1 / 47

Calorimeters

Calorimeters. Purpose of calorimeters EM Calorimeters Hadron Calorimeters. EM Calorimeters. Measure energy (direction) of electrons and photons. Identify electrons and photons. Reconstruct masses eg Z  e+ e- p 0  g g H gg Resolution important: Improve S/N

kuper
Télécharger la présentation

Calorimeters

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Calorimeters • Purpose of calorimeters • EM Calorimeters • Hadron Calorimeters T. Weidberg

  2. EM Calorimeters • Measure energy (direction) of electrons and photons. • Identify electrons and photons. • Reconstruct masses eg • Z  e+ e- • p0 g g • H gg • Resolution important: • Improve S/N • Improve precision of mass measurement. T. Weidberg

  3. EM Calorimeters • Electron and photon interactions in matter • Resolution • Detection techniques • Sampling calorimeters vs all active • Examples T. Weidberg

  4. 12.2 Charged particles in matter(Ionisation and the Bethe-Bloch Formula, variation with bg) m+ can capture e- Emc = critical energy defined via: dE/dxion.=dE/dxBrem. T. Weidberg

  5. g e- e- Ze Charged particles in matter(Bremsstrahlung = Brakeing Radiation) • Due to acceleration of incident charged particle in nuclear Coulomb field • Radiative correction to Rutherford Scattering. • Continuum part of x-ray emission spectra. • Emission often confined to incident electrons because • radiation ~ (acceleration)2 ~ mass-2. • Lorentz transformation of dipole radiation from incident particle centre-of-mass to laboratory gives narrow (not sharp) cone of blue-shifted radiation centred around cone angle of =1/. • Radiation spectrum very uniform in energy. • Photon energy limits: • low energy (large impact parameter) limited through shielding of nuclear charge by atomic electrons. • high energy limited by maximum incident particle energy. T. Weidberg

  6. 12.2 Charged particles in matter(Bremsstrahlung  EM-showers, Radiation length) • dT/dx|Brem~T (see Williams p.247)  dominates over dT/dx|ionise ~ln(T) at high T. • For electrons Bremsstrahlung dominates in nearly all materials above few 10 MeV. Ecrit(e-) ≈ 600 MeV/Z • If dT/dx|Brem~T  dT/dx|Brem=T0exp(-x/X0) • Radiation Length X0 of a medium is defined as: • distance over which electron energy reduced to 1/e. • X0~Z2 approximately. • Bremsstrahlung photon can undergo pair production (see later) and start an em-shower (or cascade) • Length scale of pair production and multiple scattering are determined by X0 because they also depend on nuclear coulomb scattering.  The development of em-showers, whether started by primary e or  is measured in X0. T. Weidberg

  7. Very Naïve EM Shower Model • Simple shower model assumes: • E0 >> Ecrit • only single Brem-g or pair production per X0 • The model predicts: • after 1 X0, ½ of E0 lost by primary via Bremsstrahlung • after next X0 both primary and photon loose ½ E again • until E of generation drops below Ecrit • At this stage remaining Energy lost via ionisation (for e+-) or compton scattering, photo-effect (for g) etc. • Abrupt end of shower happens at t=tmax = ln(E0/Ecrit)/ln2 • Indeed observe logarithmic depth dependence T. Weidberg

  8. 13.1 Photons in matter(Overview) • Rayleigh scattering • Coherent, elastic scattering of the entire atom (the blue sky) • g + atom  g + atom • dominant at lg>size of atoms • Compton scattering • Incoherent scattering of electron from atom • g + e-bound g + e-free • possible at all Eg > min(Ebind) • to properly call it Compton requires Eg>>Ebind(e-) to approximate free e- • Photoelectric effect • absorption of photon and ejection of single atomic electron • g + atom  g + e-free + ion • possible for Eg < max(Ebind) + dE(Eatomic-recoil, line width) (just above k-edge) • Pair production • absorption of g in atom and emission of e+e- pair • Two varieties: • g + nucleus  e+ + e- + nucleus (more momentum transfer to nucleusdominates) • g + Z atomic electrons  e+ + e- + Z atomic electrons • both summarised via: g + g(virtual)  e+ + e- • Needs Eg>2mec2 • Nucleus has to recoils to conserve momentum  coupling to nucleus needed  strongly Z-dependent crossection T. Weidberg

