Ingrid-Maria Gregor, DESY

Transcription

1 Particle Detectors Ingrid-Maria Gregor, DESY Thanks to: Frank Simon, Christoph Rembser, Ulrich Koetz, Daniel Pitzl, Christian Joram, Carsten Niehbuhr, Reiner Wiegler, Marc Winter, Laci Andricek, Cinzia da Via, Paula Collins, Uli Koetz, Jim Virdee, Steinar Stapnes,.

2 Disclaimer Particle Detectors are very complex, a lot of physics is behind the detection of particles: particle physics, material science, electronics, mechanics,. To get a good understanding, one needs to work on a detector project... This talk can only give a glimpse at particle detector physics, cannot cover everything Biased by my favorit detectors! Maybe not the ideal detector physicist Ingrid-Maria Gregor Maria Laach Herbstschule 2011 Slide 2

3 Overview I. Detectors in Particle Physics II. Interaction with Matter Tuesday III. Tracking Detectors Gas detectors Semiconductor trackers Wednesday IV. Calorimeters Thursday V. Examples from the real life Ingrid-Maria Gregor Maria Laach Herbstschule 2011 Slide 3

4 I. Overview: Detectors for particle physics

5 Detectors: Our Cameras for New Particles Prediction of the decay mode, signature in detector and event kinematics design of a detector Ingrid-Maria Gregor Maria Laach Herbstschule 2011 Slide 5

6 Particle Physics Detectors There is not one type of detector which provides all measurements we need -> Onion concept -> different systems taking care of certain measurement Detection of collision production within the detector volume resulting in signals due to electro-magnetic interaction Tracker: Momentum of charged particles due to magnetic field and precise measurement of track Calorimeter: Energy measurement of photons, electronics and hadrons through total absorption Myon-Detectors: Identification and precise momentum measurement of myons outside of the magnet Ingrid-Maria Gregor Maria Laach Herbstschule 2011 Slide 6

7 The Big Ones at LHC. Berlin, Bonn, DESY, Dresden, Dortmund, Freiburg, Göttingen, Heidelberg, Mainz, Mannheim, München, MPI München, Aachen, DESY, Karlsruhe Dresden, Heidelberg, MPI Heidelberg GSI Darmstadt, Frankfurt, Heidelberg, Kaiserslautern, Köln, Mannheim, Münster, Worms Ingrid-Maria Gregor Maria Laach Herbstschule 2011 Slide 7

8 Collider-Detectors: Cross Section [example CMS] High granularity for the tracker (high occupancy) Good energy resolution up to highest energies Radiation hard (hadron collider) Ingrid-Maria Gregor Maria Laach Herbstschule 2011 Slide 8

9 CMS: The heavy detector Weight: T Length: 21.5 m Diameter: 15 m Solenoid-Field: 4 T Ingrid-Maria Gregor Maria Laach Herbstschule 2011 Slide 9

10 CMS Cross Section Ingrid-Maria Gregor Maria Laach Herbstschule 2011 Slide 10 Foto: CERN

11 Particles in ATLAS Ingrid-Maria Gregor Maria Laach Herbstschule 2011 Slide 11

12 ATLAS: The largest Detector in Particle Physics 46 m 25 m Weight: t Central Solenoid: 2 T Muon-Toroid: 4 T Ingrid-Maria Gregor Maria Laach Herbstschule 2011 Slide 12 Illustration: CERN

13 ATLAS Cross Section Ingrid-Maria Gregor Maria Laach Herbstschule 2011 Slide 13 Foto: CERN

14 Size and Weight ATLAS CMS Brandenburger Tor in Berlin CMS is 30% heavier than the Eiffel tower Ingrid-Maria Gregor Maria Laach Herbstschule 2011 Slide 14

15 Magnet-Concepts: CMS -> Solenoid 35H6;E%G,6G;:E-%P,+%J! )%*+,%-. N D )5H6;E G,.4 Largest Solenoid in the world: super conducting, 4 T field encloses trackers and calorimeter 13 m long, inner radius 5.9 m, I = 20 ka, weight of coil: 220 t O% 85+H;%L,),H;6,F-%P.;47%.6-.7;%G,. Q <;5=%,::,-.E;%P.;47%.6%+;EF+6%2,=;% Q R.S;%4.).E;7%TG,-EU Q +;4$%L.HL%)5E;+.54%*F7H;E% + large homogeneous field inside coil + weak opposite >V5):4;-W% field in return yoke - size limited X (cost) - relative Xhigh material budget X &>8ABDW%R#/%!$C0/%YK$C)/%8%%%Z$J) 8[W%%%%%%%%%%?#/%I$K0/%Y!!$\)/%8%!!$ #3RW%%%%%%R#/%J$I0/%YK$\)/%8%!C$K) Ingrid-Maria Gregor Maria Laach Herbstschule 2011 Slide 15

