Progress on development of the new FDIRC PID detector

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Abstract

We present a progress status of a new concept of PID detector called FDIRC, intended to be used at the SuperB experiment, which requires π/K separation up to a few GeV/c. The new photon camera is made of the solid fused-silica optics with a volume 25× smaller and speed increased by a factor of 10 compared to the BaBar DIRC, and therefore will be much less sensitive to electromagnetic and neutron background.

Introduction

The BaBar experiment has used very successfully the DIRC PID detector (DIRC=Detector of Internally Reflected Cherenkov light) [1]. The original DIRC had one weak point: its huge photon camera, filled with 6000 l of ultra-pure water, was sensitive to electromagnetic and neutron background. This needs to be improved as the SuperB luminosity will increase 100×. Therefore we have exchanged this simple pinhole camera at the heart of the DIRC detector into a focusing camera with sophisticated, solid fused-silica optics, while shrinking it to 1/25th of its former size and increasing its speed by a factor of 10 [2]. The new detector is called FDIRC (Focused Detection of Internally Reflected Cherenkov light). Each of the 12 FDIRC Photon cameras will have 48H-8500 MaPMTs, providing excellent timing capability for single photons (σTTS140ps, TTS is a transit time spread). It is also necessary to implement new front-end electronics (FEE) with much higher timing precision (σElectronics100ps), higher hit rate capability (few MHz hit rate per pixel), and small dead time (less than 5% at 1 MHz rate) [3]. These improvements will compensate for the increase in luminosity (×100) and background between the two generations of experiments. A full scale FDIRC prototype module covering 1/12 of the barrel azimuth will be tested in the cosmic ray telescope (CRT) [4] at SLAC with 3D tracking using muons.

Section snippets

FDIRC design and construction

A very important design feature of FDIRC photon camera is to measure the Cherenkov angle with xy pixels alone (the detector coordinate system where the y-direction is vertical and the x-direction is horizontal), i.e., a high resolution timing is not needed. Time is used, however, to cut the background, perform the chromatic correction by timing, and be part of PID maximum likelihood.

Fig. 1, Fig. 2 show the new FDIRC photon camera design. The camera was designed with one important constraint:

Choice of photodetectors and electronics

There were three Hamamatsu photon detectors under consideration, the H-8500 (64 pixels) and the H-9500 (256 pixels), and very recently R-11265-00-M64 (64 pixels) multi-anode PMTs (MaPMT) by Hamamatsu. At present, our nominal choice is a 12-stage H-8500 tube. It has a sufficient single electron timing spread (σTTS140ps), it can be obtained with somewhat enhanced quantum efficiency QE of 24%, it has more acceptable uniform gain response across its face (1:2.5), and it has been studied quite

A summary of R&D results up to now

It was very important to build a first FDIRC prototype with a single bar and oil-filled photon camera [10]. We learned how to operate new fast highly pixilated detectors [7], how to design a new optics, how to do the chromatic correction by timing (this was the very first RICH detector to achieve this). To be able to do such correction, one needs to achieve a timing resolution at a level of 200–250 ps per single photon, and the photon path length needs to be longer than 2–3 m. The fact that FDIRC

Expected performance of the final FDIRC

Contributions to the Cherenkov angle resolution per photon and per track are shown in Table 1. Fig. 8 shows the expected performance relative to the BaBar DIRC for various photodetector design choices. Fig. 7 shows MC simulation of FDIRC expected Cherenkov angle resolution in CRT for H-8500 tube with 6 mm pixels and no chromatic correction. Clearly, a small pixel size and high QE efficiency is crucial to obtain the best possible performance, although a nominal design with H-8500 tube and

Goals of the final FDIRC prototype test in CRT

The FDIRC prototype will be tested at SLAC in CRT using 3D tracks. The tracking system has an angular resolution of 1.5 mrad, track position resolution of 5–6 mm, start time resolution of better than 90ps (with tracking corrections it may reach 70ps), a 1.6 GeV muon low energy cut-off thanks to a 46 in.-thick iron absorber, and a large iron area of 55 in.×90 in. allowing to reach large range of dip angles. The 3D-tracking capability is very important to understand tails of the Cherenkov angle

Conclusion

We have clearly demonstrated that the photon camera, made of solid fused silica, is buildable. Its main advantage is the radiation hardness, small size, stability and practically no maintenance. It is more expensive compared to oil-based or water-based designs, but they require a long-term maintenance, which is at the end more expensive if one integrates over 10 years of operation. And fluid-based systems represent higher risk to other subsystems.

Acknowledgments

The authors would like to thank M. McCulloch for help in final assembly of the optics and prototype, M. Zago for the mechanical design of the Fbox and the Padova University mechanical workshop personnel for Fbox construction and assembly. We also thank M. Mongelli and V. Valentini of Bari for Fbox mechanical support structure design and M. Franco for helping to construct it. This work was supported in part by the Department of Energy, Contract DEAC02-76SF00515.

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