E. Borsato

The principle of operation of a newly developed proximity focused Hybrid Photon Detector is described. The HPD characteristics, performance and calibration are reported. Results from beam tests of aerogel threshold counters read-out by HPD and the particle identification performance are presented. 1 The Hybrid Photon Detector The operating principle of the proximity focused Hybrid Photon Detector (HPD) is shown in figure 1. Light striking on a 2 mm thick quartz window hits an S20 UV photocathode that features a high quantum efficiency response peaked in the UV region (figure 2). Photoelectrons are accelerated by a uniform electric field and penetrate in the depleted region of a 300 ¯m thick 1 inch diameter silicon diode operated in reverse-bias mode. The kinetic energy is converted into a charge pulse picked up at the diode contacts. The gain is about 3300 at the operating high voltage of 15 kV . The electric output signal is subsequently amplified and conveniently shaped to be measur..

The BaBar drift chamber

The central drift chamber for the BaBar detector at the PEP-II B-factory at SLAC is a cylindrical chamber with a length of 280 cm and outer radius of 81 cm. It consists of 40 layers of small hexagonal cells arranged in 10 axial and stereo super layers. In order to minimize multiple scattering, light materials are used for the mechanical structure, and the gas mixture is helium based. The pulse height and timing electronics are mounted directly on the chamber rear end plate. A full-length prototype of the BaBar drift chamber has been built. The analysis of cosmic-ray events measures the spatial resolution averaged in the cell to be 130 μm and the dE/dx resolution to be 6.8%, meeting the performance goals for the BaBar central tracker. The mechanical assembly and stringing of the chamber was completed in December 1997 and the detector will be integrated into BaBar during summer 1998