Silicon detectors in HEP

Silicon detectors in HEP
Bluffing your way into particle physics detectors Introduction Semi-conductor physics Real Si detectors Radiation damage in Si Radiation hard sensors Novel devices/State-of-the-Art In case of any questions: Jaap Velthuis (University of Bristol)

Jaap Velthuis (University of Bristol)
Introduction Particle Physics is more than hunting for Higgs and CP violation Need to make very advanced detector systems Forefront of Engineering (stiff light weight support structures, cooling, tunnel building) High speed and radiation hard electronics Computing (web, grid, online) Accelerators (e.g. cancer therapy, diffraction) Imaging sensors (e.g. nth generation light source, medical imaging) Jaap Velthuis (University of Bristol)

Introduction Why semi-conductor devices P-N junction Particle traversing matter Scattering Signal generation Summary Baseline detector Jaap Velthuis (University of Bristol)

Why semi-conductor devices
Jaap Velthuis (University of Bristol)

Standard experiment Jaap Velthuis (University of Bristol)

The “Onion” peeled… Fundamental parameters: Charge Momentum Decay products Life time Decay vertex Mass Spin Energy Need very precise tracking close to primary vertex. Then follow track to calorimeter and measure energy. Outward Track density drops Very precise tracking Electro- magnetic calorimeter hadronic Muon chamber Jaap Velthuis (University of Bristol)

Tracking Track described by 5 parameters Modern tracking uses “Kalman Filter” Start with “proto” track Add new point Update 2 Decide to in- or exclude point based on 2 Modern Vertexing Use tracks with errors Add them to vertex Calculate 2 etc So to do good tracking and vertexing, need detectors with small error and little “deflection” Primary vertex Secondary vertex R- Jaap Velthuis (University of Bristol)

Wire chambers Traditionally tracking in wire chambers Jaap Velthuis (University of Bristol)

Wire chambers Problem in wire chambers: Wires long Many hits per wire for wires close to primary vertex (high occupancy) Leads to ambiguities in track fitting Solution: very short wires!  solid state Jaap Velthuis (University of Bristol)

Charged particle traversing matter
Energy loss described by Bethe-Bloch equation: Electric charge incident particle Atomic number/mass absorber Some constant Maximum kinetic energy which can be imparted to a free electron in a single collision Mean excitation energy Jaap Velthuis (University of Bristol)

Charged…matter dE/dx different for different particles due to different M and  Is used to identify different particles Jaap Velthuis (University of Bristol)

Charged … matter Relevant for detectors dE/dx [MeV cm2/g] Energy loss wildly varying function, MINIMUM IONIZING PARTICLE (4) Jaap Velthuis (University of Bristol)

Charged … matter Bethe-Bloch describes average energy loss Collisions stochastic nature, hence energy loss is distribution instead of number. First calculated for thin layers was Landau. Hence energy loss is Landau distributed. Signal proportional to energy loss  is most probable value Jaap Velthuis (University of Bristol)

Multiple scattering During passage through matter Coulomb scattering on nucleideviation from original track Deflection distribution Gaussian with width 0 More dense material, more scattering, shorter X0 Jaap Velthuis (University of Bristol)

Detector trade off Thick detectors (in X0) lots of energy lost  lots of signal generated But loads of scattering  bad for tracking Loads of -electrons more signal but not right direction Jaap Velthuis (University of Bristol)

Silicon trivia Silicon was discovered by Jöns Jacob Berzelius in 1824 Name from “silicis” (Latin for flint) With 25.7% second most abundant element in earth’s crust First crystalline silicon produced by Deville in 1824 Jaap Velthuis (University of Bristol)

Why solid state detectors
Small band gap  low energy required for e-h pair (3.6 eV in Si ~30 eV gas) Many e-h pairs per unit length (80/m in Si) High density  Large energy loss per unit length  Can make thin detectors with high signal Small range for -electrons  Very good spatial resolution Jaap Velthuis (University of Bristol)

Why solid state detectors
Electron and hole mobility very high Fast charge collection (~10 ns) Excellent rigidity  Self-supporting structures Possibility of creating fixed space charge by doping Jaap Velthuis (University of Bristol)

Intrinsic semi-conductors
Single non-interacting atom has set of well-defined energy levels When forming crystals, levels undergo minor shifts resulting in bands Probability for e- to occupy state given by Fermi-Dirac function Jaap Velthuis (University of Bristol)

Density of states Density of free electrons n given by product Fn(E) and density of states Jaap Velthuis (University of Bristol)

