From HEP at Tennessee
At the smallest radii from the beam line (at 4 and 7 cm at low luminosity and at 7 and 11 cm at high luminosity) the interaction region is surrounded by two barrel layers of Silicon Pixel detectors. Two endcap disks cover radii from 6cm to 15cm.
The pixel layers are composed of modular detector units. Each module consists of a thin,segmented sensor plate with highly integrated readout chips connected to them using the bump bonding technique,as shown below.
Schematic view (Figure left) of a pixel detector element. Each sensor pixel is connected via a solder bump to a pixel unit on the read-out chip,where the signal is amplified.The hit data are stored on the edge of the chip where they wait for the trigger information.
The cell size in the Pixel detectors is 100x150 µm2 .The pixels used are n-on-n devices so that,in the barrel,their response is strongly affected by the 34o Lorentz angle of the drift of electrons.The barrel Pixel geometry is deliberately arranged so that this large Lorentz angle induces significant sharing of charge across neighboring cells and this results in spatial resolutions of ≈10 and 15 µm in the φ and z coordinates respectively.
Charge sharing is induced by the Lorentz drift (see Figure right). The sensor material is silicon in which the electron drift angle is three times larger than for holes.Therefore,n-type pixels,which collect electrons,will be used. When the electrons arrive atthe pixel surface,they are spread over an r φ distance of ˜(detector thickness)x tan(34 degrees).
Motivation
- Efficient and robust pattern recognition
- high speed to resolve bunch crossing (40MHz)
- fine granularity to resolve nearby tracks;
- occupancy @ 10cm radius, 4T B-field = 0.5..1 track/cm2 << cell size
- Reconstruct narrow heavy objects
- good mass resolution requires 1-2% pT resolution at 100 GeV/c
- Tag b and tau through secondary vertices
- Radiation hardness
- up to 3*1014 neq/cm2/year at full lumi in the innermost layer