambers
System alignment
Each component of the OASys must be placed precisely at the designed position. The required accuracy of placement is better than . For this purpose, we built an OASys-component-alignment system with a small optical rotational stage. We set a fiber optic light source, a convex lens and a CCD camera at a well-focused position on a -long optical table. The light-source-mounting block with the fiber optic cable is placed on the precision optical rotational stage, which has the same
Data acquisition and online data analysis
The block diagram of the CCD readout DAQ system is shown in Fig. 3. The 56 CCD camera signals are fed into a video-signal switching multiplexer which has one video-signal output port. The multiplexer Keithley 7001 has two slots, in which we use two 40-channel multiplexer cards (Keithley 7011-S). CCD channel switching is controlled by external TTL signals. The TTL channel switching signal cable and the video-signal cable are connected between the multiplexer and the frame
Results
Sample CCD images are shown in Fig. 4, Fig. 5. Fig. 4 represents CCD images for well focused OASys beam channels with sharp focal images. In contrast, Fig. 5 represents that for weakly focused channels with typical broad focal images. Because the focal image intensity for the broad images is very low, it is difficult to recognize even the existence of the focal image. In Fig. 5, we can see a clear multi-ring interference pattern at the intensified figure. The X-dimensional and Y-dimensional
Summary
The development of a micron-precision optical alignment system (OASys) consisting of a light source, lens, and CCD camera for monitoring straightness is described. The OASys is built and installed within a precision of for the muon tracking chambers of the south muon arm used in the PHENIX experiment at RHIC in 2001–2002. Thermal movements of the chambers have been successfully monitored every hour for one year with 1– precision. As well as for the south muon arm, the north muon arm,
Acknowledgements
The authors appreciate R. Savino and BNL technical staffs for the technical support on the development of the OASys mechanics. We also thank E.G. Romero from Hytec Inc. and D. Clark from Los Alamos National Laboratory for their efforts on designing the mounting devices. Some of the authors acknowledge the support of RIKEN Special Postdoctoral Researchers Program and Technology Research Associate Program.
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