  9. Pair production Bremsstrahlung Typical Lenth = Pair Production Length L0 Typical Lenth = Radiation Length X0 e- e- g g e-* e-* e- e- Ze Ze 13.1 Photons in matter(Note on Pair Production) • Compare pair production with Bremsstrahlung • Very similar Feynman Diagram • Just two arms swapped L0=9/7 X0 T. Weidberg

  10. 13.1 Photons in matter(Crossections) • R  Rayleigh • PE  Photoeffect • C  Compton Lead Carbon • PP  Pair Production • PPE  Pair Production on atomic electrons • PN  Giant Photo-Nuclear dipole resonance T. Weidberg

  11. Transverse Shower Size • Moliere radius = 21 MeV X0/Ec Electrons Photons T. Weidberg

  12. Sampling vs All Active • Sampling: sandwich of passive and active material. eg Pb/Scintillator. • All active: eg Lead Glass. • Pros/cons • Resolution • Compactness  costs. T. Weidberg

  13. Detection Techniques • Scintillators • Ionisation chambers • Cherenkov radiation • (Wire chambers) • (Silicon) T. Weidberg

  14. Organic Scintillators (1) • Organic molecules (eg Naphtalene) in plastic (eg polysterene). • excitation  non-radiating de-excitation to first excited state  scintillating transition to one of many vibrational sub-states of the ground state. T. Weidberg

  15. Organic Scintillators (2) • gives fast scintillation light, de-excitation time O(10-8 s) • Problem is short attenuation length. • Use secondary fluorescent material to shift l to longer wavelength (more transparent). • Light guides to transport light to PMT or • Wavelength shifter plates at sides of calorimeter cell. Shift blue  green (K27)  longer attenuation length. T. Weidberg

  16. Inorganic Scintillators (1) • eg NaI activated (doped) with Thallium, semi-conductor, high density: r(NaI=3.6),  high stopping power • Dopant atom creates energy level (luminescence centre) in band-gap • Excited electron in conduction band can fall into luminescence level (non radiative, phonon emission) • From luminescence level falls back into valence band under photon emission • this photon can only be re-absorbed by another dopant atom  crystal remains transparent T. Weidberg

  17. Inorganic Scintillators (2) • High density of inorganic crystals  good for totally absorbing calorimetry even at very high particle energies (many 100 GeV) • de-excitation time O(10-6 s) slower then organic scintillators. • More photons/MeV  Better resolution. • PbWO4. fewer photons/MeV but faster and rad-hard (CMS ECAL). T. Weidberg

  18. PMT Detectors (1) • Photomultiplier: • primary electrons liberated by photon from photo-cathode (low work function, high photo-effect crossection, metal, hconversion≈¼ ) • visible photons have sufficiently large photo-effect cross-section • acceleration of electron in electric field 100 – 200 eV per stage • create secondary electrons upon impact onto dynode surface (low work function metal)  multiplication factor 3 to 5 • 6 to 14 such stages give total gain of 104 to 107 • fast amplification times (few ns)  good for triggers or veto’s • signal on last dynode proportional to #photons impacting T. Weidberg

  19. Detectors (2) • APD (Avalanche Photo Diode) • solid state alternative to PMT • strongly forward biased diode gives “limited” avalanche when hit by photon T. Weidberg

  20. 13.2 Detectors • Ionisation Chambers • Used for single particle and flux measurements • Can be used to measure particle energy up to few MeV with accuracy of 0.5% (mediocre) • Electrons more mobile then ions  medium fast electron collection pulse O(ms) • Slow recovery from ion drift T. Weidberg

  21. Resolution • Sampling fluctuations for sandwich calorimeters. • Statistical fluctuations eg number of photo-electrons or number of e-ion pairs. • Electronic noise. • Others • Non-uniform response • Calibration precision • Dead material (cracks). • Material upstream of the calorimeter. • Lateral and longitudinal shower leakage • Parameterise resolution as • a Statistical • b noise • c constant T. Weidberg

