Calorimeter for detection of the high-energy photons

Transcription

1 Calorimeter for detection of the high-energy photons

2 1. Introduction 2 1. Introduction 2. Theory of Electromagnetic Showers 3. Types of Calorimeters 4. Function Principle of Liquid Noble Gas Calorimeters 5. Energy Resolution of Sampling Calorimeters 6. ATLAS ECAL 7. Summary 8. References

3 1. Introduction 3 Calorimetry = Energie measurement using total absorption Calorimeter = Total absorptive shower counter With their help one can measure Energy, Location, Direction of high-energy Particles

4 1. Introduction 4 Calorimetry = Energie measurement using total Absorption Calorimeter = Total absorptive shower counter With their help one can measure Energy, Location, Direction of high-energy Particles Calorimetry can be used for detection of: charged particles (e ±, hadrons) neutral particles (γ, n ) Sole direct possibility to receive any kinematic information about neutral particles

5 1. Introduction 5 Energy loss of photons in matter : Photo effect (E γ < 1 MeV) Compton effect (1-10 MeV) Pair production (E γ >> 5 MeV) For high-energy photons is practically only pair production important

6 1. Introduction 6 Energy loss of e + and e - in matter: Cherenkov radiation Pb Bremsstrahlung Ionization For high-energy e - and e + is practically only Bremsstrahlung important

7 2. Theory of Electromagnetic Showers 7 Basic mechanisms of Calorimetry: (e ±, γ) (Hadrons) electromagnetic shower hadronic shower Particle shower in Calorimeter: Electromagnetic Shower (Monte Carlo Simulation)

8 2. Theory of Electromagnetic Showers 8 Basic mechanisms of Calorimetry: (e ±, γ) (Hadrons) electromagnetic shower hadronic shower Responsible processes: Pair production [γ] Bremsstrahlung [e ± ] Responsible process: Strong interaction (Inelastic collisions of Hadrons with nuclei of absorber material)

9 2. Theory of Electromagnetic Showers 9 Basic mechanisms of Calorimetry: (e ±, γ) (Hadrons) electromagnetic shower hadronic shower Responsible processes: Pair production [γ] Bremsstrahlung [e ± ] Responsible process: Strong interaction (Inelastic collisions of hadrons with nuclei of absorber material) eˉ- γ shower counter (ECAL) Hadron shower counter (HCAL)

10 2. Theory of Electromagnetic Showers Basic mechanisms of Calorimetry: 10 (e ±, γ) electromagnetic shower Responsible processes: Pair production [γ] Bremsstrahlung [e ± ] IMPORTANT FOR: H γ γ One important signature for the Higgs-Boson is a pair of high-energy photons eˉ- γ shower counter (ECAL)

11 2. Theory of Electromagnetic Showers 11 Simple shower model: alternately: Pair production Bremsstrahlung.. Process repeats until particle energies falls below E C Afterwards energy loss only through ionization Shower ceases rapidly Energy measurement principle: Shower particles are detected Sum of signals produced by all particles is proportional to the energy of the incident particle

12 2. Theory of Electromagnetic Showers 12 E 0 Shower maximum (at E E C ) n max n(x 0 )

13 2. Theory of Electromagnetic Showers 13 E 0 Shower maximum (at E E C ) Radiation length X 0 : X 0 1 n max ~ 2 2 ρ 4α Z r0 ln ρ After passage of one X 0 the e ± has only 1 e 1 1/3 2 ( Z ) Z Withal it emits on average one γ Brems with energy n(x 0 ) of its primary energy E 1 E < E γ < brems e E

14 2. Theory of Electromagnetic Showers 14 E 0 Shower maximum (at E E C ) Radiation length X 0 : X 0 1 n max ~ 2 2 ρ 4α Z r0 ln ρ 1 1/3 2 ( Z ) Z n(x 0 ) 1 After passage of one X 0 the e ± has only of its primary energy E e 1 Withal it emits on average one γ Brems with energy E < E γ < E brems e 9 High-energy photon in matter produces e + - e - pair after X 0 X 0 7

15 2. Theory of Electromagnetic Showers 15 E 0 Shower maximum (at E E C ) n(x 0 ) n max Critical Energy E C : de dx Ion de = dx Brems E c 580 ~ MeV Z Energy by which for e ± the energy loss via ionization and Bremsstrahlung will be equal. The shower begins to cease.

