By Archana SHARMA CERN Geneva Switzerland 16 June 2011 UNAL BOGOTA COLUMBIA

1. Gaseous Particle Detectors • By Archana SHARMA • CERN Geneva Switzerland • 16 June 2011 • UNAL • BOGOTA COLUMBIA

2. HIGH • ENERGY • PARTICLES • ACCELERATORS • INTERACTIONS • DETECTORS What is a TeV ?

3. The LHC is a proton proton collider 7 TeV + 7 TeV 1 TeV = 1 Tera electron volt = 1012 electron volt Rate 40 MHz The LHC will determine the Future course of High Energy Physics

4. Introduction • HEP experiments study the interactions of particles by observing collisions of particles • Result: change in direction / energy / momentum of original particles • And production of new particles

5. See http://cmsinfo.cern.ch/Welcome.html/ TOTALGaseous Detectors In CMS~ 10,000 m2

6. Just in case you wonder why? Knownparticles thatdisappeared after the Big Bang HighlyExpectedParticles Methodology E=mc2 Hypothetical Or totallyunsuspected ? SUSY

7. A Higgs Event in CMS Methodology 2 muons 2 electrons

9. The ideal detector With an “ideal” detector, we can reconstruct the interaction, i.e. obtain all possible information on it. This is then compared to theoretical predictions and ultimately leads to a better understanding of the interaction and properties of particles For all particles produced, the “ideal detector” measures energy, momentum, type by : mass, charge, life time, spin, decays

10. Negative charge Magnetic field, pointing out of the plane Positive charge Measure and derive • The mass, velocity, energy and charge (sign) • from ‘tracking’ curvature in a magnetic field • The lifetimet • from flight path before decay t

11. Different type of particles to be detected • Charged particles • e-, e+, p (protons), p, K (mesons), m (muons) • Neutral particles • g (photons), n (neutrons), K0 (mesons), • n (neutrinos, very difficult) Different particle types interact differently with matter (detector) (for example, photons do not interact with a magnetic field) Need different types of detectors to measure different types of particles

12. e- p p p g e- p Principles of detection • Interaction of a particle with detector Sensitive Material Measureable Signal • Ionization • Excitation • Particle trajectory is changed due to • Bending in a magnetic field, energy loss • Scattering, change of direction, absorption

13. Ionization signals by using • Gaseous detectors: • MWPC and its derivatives • (Multi-Wire Proportional Chambers) • Drift Chambers (DCs) • TPC (Time Projection Chamber)

14. Charged Particle Radiation Fast Electrons b-particles emitted in nuclear decay energetic electrons produced by any process Heavy Charged Particles Energetic ions: alpha particles, protons Fission products Products of nuclear reactions

15. Neutral Particles Electromagnetic Radiation X-rays emitted by re-arrangement of electron shells in atoms Gamma rays from transitions in the nucleus Neutrons Generated in various nuclear processes Often subdivided into slow and fast neutron sources

16. Electromagnetic Interaction of Particles with Matter If particle’s velocity is greater than the speed of light in the medium -> Cherenkov Radiation. When crossing the boundary between media, ~1% probability of producing a Transition Radiation X-ray. Interaction with atomic nucleus. Particle undergoes multiple scattering. Could emit a bremsstrahlung photon. Interaction with atomic electrons. Particle loses energy; atoms are excited or ionized.

17. Stopping Power Linear stopping power (S) is the differential energy loss of the particle in the material divided by the differential path length. Also called the specific energy loss. Bethe-Bloch Formula Stopping Power of muons in Copper Particle Data Group Energy loss through ionization and atomic excitation

18. Bethe-Bloch Formula m – electronic mass v – velocity of the particle (v/c = b) N – number density of atoms I – ‘Effective’ atomic excitation energy – average value found empirically Gas is represented as a dielectric medium through which the particle propagates And probability of energy transfer is calculated at different energies – Allison Cobb Particle Data Group

19. Energy Loss Function 1.6 1.5 1.4 1.3 1.2 1.1 Rel To mips Fermi Plateau Relativistic Rise 1 10 100 1000 10000 bg Minimum ionizing particles (mips)

20. Different Materials

21. Average Ionisation Energy Few eV to few tens of eV

22. Energy-loss in Tracking Chambers The Bethe Bloch Formula tool for Particle Identification

23. Straggling • Mean energy loss • Actual energy loss will scatter around the mean value • Difficult to calculate • parameterization exist in GEANT and some standalone software libraries • Form of distribution is important as energy loss distribution is often used for calibrating the detector

24. Straggling Alfons Weber

25. Electrons • Electrons are different light • Bremsstrahlung • Pair production

26. Multiple Scattering Particles not only lose energy …but also they also change direction

27. Range Integrate the Bethe-Bloch formula to obtain the range Useful for low energy hadrons and muons with momenta below a few hundred GeV Radiative Effects important at higher momenta. Additional effects at lower momenta.

