Pad HPD

The pad HPD will be housed in a cylindrical glass envelope capped with a UV-glass entrance window with a K₂CsSb photocathode. A visible light photocathode deposition facility that allows a high vacuum seal of the HPD baseplate onto the metal flange of the glass envelope has been designed. The final assembly of this apparatus was performed at CERN at the end of 1997. The signal from the photoelectrons in the silicon sensor will be detected by front end chips placed around the edge of the sensor via wires bonded to the routing lines from the pad. Focussing electrodes, fixed in the glass envelope, will demagnify the image by a factor of 2.3. The 2048 pad silicon detector contains pads of dimensions 1$\time$1mm². Successful tests have been performed on the sensor with photoelectrons up to 20 keV with (the non LHC speed) VA3 chip [7]. A signal/noise ratio of $\approx$ 10 has been achieved. Earlier tests with a 256 pad sensor had achieved a signal/noise ratio of $\approx$ 18. This difference is under investigation, but one possible reason for this degradation in performance is differences in the manufacturing of the wafer.

The SCT-128A [8] analogue chip which was developed for the ATLAS silicon tracker is being modified to achieve a noise level of $\sim$ 600e^-. Additional modifications will be needed to meet the requirements of the pad HPD for LHC-B, in particular the multiplexing properties of the chip.

  

Figure 3: Configuration 1: the 1/4 scale prototype RICH vessel