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Introduction

Introduction. Particle Detectors. Summer Student Lecture Series 2003 Christian Joram EP / TA1. From (very) basic ideas to rather complex detector systems. 1 + 1  2. Outline + approximate timing Introduction, basics Tracking (gas, solid state)

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Introduction

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  1. Introduction Particle Detectors Summer Student Lecture Series 2003 Christian Joram EP / TA1 From (very) basic ideas to rather complex detector systems 1 + 1  2 Christian Joram

  2. Outline + approximate timing Introduction, basics Tracking (gas, solid state) Scintillation and light detection Calorimetry Particle Identification Detector Systems Discussion session I Discussion session II = Detector Exhibition Introduction Thu/Fri (2x45 min) Mon/ Tue (2 x45 min) Wed (45 min) Fri, 4 June, 11:15 Tue, 8 June, 11:00 Christian Joram

  3. Literature on particle detectors Text books C. Grupen,Particle Detectors, Cambridge University Press, 1996 G. Knoll,Radiation Detection and Measurement,3rd Edition, 2000 W. R. Leo,Techniques for Nuclear and Particle Physics Experiments, 2nd edition, Springer, 1994 R.S. Gilmore,Single particle detection and measurement, Taylor&Francis, 1992 W. Blum, L. Rolandi,Particle Detection with Drift Chambers, Springer, 1994 K. Kleinknecht,Detektoren für Teilchenstrahlung, 3rd edition, Teubner, 1992 Review articles Experimental techniques in high energy physics, T. Ferbel (editor), World Scientific, 1991. Instrumentation in High Energy Physics, F. Sauli (editor), World Scientific, 1992. Many excellent articles can be found in Ann. Rev. Nucl. Part. Sci. Other sources Particle Data Book (Phys. Rev. D, Vol. 54, 1996) R. Bock, A. Vasilescu, Particle Data Briefbook http://www.cern.ch/Physics/ParticleDetector/BriefBook/ Proceedings of detector conferences (Vienna VCI, Elba, IEEE) Introduction Christian Joram

  4. Introduction “The oldest particle detector” (built many billion times) • High sensitivity to photons • Good spatial resolution • Very large dynamic range (1:1014) + automatic threshold adaptation • Energy (wavelength) discrimination • Modest speed. Data taking rate ~ 10Hz (incl. processing) retina Christian Joram

  5. Introduction Use of photographic paper as detector  Detection of photons / x-rays W. C. Röntgen, 1895 Discovery of the ‘X-Strahlen’ Photographic paper/film e.g. AgBr / AgCl AgBr + ‘energy’  metallic Ag (blackening) + Very good spatial resolution + Good dynamic range - No online recording - No time resolution Christian Joram

  6. Introduction J. Plücker 1858  J.J. Thomson 1897 Thomson’s cathode ray tube manipulation By E or B field detector accelerator From: J.J. Thomson: Cathode Rays. Philosophical Magazine, 44, 293 (1897). “… The rays from the cathode C pass through a slit in the anode A, which is a metal plug fitting tightly into the tube and connected with the earth; after passing through a second slit in another earth-connected metal plug B, they travel between two parallel aluminium plates about 5 cm. long by 2 broad and at a distance of 1.5 cm. apart; they then fall on the end of the tube and produce a narrow well-defined phosphorescent patch. A scale pasted on the outside of the tube serves to measure the deflexion of this patch….” Scintillation of glass Christian Joram

  7. Introduction E. Rutherford 1909 H. Geiger pulse The Geiger counter, later further developed and then called Geiger-Müller counter First electrical signal from a particle Christian Joram

  8. Introduction C. T. R. Wilson, 1912, Cloud chamber First tracking detector The general procedure was to allow water to evaporate in an enclosed container to the point of saturation and then lower the pressure, producing a super-saturated volume of air. Then the passage of a charged particle would condense the vapor into tiny droplets, producing a visible trail marking the particle's path. Christian Joram

  9. Introduction “progress cycle” physics theories technologies & materials knowledge / progress experiments detectors Christian Joram

  10. A W+W- decay in ALEPH e+e- (s=181 GeV)  W+W- qqmnm  2 hadronic jets + m + missing momentum _ Introduction Christian Joram

  11. Reconstructed B-mesons in the DELPHI micro vertex detector Introduction tB 1.6 ps l = ctg  500 mmg Primary Vertex Primary Vertex Christian Joram

  12. A simulated event in ATLAS (CMS) H  ZZ  4m Introduction pp collision at s = 14 TeV sinel.  70 mb Interested in processes with s  10-100 fb L = 1034 cm-2 s-1, bunch m spacing 25 ns m m m  23 overlapping minimum bias events / BC  1900 charged + 1600 neutral particles / BC Christian Joram

  13. - q Introduction Idealistic views of an elementary particle reaction q e+ Z e- time Usually we can only ‘see’ the end products of the reaction, but not the reaction itself. In order to reconstruct the reaction mechanism and the properties of the involved particles, we want the maximum information about the end products ! Christian Joram m

  14. The ‘ideal’ particle detector should provide… coverage of full solid angle (no cracks, fine segmentation measurement of momentum and/or energy detect, track and identify all particles (mass, charge) fast response, no dead time practical limitations (technology, space, budget) Particles are detected via their interaction with matter. Many different physical principles are involved (mainly of electromagnetic nature). Finally we will always observe... ionizationand excitationof matter. Introduction detector end products • charged • neutral • photons Christian Joram m