16 Magnet-Concepts: ATLAS -> Toroid *+,%-. N the largest magnet in the world N /%&%-. D )5H6;E G,.4 D )5H6;E O% 85+H;%L,),H;6,F-%P.;47%.6-.7;%G,.4 Q <;5=%,::,-.E;%P.;47%.6%+;EF+6%2,=;% Q Q R.S;%4.).E;7%TG,-EU Central Toroid field within Muon-System: 4 T +;4$%L.HL%)5E;+.54%*F7H;E% Closed field, no yoke Complex field >V5):4;-W% 2 T Solenoid-field for trackers X &>8ABDW%R#/%!$C0/%YK$C)/%8%%%Z$J)% X 8[W%%%%%%%%%%?#/%I$K0/%Y!!$\)/%8%!!$\)% X #3RW%%%%%%R#/%J$I0/%YK$\)/%8%!C$K) O%].;47%54<52-%:;+:;67.GF45+%E,%: O%9;4$%45+H;%P.;47-%,^;+%45+H;%^,4F); O%9;4$%4,<%)5E;+.54%*F7H;E Q 6,6QF6.P,+)%P.;47 Q G,):4;V%-E+FGEF+; + field always perpendicular to p + relative large field over large volume - non uniform field - complex structure >V5):4;W X (08(RW%N5++;4%5.+%E,+,.7/%R#/% _!0/%Y\$J/%8%CJ$[) Ingrid-Maria Gregor Maria Laach Herbstschule 2011 Slide 16

17 Overview I. Detectors in Particle Physics II. Interaction with Matter Tuesday III. Tracking Detectors Gas detectors Semiconductor trackers Wednesday IV. Calorimeters Thursday V. Examples from the real life Ingrid-Maria Gregor Maria Laach Herbstschule 2011 Slide 17

18 II. The Basics of All Detection Processes: Interactions with Matter

19 Interactions of heavy Particles with Matter Mean energy loss is described by the Bethe-Bloch formula Maximum kinetic energy which can be transferred to the electron in a single collision Excitation energy Density term due to polarization: leads to saturation at higher energies Shell correction term, only relevant at lower energies kinema&c term rela&vis&c rise minimum ionizing par&cles βγ 3-4 Ingrid-Maria Gregor Maria Laach Herbstschule 2011 Slide 19

20 Material Dependence of the Energy Loss Zabsorber Rule of thumb: energy loss of MIPs (βγ ~ 3): 1-2 MeV g -1 cm 2 (except H) Ingrid-Maria Gregor Maria Laach Herbstschule 2011 Slide 20

21 Particle Identification Using de/dx Energy loss depends on the particle velocity and is ~ independent of the particle s mass m. The energy loss as a function of particle momentum p = mcβγ is however depending on the particle s mass By measuring the particle momentum (deflection in the magnetic field) and measurement of the energy loss on can measure the particle mass m Nucl. Instr. and Meth. A378 (1996) 57 Particle Identification! Example: DELPHI@ LEP Ingrid-Maria Gregor Maria Laach Herbstschule 2011 Slide 21

22 A Closer Account of Energy Loss Bethe-Bloch displays only the average energy loss is a statistical process discrete scattering with different results depending on intensity of scattering primary and secondary ionisation Primary ionisation Poisson distributed Large fluctuations per reaction Secondary ionisation Created by high energetic primary electrons sometime the energy is sufficient for a clear secundary track: δ-electron Example of a delta electron in a bubble Total ionisation = primary ionisation + secondary ionisation chamber: visible path Ingrid-Maria Gregor Maria Laach Herbstschule 2011 Slide 22