Typically for intrinsic Si carrier density at 300K ~1010 cm-3 suppose strip 20m wide, 10 cm long, sensor 0.3 mm thick  S/N=410-3 Re-writing concentrations: Concentrations highly dependent on T and material So three solutions: Use high EG material Cool device down (ni at 77K ~10-20) Remove mobile carriers Jaap Velthuis (University of Bristol)

“Trick”: doping Intrinsic N-type P-type By introducing atoms with different number of valence e- can change number of free carriers E.g. P, As: 5 valence e-; donor (n-type) E.g. Al, B: 3 valence e-; acceptor (p-type) Activation energies ~0.04eV<<EG=1.12eV Jaap Velthuis (University of Bristol)

PN-junction Holes in p-type recombine with e- in n-type, creating zone without mobile carriers (depletion) Depleted silicon ideal for detector. Same signal, but no background! Note: Holes move towards p-type Electrons move towards n-type Jaap Velthuis (University of Bristol)

PN-junction Can express as function Vbias: Vjunc Depletion width Cjunc Jaap Velthuis (University of Bristol)

PN-junction By biasing detector, the depletion width can be extended over entire thickness of detector (full depletion). Important: PN junction itself is located at interface between p-strips and n-bulk. Depletion region grows from PN junction towards n-type bias contact. Typical values for full depletion V before irradiation. Jaap Velthuis (University of Bristol)

Depletion voltage Bias voltage very important: Creating large depletion zone Signal proportional depletion thickness Depletion zone also reduces background Isolating strips from each other Separating e-h pairs Depletion voltage obtained from C-V curve Jaap Velthuis (University of Bristol)

C-V curve Re-write C(Vbias) relation Plotting 1/C2 vs Vbias yields: depletion voltage Doping concentration (for asymmetric doping) Depletion voltage Jaap Velthuis (University of Bristol)

I-V curves Measure I-V to check long term stability of sensors and maximum Vbias Note current NOT zero (leakage current) If Vbias too large, get high currents (breakdown) Zener breakdown Tunnelling from occupied state in p side valence band to n side conduction band Avalanche breakdown Carriers from leakage current get so much kinetic energy that due to collisions new free carriers are generated Jaap Velthuis (University of Bristol)

Signal generation Lost energy converted into free carriers Energy needed to generate 1 e-h pair in Si is 3.6 eV Results in 8900 e-h pairs per 100 m Si for a MIP Charge cloud Gaussian with 10m E-h pairs might recombine, need (strong) field to prevent this signal loss Taken from Carriers carry kinetic energy 3/5EG Energy transferred to lattice r10 ERaman0.165 eV Jaap Velthuis (University of Bristol)

Baseline detector Need many diodes (here p-strips to n-bulk) Need reverse bias to Deplete entire sensor Separate e-h Need to readout signals from p-strips Design issues: Thick  large signal Thin  less scattering Thin  lower depletion voltage Short strips less ambiguities Strips close  very precise measurement impact position Strips far apart  less electronics hence less expensive Occupancy: fraction of strips that has been hit Jaap Velthuis (University of Bristol)

Real detectors Real sensors have much more features: Backplane contacts Guard rings Bias resistors P-strips Al readout strips Coupling capacitors … Typical scale: Sensors 6x6 cm Pitch ~100 m 512 Al strips Jaap Velthuis (University of Bristol)

Charge collection Determined by Spatial distribution of generated charge Field strength Accelerates carriers in field direction Determines time charge is moving Separation of e-h pairs “Horizontal” movement through diffusion Hall effect Jaap Velthuis (University of Bristol)

Charge collection If pitch > charge cloud all charge collected on 1 strip In this case analog signal value not importantchose digital or binary readout To do better need to share charge over more strips need pitch20m for 300 m thick sensor Problem: connecting all strips to readout channel yields too many strips Jaap Velthuis (University of Bristol)

Summary Semiconductor detectors are used close to primary vertex to Limit occupancy and reduce ambiguities Give very precise space point Energy loss described by Behte-Bloch equation Minimum ionizing particle Energy loss (=signal) is Landau distributed Particles scatter in matter, so need to have thin detectors MIP yields 8900 e-h pairs per 100 m Si Jaap Velthuis (University of Bristol)

Summary (II) Need trick to remove free charge carriers Use high band gap semiconductor Cool to cryogenic temperatures Build p-n junction and deplete detector If pitch ~ charge cloud, charge is shared. Need lots of strips. Trick intermediate strip using C-charge sharing, but non-linear charge sharing Jaap Velthuis (University of Bristol)

Silicon detectors in HEP

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