  22. Classical Pb/Scintillator T. Weidberg

  23. Lead Glass • All active • Pb Glass T. Weidberg

  24. BGO • Higher resolution T. Weidberg

  25. Liqiuid Argon • Good resolution eg NA31. T. Weidberg

  26. Fast Liquid Argon • Problem is long drift time of electrons (holes even slower). • Trick to create fast signals is fast pulse shaping. • Throw away some of the signal and remaining signal is fast (bipolar pulse shaping). • Can you maintain good resolution and have high speed (LHC)? T. Weidberg

  27. Accordion Structure Lead plates Cu/kapton electrodes for HV and signal Liquid Argon in gaps. Low C and low L cf cables in conventional LAr calorimeter. T. Weidberg

  28. Bipolar Pulse Shaping T. Weidberg

  29. T. Weidberg

  30. ATLAS Liquid Argon • Resolution • Stochastic term ~ 1/E1/2. • Noise ~ 1/E • Constant (non-uniformity over cell, calibration errors). T. Weidberg

  31. Calibration • Electronics calibration • ADC counts to charge in pC. How? • For scintillators • Correct for gain in PMT or photodiode. How? • Correct for emission and absorption in scintillator and light guides. How ? • Absolute energy scale. • Need to convert charge seen pC  E (GeV). How? T. Weidberg

  32. Hadron Calorimeters • Why you need hadron calorimeters. • The resolution problem. • e/pi ratio and compensation. • Some examples of hadron calorimeters. T. Weidberg

  33. Why Hadron Calorimeters • Measure energy/direction of jets • Reconstruct masses (eg tbW or h bbar) • Jet spectra: deviations from QCD  quark compositeness) • Measure missing Et (discovery of Ws, SUSY etc). • Electron identification (Had/EM) • Muon identification (MIPs in calorimeter). • Taus (narrow jets). T. Weidberg

  34. Hadron Interactions • Hadron interactions on nuclei produce • More charged hadrons  further hadronic interactions  hadronic cascade. • p0 gg EM shower • Nuclear excitation, spallation, fission. • Heavy nuclear fragments have short range  tend to stop in absorber plates. • n can produce signals by elastic scattering of H atoms (eg in scintillator) • Scale set by lint (eg = 17 cm for Fe, cf X0=1.76 cm)  next transparency T. Weidberg

  35. T. Weidberg

  36. Resolution for Hadron Calorimeters • e/pi ≠ 1  fluctuations in p0 fraction in shower will produce fluctuations in response (typically e/pi >1). • Energy resolution degraded and no longer scales as 1/E1/2 and response will tend be non-linear because p0fraction changes with E. T. Weidberg

  37. e/h Response vs Energy T. Weidberg

  38. Resolution Plots s(E)/E vs 1/E1/2. Fe/Scint (poor). ZEUS U/scint and SPACAL (good). T. Weidberg

  39. Compensation (1) • Tune e/pi ~= 1 to get good hadronic resolution. • U/Scintillator (ZEUS) • Neutrons from fission of U238 elastic scatter off protons in scintillator  large signals  compensate for nuclear losses. • Trade off here is poorer EM resolution. T. Weidberg

  40. Compensation (2) • Fe/Scintillator (SPACAL) • Neutrons from spallation in any heavy absorber can scatter of protons in scintillator  large signals. • If the thickness of the absorber is increased greater fraction of EM energy is lost in the passive absorber. • tune ratio of passive/active layer thickness to achieve compensation. • Needs ratio 4/1 to achieve compensation. No use for classical calorimeter with scintillator plates (why). • SPACAL: scintillating fibres in Fe absorber. T. Weidberg

  41. Scintillator Readout T. Weidberg

  42. SPACAL 1 mm scintillating fibres in Fe T. Weidberg

  43. T. Weidberg

  44. T. Weidberg

  45. Compensation (3) • Software weighting (eg H1) • EM component localized  de-weight large local energies • Very simplified: T. Weidberg

  46. Fine grain Fe/Scintillator Calorimeter (WA1) • With weighting resolution improved. T. Weidberg

  47. H1 Hadronic resolution with weighting Standard H1 weighting Improved (Cigdem Issever) T. Weidberg

More Related