u Basic principles n Interaction of charged particles

u Basic principles n Interaction of charged particles and photons n Electromagnetic cascades n Nuclear interactions n Hadronic cascades u Homogeneous calorimeters u Sampling calorimeters CERN Summer Student Lectures 2003 Particle Detectors Christian Joram

Calorimetry u Calorimetry: Energy measurement by total absorption, combined with spatial reconstruction. u Calorimetry is a “destructive” method u Detector response E u Calorimetry works both for charged (e and hadrons) and neutral particles (n, g) u Basic mechanism: formation of electromagnetic or hadronic showers. u Finally, the energy is converted into ionization or excitation of the matter. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram

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 m (>1000 Ge. V) For electrons: radiation length [g/cm 2] CERN Summer Student Lectures 2003 Particle Detectors Christian Joram e-

Interaction of charged particles (Leo) energy loss (radiative + ionization) of electrons and protons in copper Critical energy Ec For electrons one finds approximately: density effect of d. E/dx(ionisation) ! Ec(e-) in Fe(Z=26) = 22. 4 Me. V For muons Ec(m) in Fe(Z=26) 1 Te. V CERN Summer Student Lectures 2003 Particle Detectors Christian Joram

Interaction of photons In order to be detected, a photon has to create charged particles and/or transfer energy to charged particles u 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) CERN Summer Student Lectures 2003 Particle Detectors Christian Joram

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

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

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

Interaction of photons u Pair production Only possible in the Coulomb field of a nucleus (or an electron) if Cross-section (high energy approximation) independent of energy ! Energy sharing between e+ and e- becomes asymmetric at high energies. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram

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

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

Reminder: basic electromagnetic interactions g e+ / e Ionisation § Photoelectric effect d. E/dx § E E Bremsstrahlung § Compton effect d. E/dx § E E § Pair production E CERN Summer Student Lectures 2003 Particle Detectors Christian Joram 12

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

Electromagnetic cascades Longitudinal shower development: Shower maximum at 95% containment Size of a calorimeter grows only logarithmically with E 0 Transverse shower development: 95% of the shower cone is located in a cylinder with radius 2 RM Molière radius 6 Ge. V/c e- Longitudinal and transverse development scale with X 0, R M (C. Fabjan, T. Ludlam, CERN-EP/82 -37) 8 cm Example: E 0 = 100 Ge. V in lead glass Ec=11. 8 Me. V tmax 13, t 95% 23 X 0 2 cm, RM = 1. 8·X 0 3. 6 cm CERN Summer Student Lectures 2003 Particle Detectors Christian Joram 46 cm

Energy resolution u Energy resolution of a calorimeter (intrinsic limit) 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 E 0 More general: Stochastic term Constant term Noise term Inhomogenities Bad cell intercalibration Non-linearities Electronic noise radioactivity pile up Quality factor ! CERN Summer Student Lectures 2003 Particle Detectors Christian Joram

Interaction of charged particles Nuclear Interactions The interaction of energetic hadrons (charged or neutral) is determined by inelastic nuclear processes. p, n, p, K, … multiplicity ln(E) pt 0. 35 Ge. V/c Excitation and finally breakup up nucleus fragments + production of secondary particles. For high energies (>1 Ge. V) the cross-sections depend only little on the energy and on the type of the incident particle (p, p, K…). In analogy to X 0 a hadronic absorption length can be defined CERN Summer Student Lectures 2003 Particle Detectors Christian Joram

I. 2. 1 Interaction of charged particles For Z > 6: la > X 0 la and X 0 in cm X 0, la [cm] la X 0 Z CERN Summer Student Lectures 2003 Particle Detectors Christian Joram

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

Hadronic cascades • Longitudinal shower development For Iron: a = 9. 4, b=39 la =16. 7 cm (C. Fabjan, T. Ludlam, CERN-EP/82 -37) E =100 Ge. V t 95% 80 cm • Lateral shower development The shower consists of core + halo. 95% containment in a cylinder of radius l. I. Hadronic showers are much longer and broader than electromagnetic ones ! CERN Summer Student Lectures 2003 Particle Detectors Christian Joram

Calorimetry Calorimeter types u Homogeneous calorimeters: Detector = absorber good energy resolution limited spatial resolution (particularly in longitudinal direction) only used for electromagnetic calorimetry u Sampling calorimeters: 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

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

Homogeneous calorimeters Examples OPAL Barrel + end-cap: lead glass + pre-sampler (OPAL collab. NIM A 305 (1991) 275) 10500 blocks (10 x 37 cm 3, 24. 6 X 0), PM (barrel) or PT (end -cap) readout. Spatial resolution (intrinsic) 11 mm at 6 Ge. V BGO E. M. Calorimeter in L 3 (L 3 collab. NIM A 289 (1991) 53) 11000 crystals, 21. 4 X 0, temperature monitoring + control system light output -1. 55% / ºC E/E < 1% for E > 1 Ge. V spatial resolution < 2 mm (E >2 Ge. V) Partly test beam results ! CERN Summer Student Lectures 2003 Particle Detectors Christian Joram

Homogeneous calorimeters NA 48: LKr Ionisation chamber (T = 120 K) no metal absorbers quasi homogenous ! Cu-Be ribbon electrode prototype full device (prel. ) x, y 1 mm t 230 ps (V. Marzulli, NIM A 384 (1996) 237, M. Martini et al. , VII International Conference on Calorimetry, Tuscon, 1997) CERN Summer Student Lectures 2003 Particle Detectors 97 run: reduced performance due to problems with blocking capacitors lower driftfield: 1. 5 k. V/cm rather than 5 k. V/cm Christian Joram

Homogeneous calorimeters The NA 48 LKr calorimeter prior to installation in the cryostat. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram

Homogeneous calorimeters One half of the NA 48 LKr calorimeter. CERN Summer Student Lectures 2003 Particle Detectors Christian Joram

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

Sampling calorimeters u ATLAS electromagnetic Calorimeter Accordion geometry absorbers immersed in Liquid Argon (RD 3 / ATLAS) Liquid Argon (90 K) + lead-steal absorbers (1 -2 mm) + multilayer copper-polyimide readout boards Ionization chamber. 1 Ge. V E-deposit 5 x 106 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 Ge. V (ATLAS TDR) Spatial and angular uniformity 0. 5% Spatial resolution 5 mm / E 1/2 CERN Summer Student Lectures 2003 Particle Detectors Christian Joram

Sampling calorimeters u 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 CERN Summer Student Lectures 2003 Particle Detectors Christian Joram 28

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