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Basic principles Interaction of charged particles and photons Electromagnetic cascades

Basic principles Interaction of charged particles and photons Electromagnetic cascades Nuclear interactions Hadronic cascades Homogeneous calorimeters Sampling calorimeters. Calorimetry. Calorimetry: Energy measurement by total absorption , combined with spatial reconstruction.

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Basic principles Interaction of charged particles and photons Electromagnetic cascades

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  1. Basic principles Interaction of charged particles and photons Electromagnetic cascades Nuclear interactions Hadronic cascades Homogeneous calorimeters Sampling calorimeters Calorimetry Christian Joram

  2. Calorimetry: Energy measurement by total absorption, combined with spatial reconstruction. Calorimetry is a “destructive” method Detector response  E Calorimetry works both for  charged (e and hadrons)  and neutral particles (n,g) Basic mechanism: formation of  electromagnetic or hadronic showers. Finally, the energy is converted into ionization or excitation of the matter. Calorimetry Christian Joram

  3. Energy loss by Bremsstrahlung Radiation of real photons in the Coulomb field of the nuclei of the absorber Effect plays a role only for e± and ultra-relativistic m (>1000 GeV) For electrons: Interaction of charged particles e- radiation length [g/cm2] Christian Joram

  4. Critical energy Ec For electrons one finds approximately: density effect of dE/dx(ionisation) ! Ec(e-) in Fe(Z=26) = 22.4 MeV For muons Ec(m) in Fe(Z=26)  1 TeV Interaction of charged particles (Leo) energy loss (radiative + ionization) of electrons and protons in copper Christian Joram

  5. Interaction of photons In order to be detected, a photon has to create charged particles and/or transfer energy to charged particles Photo-electric effect: Only possible in the close neighborhood of a third collision partner  photo effect releases mainly electrons from the K-shell. Cross section shows strong modulation if Eg Eshell At high energies (1) Interaction of photons Christian Joram

  6. Interaction of photons Application of photo effect in medicine Detection of tumors with SPECT = Single Photon Emission Computed Tomography PMT PMT PMT scintillator Eg =141 keV d Pb collimator L 99Tc q w = photo effect + production of scintillation light • Problem: • spatial resolution s ~ w/L·d (typical 5 -10 mm) • efficiency e ~ (w/L)2 (typical 2·10-4) Christian Joram

  7. Compton scattering: Assume electron as quasi-free. Cross-section: Klein-Nishina formula, at high energies approximately Atomic Compton cross-section: Interaction of photons Christian Joram

  8. Interaction of photons Application of Compton effect in medicine A Compton camera = an electronically collimated SPECT device (Concept currently under development) 2nd detector measures position of Compton scattered g scintillator qg 1st detector (Silicon) measures position of and E-deposition by Compton electron Photon trajectory known apart from azimutal angle f  Ambiguity. e- d Reconstruction: Gamma source lies in the crossing points of Compton cones Expect efficiency increase by factor 10-50 compared to collimated SPECT. Christian Joram

  9. Pair production Only possible in the Coulomb field of a nucleus (or an electron) if Cross-section (high energy approximation) Energy sharing between e+ and e- becomes asymmetric at high energies. Interaction of photons independent of energy ! Christian Joram

  10. Standard PET geometry Positron annihilation Pair production Application of positron annihilation in medicine PET = Position Emission Tomography Use 18F labeled radiotracer 18F  18O + e+ e++ e-  2 g (2 x 511 keV) 2 g’s are detected by 2 scintillators in coincidence. 18F lies somewhere on this line of record! Need many lines of record + tomographic reconstruction. Christian Joram

  11. In summary: Interaction of photons m: mass attenuation coefficient photo effect 1 MeV pair production Rayleigh scattering (no energy loss !) Compton scattering (PDG) Christian Joram

  12. Reminder: basic electromagnetic interactions e+ / e- g • Ionisation • Bremsstrahlung • Photoelectric effect • Compton effect • Pair production s dE/dx E E s dE/dx E E s E Christian Joram

  13. Electromagnetic cascades Electromagnetic Cascades (showers) Electron shower in a cloud chamber with lead absorbers Simple qualitative model Consider only Bremsstrahlung and pair production. Assume: X0 = lpair g Process continues until E(t)<Ec After t = tmax the dominating processes are ionization, Compton effect and photo effect absorption. Christian Joram

  14. Longitudinal shower development: Shower maximum at 95% containment Size of a calorimeter grows only logarithmically with E0 Electromagnetic cascades Transverse shower development: 95% of the shower cone is located in a cylinder with radius 2 RM Molière radius Longitudinal and transverse development scale with X0, RM 6 GeV/c e- (C. Fabjan, T. Ludlam, CERN-EP/82-37) • Example: E0 = 100 GeV in lead glass • Ec=11.8 MeV  tmax 13, t95% 23 X0  2 cm, RM = 1.8·X0  3.6 cm 8 cm 46 cm Christian Joram

  15. Energy resolution of a calorimeter (intrinsic limit) Energy resolution total number of track segments holds also for hadron calorimeters Also spatial and angular resolution scale like 1/E Relative energy resolution of a calorimeter improves with E0 More general: Constant term Inhomogenities Bad cell inter-calibration Non-linearities Noise term Electronic noise radioactivity pile up Stochastic term Quality factor ! Christian Joram