28. Radiation Length Mean distance over which an electron loses all but 1/e of its energy through bremsstralung Energy Loss in Lead also 7/9 of the mean free path for electron-positron pair production by a high energy photon

29. Electron Critical Energy Energy loss through bremsstrahlung is proportional to the electron energy Ionization loss is proportional to the logarithm of the electron energy Critical energy (Ec) is the energy at which the two loss rates are equal Electron in Copper: Ec = 20 MeV Muon in Copper: Ec = 400 GeV!

30. Photon Pair Production Probability that a photon interaction will result in a pair production Differential Cross-section Total Cross-section What is the minimum energy for pair production?

31. Electromagnetic cascades Visualization of cascades developing in the CMS electromagnetic and hadronic calorimeters

32. Muon Energy Loss For muons the critical energy (above which radiative processes are more important than ionization) is at several hundred GeV. Pair production, bremsstrahlung and photonuclear Ionization energy loss Mean range

33. Muon Tomography Luis Alvarez used the attenuation of muons to look for chambers in the Second Giza Pyramid He proved that there are no chambers present

34. X-Ray Radiography for airport security

35. Signals from Particles in a Gas Detector • Signals in particle detectors are mainly due to ionisation • And excitation in a sensitive medium – gas • Also: • Direct light emission by particles travelling faster than the speed of light in a medium • Cherenkov radiation • Similar, but not identical • Transition radiation

36. Cerenkov Radiation • Moving charge in dielectric medium • Wave front comes out at certain angle slow fast

37. Transition Radiation • Transition radiation is produced, when a relativistic particle traverses an inhomogeneous medium • Boundary between different materials with different diffractive index n. • Strange effect • What is generating the radiation? • Accelerated charges

38. Transition Radiation (2) Before the charge crosses the surface,apparent charge q1 with apparent transverse vel v1 After the charge crosses the surface,apparent charges q2 and q3 with apparent transverse vel v2 and v3

39. From Interactions to Detectors

42. Multiwire Proportional Chamber

43. Multiwire Proportional Chamber and derivatives

44. Signal Creation Charged particles traversing matter leaving excited atoms, electron or holes and ions behind. These can be detected using either excitation or ionization. Excitation Photons emitted by excited atoms can be detected by photomultipliers or semiconductor photon detectors Ionization If an electric field is applied in the detector volume, the movement of the electrons and ions induces a signal on metal electrodes. Signals are read out using appropriate readout electronics

45. Signal Induction A point charge above a grounded metal plate induces a surface charge. q Total induced charge –q. q Different charge positions results in different charge distributions but the total charge stays –q. -q -q

46. Signal Induction for Moving Charges Segment the grounded metal plate into grounded individual strips. q The surface charge density from the moving charge does not change with respect to the infinite metal plate. q -q The charge on each strip depends on the charge position. -q If the charge is moving,current flows between the strips and ground.

47. Charge Generation in a Gas Amount of ionization produced in a gas is not very great. A minimum ionizing particle (m.i.p.) typically produces 30 ion pairs per cm from primary ionization in commonly used gases (e.g. Argon) The total ionization is ~100 ion pairs per cm including the secondary ionization caused by faster primary electrons. Primary ionization Secondary ionization

48. Charge Collection Cathode Charge is produced near the track. Electric Field Apply an electric field to move charge to electrodes. Charge is accelerated by the field, but loses energy through collisions with gas molecules. Overall, steady drift velocity of electrons towards anode and positive ions towards the cathode. Anode

49. Ion Mobility Ions drift slowly because of their large mass and scattering cross-section. Similar spectrum to the Maxwell energy distribution of the gas molecules. Average drift velocity (W+) increases with the field strength (E) and decreases as the gas pressure, P, increases. A pressure increase leads to a shorter mean free path (distance during which an ion is accelerated before losing its energy in a collision). The ion mobility, μ+, defined as μ+=W+(P/E), is constant for a given ion type in a given gas.

50. Electron Drift Velocity The dependence of the electron drift velocity on the electric field varies with the type of gas used.