  15. Definitions and units Some important definitions and units • energy E: measure in eV • momentum p: measure in eV/c • mass mo: measure in eV/c2 1 eV is a tiny portion of energy. 1 eV = 1.6·10-19 J mbee = 1g = 5.8·1032 eV/c2 vbee= 1m/s  Ebee = 10-3 J = 6.25·1015 eV ELHC = 14·1012 eV To rehabilitate LHC… Total stored beam energy: 1014 protons * 14·1012 eV  1·108 J this corresponds to a mtruck = 100 T vtruck = 120 km/h Christian Joram

  16. Definitions and units The concept of cross sections Cross sections sor differential cross sections ds/dW are used to express the probability of interactions between elementary particles. Example:2 colliding particle beams beam spot areaA F1= N1/t F2= N2/t What is the interaction rate Rint. ? Rint N1N2 / (A ·t) = s · L shas dimension area ! Practical unit: 1 barn (b) = 10-24 cm2 LuminosityL[cm-2 s-1] Example:Scattering from target scattered beam solid angle elementdW target q incident beam .nA= area density of scattering centers in target Nscat(q) Ninc·nA ·dW = ds/dW (q)·Ninc·nA·dW Christian Joram

  17. Momentum measurement Tracking Multiple scattering Bethe-Bloch formula / Landau tails Ionization of gases Wire chambers Drift and diffusion in gases Drift chambers Micro gas detectors Silicon as a detection medium Silicon detectors strips/pixels Christian Joram

  18. Momentum measurement Christian Joram

  19. Momentum measurement the sagitta s is determined by 3 measurements with error s(x): for N equidistant measurements, one obtains (R.L. Gluckstern, NIM 24 (1963) 381) ex: pT=1 GeV/c, L=1m, B=1T, s(x)=200mm, N=10 Momentum measurement (for N  10) (s  3.75 cm) Christian Joram

  20. Scattering An incoming particle with charge z interacts with a target of nuclear charge Z. The cross-section for this e.m. process is Average scattering angle Cross-section for infnite ! Multiple Scattering Sufficiently thick material layer  the particle will undergo multiple scattering. Multiple Scattering Rutherford formula Christian Joram

  21. Back to momentum measurements: What is the contribution of multiple scattering to ? Momentum measurement Approximation X0 is radiation length of the medium (discuss later) remember independent of p ! More precisely: ex: Ar (X0=110m), L=1m, B=1T Christian Joram

  22. Detection of charged particles How do they loose energy in matter ? Discrete collisions with the atomic electrons of the absorber material. Collisions with nuclei not important (me<<mN). If are big enough  ionization. Interaction of charged particles q e - Instead of ionizing an atom, under certain conditions the photon can also escape from the medium.  Emission of Cherenkov and Transition radiation. (See later). Christian Joram

  23. dE/dx in [MeV g-1 cm2] Bethe-Bloch formula only valid for “heavy” particles (mmm). dE/dx depends only on b, independent of m ! First approximation: medium simply characterized by ~ electron density Bethe-Bloch formula Average differential energy loss Ionisation only Bethe - Bloch formula Z/A = 1 “Fermi plateau” Z/A~0.5 “relativistic rise”   3-4 minimum ionizing particles, MIPs “kinematical term” Christian Joram

  24. Landau tails Real detectors (limited granularity) can not measure <dE/dx> ! It measures the energy DE deposited in a layer of finite thickness dx. For thin layers (and low density materials):  Few collisions, some with high energy transfer.  Energy loss distributions show large fluctuations towards high losses: ”Landau tails” e- <DE> DE For thick layers and high density materials:  Many collisions.  Central Limit Theorem  Gaussian shape distributions. e- <DE> DE Christian Joram

  25. Gas detectors Fast charged particles ionize the atoms of a gas. Often the resulting primary electron will have enough kinetic energy to ionize other atoms. Ionization of gases Primary ionization Total ionization 10 - 40 pairs/cm DE/pair ~ 20 - 40 eV Assume detector, 1 cm thick, filled with Ar gas: 1 cm ~ 100 e-ion pair  100 electron-ion pairs are not easy to detect! Noise of amplifier 1000 e- (ENC) ! We need to increase the number of e-ion pairs. Christian Joram

  26. Gas amplification Consider cylindrical field geometry (simplest case): Proportional Counter C = capacitance / unit length Electrons drift towards the anode wire ( stop and go! More details in next lecture!). Close to the anode wire the field is sufficiently high (some kV/cm), so that e- gain enough energy for further ionization  exponential increase of number of e--ion pairs. Christian Joram

  27. Proportional Counter a: First Townsend coefficient (e--ion pairs/cm) l: mean free path Gain (F. Sauli, CERN 77-09) (O. Allkofer, Spark chambers, Theimig München, 1969) e Christian Joram

  28. Signal formation Avalanche formation within a few wire radii and within t < 1 ns! Signal induction both on anode and cathode due to moving charges (both electrons and ions). Proportional Counter (F. Sauli, CERN 77-09) Electrons collected by anode wire, i.e. dr is small (few mm). Electrons contribute only very little to detected signal (few %). Ions have to drift back to cathode, i.e. dr is big. Signal duration limited by total ion drift time ! (F. Sauli, CERN 77-09) Need electronic signal differentiation to limit dead time. Christian Joram

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