  16. Nuclear Interactions The interaction of energetic hadrons (charged or neutral) is determined by inelastic nuclear processes. For high energies (>1 GeV) the cross-sections depend only little on the energy and on the type of the incident particle (p, p, K…). In analogy to X0 a hadronic absorption length can be defined Interaction of charged particles p,n,p,K,… multiplicity ln(E) pt 0.35 GeV/c Excitation and finally breakup up nucleus  nucleus fragments + production of secondary particles. Christian Joram

  17. I.2.1 Interaction of charged particles For Z > 6: la > X0 la and X0 in cm la X0, la [cm] X0 Z Christian Joram

  18. Hadronic casacdes Hadronic + electromagnetic component Large energy fluctuations  limited energy resolution Hadronic cascades Various processes involved. Much more complex than electromagnetic cascades. (Grupen) charged pions, protons, kaons …. Breaking up of nuclei (binding energy), neutrons, neutrinos, soft g’s muons ….  invisible energy neutral pions  2g  electromagnetic cascade example 100 GeV: n(p0)18 Christian Joram

  19. Longitudinal shower development Hadronic cascades For Iron: a = 9.4, b=39 la =16.7 cm E =100 GeV  t95%  80 cm (C. Fabjan, T. Ludlam, CERN-EP/82-37) • Lateral shower development The shower consists of core + halo. 95% containment in a cylinder of radius lI. Hadronic showers are much longer and broader than electromagnetic ones ! Christian Joram

  20. Calorimeter types Homogeneous calorimeters: Sampling calorimeters: Calorimetry  Detector = absorber  good energy resolution  limited spatial resolution (particularly in longitudinal direction)  only used for electromagnetic calorimetry  Detectors and absorber separated  only part of the energy is sampled.  limited energy resolution  good spatial resolution  used both for electromagnetic and hadron calorimetry Christian Joram

  21. Homogeneous calorimeters Two main types: Scintillator crystals or “glass” blocks (Cherenkov radiation).  photons. Readout via photomultiplier, -diode/triode Scintillators (crystals) Cherenkov radiators Homogeneous calorimeters Relative light yield: rel. to NaI(Tl) readout with PM (bialkali PC) Relative light yield: rel. to NaI(Tl) readout with PM (bialkali PC) Christian Joram

  22. Examples OPAL Barrel + end-cap: lead glass + pre-sampler BGO E.M. Calorimeter in L3 Homogeneous calorimeters (OPAL collab. NIM A 305 (1991) 275) 10500 blocks (10 x 10 x 37 cm3, 24.6 X0), PM (barrel) or PT (end-cap) readout. Spatial resolution (intrinsic)  11 mm at 6 GeV (L3 collab. NIM A 289 (1991) 53) 11000 crystals, 21.4 X0, temperature monitoring + control system light output -1.55% / ºC E/E < 1% for E > 1 GeV spatial resolution < 2 mm (E >2 GeV) Partly test beam results ! Christian Joram

  23. NA48: LKr Ionisation chamber (T = 120 K) no metal absorbers  quasi homogenous ! prototype full device (prel.) Homogeneous calorimeters Cu-Be ribbon electrode x,y  1 mm t  230 ps 97 run: reduced performance due to problems with blocking capacitors  lower driftfield: 1.5 kV/cm rather than 5 kV/cm (V. Marzulli, NIM A 384 (1996) 237, M. Martini et al., VII International Conference on Calorimetry, Tuscon, 1997) Christian Joram

  24. Homogeneous calorimeters The NA48 LKr calorimeter prior to installation in the cryostat. Christian Joram

  25. Homogeneous calorimeters One half of the NA48 LKr calorimeter. Christian Joram

  26. Sampling calorimeters Absorber + detector separated  additional sampling fluctuations Sampling calorimeters Detectable track segments • MWPC, streamer tubes • warm liquids TMP = tetramethylpentane, TMS = tetramethylsilane • cryogenic noble gases: mainly LAr (Lxe, LKr) • scintillators, scintillation fibres, silicon detectors Christian Joram

  27. ATLAS electromagnetic Calorimeter Accordion geometry absorbers immersed in Liquid Argon Sampling calorimeters (RD3 / ATLAS) Liquid Argon (90K) + lead-steal absorbers (1-2 mm) + multilayer copper-polyimide readout boards  Ionization chamber. 1 GeV E-deposit  5 x106 e- • Accordion geometry minimizes dead zones. • Liquid Ar is intrinsically radiation hard. • Readout board allows fine segmentation (azimuth, pseudo-rapidity and longitudinal) acc. to physics needs Test beam results, e- 300 GeV (ATLAS TDR) Spatial and angular uniformity 0.5% Spatial resolution  5mm / E1/2 Christian Joram

  28. Sampling calorimeters • CMS Hadron calorimter Cu absorber + scintillators 2 x 18 wedges (barrel) + 2 x 18 wedges (endcap)  1500 T absorber Scintillators fill slots and are read out via fibres by HPDs Test beam resolution for single hadrons Christian Joram

  29. Sampling calorimeters 4 scintillating tiles of the CMS Hadron calorimeter Christian Joram

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