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Jaap Velthuis (University of Bristol)1 Silicon detectors in HEP Introduction Semi-conductor physics Real Si detectors Radiation damage in Si Radiation.

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Presentation on theme: "Jaap Velthuis (University of Bristol)1 Silicon detectors in HEP Introduction Semi-conductor physics Real Si detectors Radiation damage in Si Radiation."— Presentation transcript:

1 Jaap Velthuis (University of Bristol)1 Silicon detectors in HEP 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: Bluffing your way into particle physics detectors

2 Jaap Velthuis (University of Bristol)2 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. n th generation light source, medical imaging)

3 Jaap Velthuis (University of Bristol)3 Introduction Why semi-conductor devices P-N junction Particle traversing matter –Scattering –Signal generation Summary Baseline detector

4 Jaap Velthuis (University of Bristol)4 Why semi-conductor devices

5 Jaap Velthuis (University of Bristol)5 Standard experiment

6 Jaap Velthuis (University of Bristol)6 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. Very precise tracking Electro- magnetic calorimeter hadronic calorimeter Muon chamber Outward Track density drops

7 Jaap Velthuis (University of Bristol)7 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 R- Primary vertex Secondary vertex

8 Jaap Velthuis (University of Bristol)8 Wire chambers Traditionally tracking in wire chambers

9 Jaap Velthuis (University of Bristol)9 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

10 Jaap Velthuis (University of Bristol)10 Charged particle traversing matter Energy loss described by Bethe-Bloch equation: Some constant Atomic number/mass absorber Electric charge incident particle Mean excitation energy Maximum kinetic energy which can be imparted to a free electron in a single collision

11 Jaap Velthuis (University of Bristol)11 Charged…matter dE/dx different for different particles due to different M and Is used to identify different particles

12 Jaap Velthuis (University of Bristol)12 Charged … matter Relevant for detectors dE/dx [MeV cm 2 /g] Energy loss wildly varying function, –MINIMUM IONIZING PARTICLE ( 4)

13 Jaap Velthuis (University of Bristol)13 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

14 Jaap Velthuis (University of Bristol)14 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 X 0

15 Jaap Velthuis (University of Bristol)15 Detector trade off Thick detectors (in X 0 ) lots of energy lost lots of signal generated But loads of scattering bad for tracking Loads of -electrons more signal but not right direction

16 Jaap Velthuis (University of Bristol)16 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 earths crust First crystalline silicon produced by Deville in 1824

17 Jaap Velthuis (University of Bristol)17 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

18 Jaap Velthuis (University of Bristol)18 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

19 Jaap Velthuis (University of Bristol)19 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

20 Jaap Velthuis (University of Bristol)20 Intrinsic semi-conductors Density of states Density of free electrons n given by product F n (E) and density of states

21 Jaap Velthuis (University of Bristol)21 Intrinsic semi-conductors Typically for intrinsic Si carrier density at 300K ~10 10 cm -3 suppose strip 20 m wide, 10 cm long, sensor 0.3 mm thick S/N= Re-writing concentrations: –Concentrations highly dependent on T and material So three solutions: –Use high E G material –Cool device down (n i at 77K ~ ) –Remove mobile carriers

22 Jaap Velthuis (University of Bristol)22 Trick: doping 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<

23 Jaap Velthuis (University of Bristol)23 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

24 Jaap Velthuis (University of Bristol)24 PN-junction Can express as function V bias : – V junc –Depletion width –C junc

25 Jaap Velthuis (University of Bristol)25 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.

26 Jaap Velthuis (University of Bristol)26 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

27 Jaap Velthuis (University of Bristol)27 C-V curve Re-write C(V bias ) relation Plotting 1/C 2 vs V bias yields: –depletion voltage –Doping concentration (for asymmetric doping) Depletion voltage

28 Jaap Velthuis (University of Bristol)28 I-V curves Measure I-V to check long term stability of sensors and maximum V bias Note current NOT zero (leakage current) If V bias 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

29 Jaap Velthuis (University of Bristol)29 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 Carriers carry kinetic energy 3/5E G Energy transferred to lattice r 10 E Raman eV Taken from

30 Jaap Velthuis (University of Bristol)30 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

31 Jaap Velthuis (University of Bristol)31 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

32 Jaap Velthuis (University of Bristol)32 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

33 Jaap Velthuis (University of Bristol)33 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

34 Jaap Velthuis (University of Bristol)34 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

35 Jaap Velthuis (University of Bristol)35 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

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