CCD Vertex Detector R&D Project

Project Overview

As a candidate of the vertex detector for ILC, we propose a vertex detector based on fine pixel CCDs (FPCCD). In this idea, the hit signal is accumulated during a train and readout in 200ms interval between trains. We would carry out the R&D program including the following items:
  • Development of the new FPCCD sensors
  • Readout ASICs for FPCCD readout
  • Fabrication and evaluation of the prototype ladders


At ILC, 2820 bunches of electron and positron beam make collisions successively with 337ns bunch intervals. This succession of 2820 bunches is called "train", and trains are repeated at a rate of 5Hz.

Due to low energy electron/positron beam background, the hit rate of the innermost layer of the vertex detector is estimated to be ≈1.5 hit/cm2 at R=2.0 cm and B=3T for one bunch crossing (BX). If the hits are accumulated for one train, the hit density becomes very high, and the pixel occupancy for a pixel detector with 25μm pixel size exceeds 10%, which is not acceptable.

One method to keep the pixel occupancy acceptable level (≈0.5%) is to read out the sensors more than 20 times in one train. There are R&D activities by several groups based on this method. This method requires very fast readout speed, and the feasibility has not been demonstrated yet.

In the FPCCD vertex detector, we use very fine (≈5μm) pixel CCDs. The pixel occupancy will be less than 0.5% even if the hit signal is accumulated during a total train of 2820 bunches. In order to suppress the number of hit pixels due to diffusion in the epitaxial layer, the sensitive layer of the FPCCD should be fully depleted.

Another advantage of the FPCCD vertex detector is that it is completely free from electro-magnetic interference (EMI) caused by the beame. At the SLD, an experiment executed at SLC which is the first linear collider in the world, the EMI caused by the beam affected the readout of the vertex detector. For the FPCCD vertex detector, however, the EMI would cause no effect because the detector is read out between trains.

Recently, CCDs with the pixel size less than 5μm are used for digital cameras. So, we believe the FPCCD is technically the most feasible option for the vertex detector.