Practical experience: Pixel Detectors

The NIKHEF is closely involved in the development of various parts of the ATLAS detector.  This detector will investigate high energy proton-proton collisions at the Large Hadron Collider (LHC) and will become operational in 2005 at CERN in Geneva.  At the very centre of ATLAS, the vertex and tracking detector are located ('Inner Detector').   Purpose of this detector is to reconstruct the tracks of charged particles, which are produced at the collisions, in 3-dimensional space.  In addition, the position of the proton-proton collision and momentum of the particles can be precisely determined. Position determination is done through semiconductor sensors based on silicon.  Both pixel and strip geometries are applied.  A charged particle traverses the silicon and ionises the material. The so formed free electrons are captured and fed into an amplifier.

Close to the interaction point, the density of particles will be highest and therefore, the demand s to the detector resolution will be most stringent.  Also because of this, a pixel geometry is preferred above a strip geometry so that with only one single detection layer two coordinates in space can be determined.  Next to the determination of the trajectory of the charged particle, the pixel detector shows its importance in the determination of the 'lifetime' of a particle.   A long living particle with a speed close to the speed of light will travel a certain distance before it disintegrates !  A displaced vertex occurs.  These 'secondary' vertices are characterising the occurrence of certain heavy particles which play an important role in the hunting for the Higgs particle and the study of CP-violation in the B-meson system.

The pixel detector consists of a 'sandwich' of two semiconductor substrates (chips) which are electrically connected.  Each chip contains about 2000 pixel cells with dimensions 50 * 500 mm and a resolution of 8 by 400 mm.   The information each pixel cell obtains, can be readout.  The readout chip therefore consists of an amplifier, a discriminator and a local memory unit.  The NIKHEF infrastructure is well suited for both the design work on sensor and electronics as well as testing the complete detector.

Tasks for the student include:

	calibration studies on prototype pixel detectors
	verification of the threshold uniformity of the discriminators
	timing measurements, efficiency
	temperature dependence
	computer simulations

The possibility exists that part of the job will possibly be carried out at CERN.

Further information can be obtained from:

	Prof. Dr. Ing. B. van Eijk
	Dr. E. Heijne (CERN and NIKHEF, tel. 020-592 2152, email x09@nikhef.nl)

Latest change: 12 January 1999

nederlandse versie

email: vaneijk@nikhef.nl