1. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram IV/1   Basic principles Interaction of charged particles and photons Electromagnetic cascades Nuclear interactions Hadronic cascades   Homogeneous calorimeters   Sampling calorimeters

2. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram IV/2 Calorimetry   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,  )   Basic mechanism: formation of  electromagnetic  or hadronic showers.   Finally, the energy is converted into ionization or excitation of the matter.

3. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram IV/3 Interaction of charged particles 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  (>1000 GeV) For electrons: radiation length [g/cm 2 ] e-e-

4. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram IV/4 Critical energy E c For electrons one finds approximately: density effect of dE/dx(ionisation) ! E c (e - ) in Fe(Z=26) = 22.4 MeV For muons E c (  ) in Fe(Z=26)  1 TeV Interaction of charged particles energy loss (radiative + ionization) of electrons and protons in copper (Leo)

5. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram IV/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 E   E shell At high energies (  1) Interaction of photons

6. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram IV/6 scintillator Application of photo effect in medicine Detection of tumors with SPECT = Single Photon Emission Computed Tomography Pb collimator PMT = photo effect + production of scintillation light 99 Tc E  =141 keV Problem:  spatial resolution  ~ w/L·d (typical 5 -10 mm)  efficiency  ~  w/L) 2 (typical 2·10 -4 ) d w L  Interaction of photons

7. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram IV/7   Compton scattering: Assume electron as quasi-free. Cross-section: Klein-Nishina formula, at high energies approximately Atomic Compton cross-section: Interaction of photons

8. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram IV/8 scintillator d Application of Compton effect in medicine A Compton camera = an electronically collimated SPECT device  e-e- 1 st detector (Silicon) measures position of and E-deposition by Compton electron Photon trajectory known apart from azimutal angle  Ambiguity. 2 nd detector measures position of Compton scattered  Reconstruction: Gamma source lies in the crossing points of Compton cones Expect efficiency increase by factor 10-50 compared to collimated SPECT. (Concept currently under development) Interaction of photons

9. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram IV/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 !

10. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram IV/10 Standard PET geometry Pair production Positron annihilation Application of positron annihilation in medicine PET = Position Emission Tomography Use 18 F labeled radiotracer 18 F  18 O + e + e + + e -  2  (2 x 511 keV) 2  ’s are detected by 2 scintillators in coincidence. 18 F lies somewhere on this line of record! Need many lines of record + tomographic reconstruction.

11. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram IV/11 In summary: Interaction of photons (PDG)  : mass attenuation coefficient 1 MeV photo effect Rayleigh scattering (no energy loss !) Compton scattering pair production

12. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram IV/12 Reminder: basic electromagnetic interactions e + / e -  Ionisation  Bremsstrahlung  Photoelectric effect  Compton effect  Pair production E E E E E  dE/dx   

13. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram IV/13 Electromagnetic cascades  Electron shower in a cloud chamber with lead absorbers Consider only Bremsstrahlung and pair production. Assume: X 0 = pair Process continues until E(t)<E c After t = t max the dominating processes are ionization, Compton effect and photo effect  absorption. Electromagnetic Cascades (showers) Simple qualitative model

14. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram IV/14 Electromagnetic cascades Longitudinal shower development: Shower maximum at 95% containment Size of a calorimeter grows only logarithmically with E 0 Example: E 0 = 100 GeV in lead glass E c =11.8 MeV  t max  13, t 95%  23 X 0  2 cm, R M = 1.8·X 0  3.6 cm Longitudinal and transverse development scale with X 0, R M (C. Fabjan, T. Ludlam, CERN-EP/82-37) 6 GeV/c e - Transverse shower development: 95% of the shower cone is located in a cylinder with radius 2 R M Molière radius 46 cm 8 cm

15. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram IV/15 Energy resolution   Energy resolution of a calorimeter (intrinsic limit) total number of track segments holds also for hadron calorimeters Stochastic term More general: Constant term Inhomogenities Bad cell inter- calibration Non-linearities Noise term Electronic noise radioactivity pile up Quality factor ! Also spatial and angular resolution scale like 1 /  E Relative energy resolution of a calorimeter improves with E 0

16. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram IV/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, , K…). In analogy to X 0 a hadronic absorption length can be defined Interaction of charged particles multiplicity  ln(E) p t  0.35 GeV/c Excitation and finally breakup up nucleus  nucleus fragments + production of secondary particles. p,n, ,K,…

17. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram IV/17 I.2.1 Interaction of charged particles X0X0 a X 0, a [cm] Z For Z > 6: a > X 0 a and X 0 in cm

18. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram IV/18 Hadronic cascades Hadronic casacdes Hadronic + electromagnetic component Large energy fluctuations  limited energy resolution neutral pions  2   electromagnetic cascade charged pions, protons, kaons …. Breaking up of nuclei (binding energy), neutrons, neutrinos, soft  ’s muons ….  invisible energy example 100 GeV: n(  0 )  18 Various processes involved. Much more complex than electromagnetic cascades. (Grupen)

19. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram IV/19 Hadronic cascades Longitudinal shower development Lateral shower development The shower consists of core + halo. 95% containment in a cylinder of radius  I. For Iron: a = 9.4, b=39 a =16.7 cm E =100 GeV  t 95%  80 cm Hadronic showers are much longer and broader than electromagnetic ones ! (C. Fabjan, T. Ludlam, CERN-EP/82-37)

20. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram IV/20 Calorimetry Calorimeter types   Homogeneous calorimeters:   Sampling calorimeters:  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

21. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram IV/21 Homogeneous calorimeters Two main types: Scintillator crystals or “glass” blocks (Cherenkov radiation).  photons. Readout via photomultiplier, -diode/triode   Scintillators (crystals)   Cherenkov radiators Relative light yield: rel. to NaI(Tl) readout with PM (bialkali PC)

22. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram IV/22 Homogeneous calorimeters Examples OPAL Barrel + end-cap: lead glass + pre-sampler BGO E.M. Calorimeter in L3  10500 blocks (10 x 10 x 37 cm 3, 24.6 X 0 ), PM (barrel) or PT (end-cap) readout. Spatial resolution (intrinsic)  11 mm at 6 GeV (OPAL collab. NIM A 305 (1991) 275) 11000 crystals, 21.4 X 0, temperature monitoring + control system light output -1.55% / ºC  E /E 1 GeV spatial resolution < 2 mm (E >2 GeV) (L3 collab. NIM A 289 (1991) 53) Partly test beam results !

23. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram IV/23 Homogeneous calorimeters NA48: LKr Ionisation chamber (T = 120 K) no metal absorbers  quasi homogenous ! prototype full device (prel.) (V. Marzulli, NIM A 384 (1996) 237, M. Martini et al., VII International Conference on Calorimetry, Tuscon, 1997)  x,y  1 mm  t  230 ps Cu-Be ribbon electrode 97 run: reduced performance due to problems with blocking capacitors  lower driftfield: 1.5 kV/cm rather than 5 kV/cm

24. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram IV/24 Homogeneous calorimeters The NA48 LKr calorimeter prior to installation in the cryostat.

25. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram IV/25 Homogeneous calorimeters One half of the NA48 LKr calorimeter.

26. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram IV/26 Sampling calorimeters Absorber + detector separated  additional sampling fluctuations Detectable track segments MWPC, streamer tubes warm liquids TMP = tetramethylpentane, TMS = tetramethylsilane cryogenic noble gases: mainly LAr (Lxe, LKr) scintillators, scintillation fibres, silicon detectors

27. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram IV/27 Sampling calorimeters   ATLAS electromagnetic Calorimeter Accordion geometry absorbers immersed in Liquid Argon (RD3 / ATLAS) Liquid Argon (90K) + lead-steal absorbers (1-2 mm) + multilayer copper-polyimide readout boards  Ionization chamber. 1 GeV E-deposit  5 x10 6 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 / E 1/2

28. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram IV/28  CMS Hadron calorimter Cu absorber + scintillators Sampling calorimeters Scintillators fill slots and are read out via fibres by HPDs 2 x 18 wedges (barrel) + 2 x 18 wedges (endcap)  1500 T absorber Test beam resolution for single hadrons

29. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram IV/29 4 scintillating tiles of the CMS Hadron calorimeter Sampling calorimeters