Calorimeters

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

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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

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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

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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