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

2. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram I/2 Introduction 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 Thu/Fri (2x45 min) Mon/ Tue (2 x45 min) Wed (45 min) Fri, 4 June, 11:15 Tue, 8 June, 11:00

3. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram I/3 Introduction 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)

4. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram I/4 retina “The oldest particle detector” (built many billion times) High sensitivity to photons Good spatial resolution Very large dynamic range (1:10 14 ) + automatic threshold adaptation Energy (wavelength) discrimination Modest speed. Data taking rate ~ 10Hz (incl. processing) Introduction

5. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram I/5 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 Introduction

6. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram I/6 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….” J. Plücker 1858  J.J. Thomson 1897 accelerator manipulation By E or B field detector Thomson’s cathode ray tube Scintillation of glass Introduction

7. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram I/7 E. RutherfordH. Geiger1909 The Geiger counter, later further developed and then called Geiger-Müller counter First electrical signal from a particle pulse Introduction

8. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram I/8 C. T. R. Wilson, 1912, Cloud chamber 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. First tracking detector Introduction

9. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram I/9 physics theories experiments technologies & materials knowledge / progress Introduction “progress cycle” detectors

10. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram I/10 Introduction A W + W - decay in ALEPH e + e - (  s=181 GeV)  W + W -  qq    2 hadronic jets +  missing momentum _

11. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram I/11 Introduction Reconstructed B-mesons in the DELPHI micro vertex detector  B  1.6 ps l = c   500  m  Primary Vertex Primary Vertex

12. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram I/12 Introduction A simulated event in ATLAS (CMS) H  ZZ  4  pp collision at  s = 14 TeV  inel.  70 mb Interested in processes with  fb  23 overlapping minimum bias events / BC  1900 charged + 1600 neutral particles / BC L = 10 34 cm -2 s -1, bunch spacing 25 ns    

13. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram I/13 time e-e- e+e+ q q - Z Introduction Idealistic views of an elementary particle reaction 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 ! m

14. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram I/14 Introduction 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... ionizationexcitation ionization and excitation of matter. end products charged neutral photons detector m

15. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram I/15 Definitions and units Some important definitions and units  energy E: measure in eV  momentum p: measure in eV/c  mass m o : measure in eV/c 2 1 eV is a tiny portion of energy. 1 eV = 1.6·10 -19 J m bee = 1g = 5.8·10 32 eV/c 2 v bee = 1m/s  E bee = 10 -3 J = 6.25·10 15 eV E LHC = 14·10 12 eV To rehabilitate LHC… Total stored beam energy: 10 14 protons * 14·10 12 eV  1·10 8 J this corresponds to a m truck = 100 T v truck = 120 km/h

16. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram I/16 The concept of cross sections Definitions and units Cross sections  or differential cross sections d  d  are used to express the probability of interactions between elementary particles. Example: 2 colliding particle beams    = N 1 /t    = N 2 /t What is the interaction rate R int. ? beam spot area A R int       ·  t) =  · L Luminosity L [cm -2 s -1 ] Example: Scattering from target  solid angle element d  incident beam scattered beam target N scat (  )  N inc · n A · d  = d  d  (  )  ·  N inc · n A · d   has dimension area ! Practical unit: 1 barn (b) = 10 -24 cm 2. n A = area density of scattering centers in target

17. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram I/17 Multiple scattering Bethe-Bloch formula / Landau tails Ionization of gases Wire chambers Drift and diffusion in gases Drift chambers Micro gas detectors Silicon detectors strips/pixels Momentum measurement Silicon as a detection medium

18. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram I/18 Momentum measurement

19. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram I/19 Momentum measurement the sagitta s is determined by 3 measurements with error  (x): for N equidistant measurements, one obtains (R.L. Gluckstern, NIM 24 (1963) 381) ex: p T =1 GeV/c, L=1m, B=1T,  (x)=200  m, N=10 (for N   10) (s  3.75 cm)

20. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram I/20 Multiple Scattering 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. Rutherford formula

21. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram I/21 Momentum measurement Back to momentum measurements: What is the contribution of multiple scattering to ? independent of p ! Approximation X 0 is radiation length of the medium (discuss later) ex: Ar (X 0 =110m), L=1m, B=1T remember More precisely:

22. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram I/22 Interaction of charged particles 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 (m e <<m N ).   If are big enough  ionization. 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).

23. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram I/23 Bethe-Bloch formula   dE/dx in [MeV g -1 cm 2 ]   Bethe-Bloch formula only valid for “heavy” particles (m  m  ).   dE/dx depends only on  independent of m !   First approximation: medium simply characterized by ~ electron density Average differential energy loss Ionisation only  Bethe - Bloch formula Z/A~0.5 Z/A = 1 “relativistic rise” “kinematical term”   3-4 minimum ionizing particles, MIPs “Fermi plateau”

24. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram I/24 Landau tails 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” For thick layers and high density materials:  Many collisions.  Central Limit Theorem  Gaussian shape distributions. Real detectors (limited granularity) can not measure ! It measures the energy  E deposited in a layer of finite thickness  x. <E><E> EE e-e- e-e- <E><E> EE

25. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram I/25 Ionization of gases 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. Primary ionizationTotal ionization 10 - 40 pairs/cm  E/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.

26. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram I/26 Proportional Counter Gas amplification Consider cylindrical field geometry (simplest case): 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.

27. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram I/27 Proportional Counter  : First Townsend coefficient (e - -ion pairs/cm) (O. Allkofer, Spark chambers, Theimig München, 1969) : mean free path Gain (F. Sauli, CERN 77-09) 

28. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram I/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 Electrons collected by anode wire, i.e. dr is small (few  m). 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 ! Need electronic signal differentiation to limit dead time. (F. Sauli, CERN 77-09)