From HEP at Tennessee
Excellent particle identification for hadrons and leptons over a large range of solid angle and momentum is an essential requirement for meeting the physics objectives of BABAR. In particular, measurements of CP violation require particle identification (PID), both to reconstruct exclusive final states and to tag the quark content of the other B in the event. Information from the drift chamber, calorimeter, and the instrumented flux return can be used to identify most of the leptons and many of the hadrons. However, these systems are not sufficient to distinguish charged pions from kaons with momenta greater than about 0.7 GeV/c, or protons above 1.3 GeV/c, as is required to obtain efficient tagging and event reconstruction.
To meet these requirements, a new type of PID system has been chosen for BABAR. This barrel region detector is called DIRC for "Detection of Internally Reflected Cherenkov light". It is a ring imaging Cherenkov detector based on total internal reflection and uses long, rectangular bars made from synthetic fused silica ("quartz") as both radiator and light guide. It is thin (both in radius and radiation length), fast, robust, and tolerant of background.
How does the DIRC work?
A charged particle traversing a DIRC quartz bar with velocity |beta| in a medium of refractive index n produces Cherenkov light if |n*beta|>1. The DIRC Cherenkov radiators are 4.9 m-long rectangular quartz bars oriented parallel to the z axis of the detector. Through internal reflections, the Cherenkov light from the passage of a particle through the DIRC is carried to the ends of the bar.
The DIRC uses as a radiator 144 quartz bars arranged in a 12-sided polygon around the beam line. This maximizes azimuthal coverage, simplifies construction, and minimizes edge effects. For sufficiently fast charged particles, some part of the Cherenkov radiation cone emitted by the particle |theta_c-equation|, (with n=1.473) is captured by internal reflection in the bar and transmitted to the photon detector array located at the backward end of the detector. (Forward-going light is first reflected from a mirror located on the end of the bar.) The high optical quality of the quartz preserves the angle of the emitted Cherenkov light. The measurement of this angle, in conjunction with knowing the track angle and momentum from the drift chamber, allows a determination of the particle mass. An advantage of the DIRC for an asymmetric collider is that the high momentum tracks are boosted forward, which causes a much higher light yield than for particles at normal incidence. This is due to two effects: the longer path length in the quartz and a larger fraction of the produced light being internally reflected in the bar.
Each quartz bar is 1.7 cm thick, 3.5 cm wide, and 490 cm long, and is constructed by gluing end-to-end four shorter bars. The total radial thickness of the DIRC, including quartz thickness, sagitta from the polygonal shape, mechanical supports, and a 1 cm clearance on each side, is 10 cm. This material represents 0.19 |X_0| (radiation length) at normal incidence. An effort has been made to minimize both the radial thickness and the amount of material, since these increase the radius and cost of the barrel calorimeter while degrading its performance for soft photons.
A quartz "wedge" is glued to the readout end of each bar. It reflects the lower Cherenkov ring image onto the upper one, reducing the number of PMTs needed by 50%. The wedge is a 9 cm long block of synthetic fused silica with the same width as the bars, and a trapezoidal profile (2.7 cm high at the bar end and 7.9 cm high at the quartz window which provides the interface to the water). Total internal reflection on all sides of the quartz wedge provides nearly lossless reflection.
The photon detector consists of about 11,000 conventional 2.5 cm-diameter phototubes. They are organized in a close-packed array at a distance of about 120 cm from the end of the radiator bars. The phototubes, together with modular bases, are located in a gas-tight volume as protection against helium leaks from the drift chamber.
