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

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.

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-

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)

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

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 mm)  efficiency  ~  w/L) 2 (typical 2·10 -4 ) d w L  Interaction of photons

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

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 compared to collimated SPECT. (Concept currently under development) Interaction of photons

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 !

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.

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

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   

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

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

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

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,…

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

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)

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)

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

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)

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  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) 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 !

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

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

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

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

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 GeV (ATLAS TDR) Spatial and angular uniformity  0.5% Spatial resolution  5mm / E 1/2

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

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