The ATLAS experiment is the largest of the four experiments that has just begun recording data from collisions produced by the LHC. Since detector installation was completed in 2008, it has been commissioned using first cosmic-ray [128, 5] and, more recently, collision data [6]. Figure 4.10 (a)
Figure 4.9: Illustration of the configuration of the 16 scintillators in each of the two MBTS disks.
shows the very first LHC collision event recorded by ATLAS and Fig. 4.10 (b) one of the first events recorded by ATLAS for collisions at centre of mass energy of 7 TeV. More than 97% of the read-out channels of each sub-detector are operational.
(a)√
s= 900GeV
(b)√
s= 7TeV
Figure 4.10: a) The first LHC collision event recorded by ATLAS on 23 November 2009. The tracks for the charged particles are straight because the magnetic field of the solenoid was off. b) One of the first collision events at 7 TeV recorded with the full ATLAS detector on from 30 March 2010.
Both events are examples of inelastic proton-proton collisions.
Chapter 5
The ATLAS Pixel Detector
5.1 Silicon Detectors
Silicon detectors measure the passage of charged particles to extremely high spatial precision. They are therefore typically situated close to the interaction point where the particle density is highest.
The closer the first layer of silicon to the interaction point, the better the track parameter resolution, but the higher the radiation dose. Therefore the optimal detector placement involves balancing the performance with the detector lifetime.
Silicon can be either positively or negatively doped by introducing impurities into the silicon lattice. Positively doped silicon (p-type) is typically produced by introducing boron atoms. As boron has three valence electrons in comparison to silicon’s four, boron borrows an electron from the lattice to fill its valence bonds. The result is a missing negative charge, which is called ahole.
Negatively doped silicon (n-type) is typically produced by introducing phosphorus atoms, which have five valence electrons. In this case, an electron is released which can migrate through the lattice.
A pn junction, illustrated in Fig. 5.1, is created from a piece of p-doped and a piece of n-doped silicon. Free holes and electrons, which drift due to thermal diffusion, can pass through the junction.
This creates an excess of negative charge on the p-side and an excess of positive charge on the n-side.
As the excess charge increases, an electrical potential builds up. Once this potential exceeds the energy needed for electrons and holes to cross the barrier, the flow of charge stops. The region near the junction is depleted of mobile charge carriers and is called thedepletion region.
An external voltage applied across a pn junction is called a bias voltage. The bias voltage is
Donor atom Acceptor atom
Free electron Free hole
n-type p-type n-type p-type
Depletion Zone
Figure 5.1: Illustration of pn junction
normally large enough that the depletion zone extends across the whole sensor so that there are no free charge carriers. The voltage can be applied either with or against the flow of charge. A forward bias, which has the positive supply on the p-side and negative supply on the n-side, yields a large flow of charge. A reverse bias extends the depletion zone such that the charge flow with a reverse bias is very small and called theleakage current. Silicon detectors typically use sensors made from reverse biased pn junctions.
An ionising particle, such as a charged pion or a muon, propagating through a silicon detector ionises the silicon atoms and produces pairs of electrons and holes along its trajectory. The number of electron-hole pairs produced is proportional to the energy lost by the particle. The externally applied electric field makes the electrons and holes move in opposite directions and pulls them to the sensor surface. The charge drifts to the surface and produces a pulse of current through induction, which is detected using charge sensitive electronics. The integral of the pulse is proportional to the amount of charge deposited by the ionising particle.
5.1.1 Energy Loss of Charged Particles in Matter
Charged particles traversing matter lose energy through interactions [31]. These occur via different processes including inelastic scattering from atomic electrons, elastic scattering from nuclei, emission of Cherenkov radiation, nuclear interactions and bremsstrahlung.
The amount of energy lost by a particle passing through matter depends on the particle type and energy. At the energies typical of particle physics experiments, electrons and positrons typically lose most of their energy through bremsstrahlung, while for heavier particles it occurs mostly through inelastic collisions. The Bethe-Bloch equation describes the mean rate of energy loss of moderately relativistic charged heavy particles with the precision of a few percent:
− dE
dz
=Kz2Z A
1 β2
1
2ln2mec2β2γ2Tmax
I2 −β2−δ(βγ) 2
(5.1) It accounts for energy lost through inelastic, elastic and Cherenkov processes but the accuracy can be improved by including corrections for the density and shell effects. Figure 5.2 shows the average energy lost by muons as a function of their momentum. It is large for very low momentum particles but falls rapidly with increasing momentum before reaching a minimum. A particle with this minimum energy is typically referred to as a minimum ionising particle (MIP). For larger momenta, the energy loss rises slowly, flattens out and then rises steeply due to radiative energy losses. This region is referred to as the relativistic rise.
Muon momentum 1
10 100
Stopping power [MeV cm2/g] Lindhard- Scharff
Bethe-Bloch Radiative
Radiative effects reach 1%
µ+ on Cu
Without δ Radiative
losses
0.001 0.01 0.1 1 10 βγ 100 1000 104 105 106
[MeV/c] [GeV/c]
100 10
1
0.1 1 10 100 1 10 100
[TeV/c] Anderson-
Ziegler
Nuclear losses
Minimum ionization
Eµc µ−
Figure 5.2: The average energy lost by muons in copper as a function ofβγ=p/M cfrom [31]