23 Energy Loss in Thin Layers In case of thin detectors the variation width within the energy transfer of the reactions leads to a large variation of the energy loss: A broad maximum: collisions with little energy loss A long tail towards higher energy loss: few collisions with large energy loss, δ-electrons most probable peak! The Landau distribution is used in physics to describe the fluctuations in the energy loss of a charged particle passing through a thin layer of matter Thin absorber: <de> < ~10Tmax Ingrid-Maria Gregor Maria Laach Herbstschule 2011 Slide 23

24 Electrons: Energy Loss Ionization loss by electrons (positrons) differs because of the kinematics, spin and the identity of the incident electron with the electrons which it ionizes. Bremsstrahlung is dominating at high energies At low energies: ionisation, additional scattering Critical energy: the energy at which the losses due to ionisation and Bremsstrahlung are equal de dx (E c)= de dx (E c) For electrons approximately: E solid+liq c = 610MeV Z E gas c = 710MeV Z Ingrid-Maria Gregor Maria Laach Herbstschule 2011 Slide 24

25 Multiple Scattering! Charged particles are forced to deviate from a straight track when moving through a medium: multiple scattering due to Coulomb field θ 0 = θ rms plane = 1 2 θ rms space θ 0 = 13.6 MeV β cp z x/x 0 [ ln(x/x 0 )] relevant for relativistic particles, for material thickness from 10-3 X0 bis 100 X0 Ingrid-Maria Gregor Maria Laach Herbstschule 2011 Slide 25

26 Electrons and Photons: Radiation Length An important parameter for particle detectors: Radiation length Defines the amount of material a particle has to travel through: until the energy of an electron is reduced by Bremsstrahlung to 1/e of its original energy empirical: X 0 = A Z(1+Z) ln(287/ Z) g cm 2 A Z 2 The radiation length is also an important quantity in multiple scattering A very important number when building detectors, one always has to keep in mind how much material is within the detector volume Usually quoted in [g/cm 2 ], typical values are: Air: g/cm 2, ->~ 300 m Water: g/cm 2, -> ~ 36 cm Aluminium: g/cm 2, -> 8.9 cm Tungsten: 6.76 g/cm 2, -> 0.35 cm Ingrid-Maria Gregor Maria Laach Herbstschule 2011 Slide 26

27 Photons: Interactions Photons appear in detector systems as primary photons, they are created in bremsstrahlung and de-excitations, and they are used for medical applications, both imaging and radiation treatment. ν Photo-Effect Compton-Scattering Pair creation e - ν e - e+ ν Only possible in the close neighborhood of a third collision partner photo effect releases mainly electrons from the K- shell. e - γ + e γ + e ν Elastic scattering of a photon with a free electron E γ = 1 1+ɛ(1 cos θ γ ) Nucleus e - Only possible in the Coulomb field of a nucleus (or an electron) if E γ 2m e c 2 ~1.022 MeV Reduction of photon intensity with passage through matter: I(x) =I 0 e µx Ingrid-Maria Gregor Maria Laach Herbstschule 2011 Slide 27

28 Photons in Matter Paarerzeugung im Kernfeld pair creation in electron field At high energies paar creation is dominating Low energies: Photo electric effect Coherent scattering: Rayleigh scattering Compton scattering Nucleus excitation Ingrid-Maria Gregor Maria Laach Herbstschule 2011 Slide 28

29 Energy Loss via Photon Emission Two important effects: Cherenkov radiation Sonic boom for charged particles Transition Radiation is produced by relativistic charged particles when they cross the interface of two media of different dielectric constants significant radiation only at large γ (O ~ 1000) in the kev range. Very useful for electron/pion separation Both effects are not really contributing to the energy loss of the particles! Ingrid-Maria Gregor Maria Laach Herbstschule 2011 Slide 29

30 Cherenkov-Radiation Emission of photons when a charged particle is faster than speed of light within a medium: constructive interference Emission under a characteristic Angle: cosθ c = c t / n vt = 1 nβ wave length maximum within UV Ingrid-Maria Gregor Maria Laach Herbstschule 2011 Slide 30

31 A Short Summary Ingrid-Maria Gregor Maria Laach Herbstschule 2011 Slide 31

32 A Short Summary e + e - K - decay π - K - µ - δ-electrons e - here: no individual Clusters, high de/dx, low:βγ Ingrid-Maria Gregor Maria Laach Herbstschule 2011 Slide 32

33 A Short Word on Neutrinos* * only true for experiments at accelerators Claus Grupen, Particle Detectors, Cambridge University Press, Cambridge 1996 (455 pp. ISBN ) RN Ingrid-Maria Gregor Maria Laach Herbstschule 2011 Slide 33