16 2. Theory of Electromagnetic Showers 16 E 0 Shower maximum (at E E C ) n max = ln E 0 ln ln 2 E c Critical Energy E C : de dx n max Brems Longitudinal shower depth => Calorimeter size increases only logarithmically with the primary energy of the incident particle! Ion de = dx E c 580 ~ Energy by which for e ± the energy loss via ionization and Bremsstrahlung will be equal. The shower begins to cease. Z n(x 0 ) MeV

17 2. Theory of Electromagnetic Showers 17 E 0 Shower maximum (at E E C ) n max = ln E 0 ln ln 2 E c n max n(x 0 ) Another characteristic parameter Average energy perparticle after n generations of X 0 : E E 2 0 ( n) = n Number of particles at shower maximum: E N ( n ) = 0 max E c Total number of particles in shower after n generations: N( n) = 2 n

18 2. Theory of Electromagnetic Showers 18 Transverse shower development: Longitudinal shower development: e-m particle in Pb R M 21MeV E c X 0 R(90%) = 1 R M R(95%) = 2 R M R(99%) = 3 R M R L L(95%) = n max Z (X0)

19 2. Theory of Electromagnetic Showers 19 Typical values for X 0, E C and R M of materials used in calorimeter

20 3. Types of Calorimeters 20 Homogeneous Calorimeters: absorber is simultaneously active medium Crystal and plastic scintillators Liquid scintillator detectors Lead glass Cherenkov counter + Good energy resolution Not optimal spatial resolution Difficult segmentation (detection principle is light)

21 3. Types of Calorimeters 21 Sampling Calorimeters: Alternating layers of absorber and active material Sandwich of absorbers and scintillators Sandwich of absorbers and gas ionization chambers Liquid noble gas calorimeters Reduced energy resolution + More flexible in design trough segmentation + More compact dimensions Only part of the deposited energy is actually detected => sampling

22 3. Types of Calorimeters 22 detection principle is light Possible setups of Sampling-Calorimeters detection principle is ionization

23 4. Liquid Noble Gas Calorimeters 23 Basic principle Zählgas Ionization chamber two conducting electrodes with high voltage between filled with electric neutral gas E 0 gas ionized by incident radiation created ions and dissociated electrons move to the electrodes

24 4. Liquid Noble Gas Calorimeters 24 Liquid Gas Calorimeter A row of Ionization chambers Liquid Gas as active medium Absorber plates used as electrodes Advantage: good spatial resolution Biggest disadvantage : long accumulation time prevention of high count rates Solution?

25 4. Liquid Noble Gas Calorimeters 25 Accordion Calorimeter Short charge accumulation time (nsrange) short drift ways => short delay times, high count rates Various segmentation possible. => gathering of all solid angles Very good spatial resolution Readout on the end, direct on elektrodes => minimisation of electronic noise => no dead zones

26 σ E E = 5. Energy Resolution 26 a E b c E Energy resolution of sampling calorimeters Stochastic term a statistical fluctuations of the number of shower particles fluctuations of the number of electrons crossing active layer (sampling-fluctuations) Constant term b Longitudinal leakage and transversal leakage influence Reactions before particle enter into calorimeter Inhomogeneities of detector sensitivity Calibration bugs between individual cells(it is very difficult to perfectly synchronize thousands of detector cells between them) Rauschterm c Noise of the electronic (dominates at low energies).

27 Weight: 7000 ton Magnetic field: 2T 6. ATLAS OVERVIEW 27

28 6. ATLAS CAL. SYSTEM 28 HCAL ECAL ECAL: Liquid Argon (LAr) HCAL: Lead absorbers + Plastic scintillators

29 47cm 6. ATLAS ECAL read out channels

30 6. ATLAS ECAL 30 Energy Resolution σ E E = 10,6% E[ GeV ] 170MeV 0,4% E[ GeV ] Corresponds very good with expectations from simulation Good linearity between 20 GeV and 250 GeV. Better than 0.2 %

31 7. Summary 31 Electromagnetic calorimeters measure the energy of high-energy photons, electrons and positrons via total absorption. There is two types of calorimeters: homogeneous- and samplingcalorimeters. Absorbers: lead, steel. Active mediums: scintillators, lead glass counter, gases, liquid noble gases. In ATLAS detector (at LHC) is deployed the liquid argon sampling ECAL with an accordion geometry. With help of ATLAS ECAL one tries to detect Higgs Boson via searching for two high-energy photons (because of H γ γ )

32 8. References 32 Konrad Kleinknecht: Detektoren für die Teilchenstrahlung, 4. Auflage 2005 Claus Grupen: "Teilchendetektoren", 1993 Lecture: Teilchenphysik mit höchstenergetischen Beschleunigern Prof. Dr. Siegfried Bethke TU München Lecture: "The Physics of Particle Detectors" Prof. H.-C. Schultz-Coulon, Uni Heidelberg Particle Data Group Images:

33 Thank you 6. for ATLAS your attention!