34 Overview I. Detectors in Particle Physics II. Interaction with Matter Tuesday III. Tracking Detectors Gas detectors Semiconductor trackers Wednesday IV. Calorimeters Thursday V. Examples from the real life Ingrid-Maria Gregor Maria Laach Herbstschule 2011 Slide 34

35 Resolution of Tracking Detectors An important figure of merit is the resolution of a tracking detector Depending on detector geometry and charge collection Pitch (distance between channels) Charge sharing between channels Simple case: all charge is collected by one strip Traversing particle creates signal in hit strip Flat distribution along strip pitch; no area is pronounced Probability distribution for particle passage: The reconstructed point is always the middle of the strip: Ingrid-Maria Gregor Maria Laach Herbstschule 2011 Slide 35

36 Resolution of Tracking Detectors II Calculating the resolution orthogonal to the strip: Resulting in a general term (also valid for wire chambers): very important! For a silicon strip detector with a strip pitch of 80 µm this results in a minimal resolution of ~23µm In case of charge sharing between the strip (signal size decreasing with distance to hit position) Resolution improved by center of gravity calculation Ingrid-Maria Gregor Maria Laach Herbstschule 2011 Slide 36

37 Tracking: Determination of the Momentum in Magnetic Field A tracking detector is typically placed within a B-field to enable momentum measurements Charged particles are deflected in a magnetic field: takes only effect on the componente perpendicular to the field Radius of the circular path is proportional to the transversal momentum: p T GeV/c =0.3 B T r m parallel to the field is no deflection: particle is moving on a helix, the radius is determined by the field and pt Magnetic Field Ingrid-Maria Gregor Maria Laach Herbstschule 2011 Slide 37

38 Determination of the Momentum in Magnetic Field II In real applications usually only slightly bent track segments are measured Figure of merit: Sagitta Segment of a circle: s = r r 2 L2 4 r = s 2 + L2 8s L2 8s (s L) With the radius-momentum-b-field relation: In general, for many measurement points: r = p T 0.3 B s = 0.3 BL2 8 p T σ(p T ) p T = σ(x) 0.3 BL 2 720/(N + 4) pt Je larger the magnetic field B, the length L and the number of measurement points, and the better the spacial resolution, the better is the momentum resolution ex.: N =7, L = 0.5, B = 2T, σ(x) = 20 µm, pt = 5 GeV/c: Δpt /pt = 0.5 %, r = 8.3 m, s = 3.75 mm Ingrid-Maria Gregor Maria Laach Herbstschule 2011 Slide 38

39 Impuls resolution: Spacial resolution and multiple scattering Two component are influencing the momentum resolution σ(pt)/pt of a tracking system: Inaccuracy of the tracking detector: σ(p T ) p T Influence of the particle due to MS: θ 1 p Known: and therefore and therefore also the spacial imprecision: σ(p T ) p T σ(p T ) p T σ(x) MS p T MS = const σ(x) MS 1 p The measurement of low momentum particles is limited by multiple scattering! At higher momenta the spacial resolution of the detector is dominating! Ingrid-Maria Gregor Maria Laach Herbstschule 2011 Slide 39

40 Summary Part 1 Ionization and Excitation: Charged particles traversing material are exciting and ionizing the atoms. The average energy loss of the incoming particle by this process is to a good approximation described by the Bethe Bloch formula. The energy loss fluctuation is well approximated by the Landau distribution. Multiple Scattering and Bremsstrahlung: The incoming particles are scattering off the atomic nuclei which are partially shielded by the atomic electrons. Measuring the particle momentum by deflection of the particle trajectory in the magnetic field, this scattering imposes a lower limit on the momentum resolution of the spectrometer. The deflection of the particle on the nucleus results in an acceleration that causes emission of Bremsstrahlungs-Photons. These photons in turn produced e+e- pairs in the vicinity of the nucleus. tomorrow we will finally talk about detectors.. Ingrid-Maria Gregor Maria Laach Herbstschule 2011 Slide 40

41 Bremsstrahlung Overview e + /e Ionisation de dx γ Photoelectric Effect σ de dx E Bremsstrahlung Compton Effect E σ E E Pair Production σ Ingrid-Maria Gregor Maria Laach Herbstschule 2011 Slide 41 E