Scintillation Light and Photon Detectors

Transcription

1 Scintillation Light and Photon Detectors Urs Langenegger (Paul Scherrer Institute) Fall 2014 Inorganic scintillators Organic scintillators Photon detectors

2 References Books Title Author Remarks Teilchendetektoren C. Grupen pedagogical, out of print (in German) Detectors for Particle Radiation K. Kleinknecht quite compact (also in German) Experimental Techniques in HEP T. Ferbel (ed) chapters on calorimetry Techniques for Nuclear and Particle W. Leo hands-on Physics Experiments Review of Particle Physics PDG 2012 chapters 30, 31, and 32 The PMT Handbook Hamatsu Co. Photon is our Business Review articles and other resources Title Author Reference Calorimetry for HEP Fabjan and Gianotti RMP, 75, (2001) Advances in Hadron Calorimetry Wigmans AR, 41, 133 (1991) Particle detector briefbook Bock Photodetection in the LHC experiments Joram NIM, A695, 13 (2012) RMP = Reviews of Modern Physics AR = Annual Reviews of Nucl. and Part. Sciences NIM = Nuclear Instruments and Methods in Physics Research 2

3 Scintillation/photon detectors Photon production in the scintillator Light guide Light readout production of primary photo-electron or electron-hole pair amplification of signal measurement of secondary electrons 3

4 Scintillation detectors II Scintillation light recombination (not Cherenkov light) luminescence: fluorescence (10 8 s), resp phosphorescence (delayed) Historic application in particle physics 1903 Crookes: ZnS screen in darkened room, observed with microscope α-particle detection 1944 Curran and Baker: combination with photomultiplier tubes Very diverse applications calorimetry energy measurement time-of-flight particle identification fibers tracking counters trigger or veto modern neutrino physics: KamLAND (ν oscillations), Borexino (solar ν) very low energy threshold self shielding 4

5 Scintillators 2 categories and 3 scintillation mechanisms Category Form Scintillation mechanisms inorganic crystal excitons in lattice liquid noble gases molecular formation and de-excitation organic plastic molecular de-excitation Peculiarities: short rise time possible ( 100 ps to µs) light signal proportional to energy deposition possibility of particle identification with pulse shape Desired characteristics: high efficiency for conversion of excitation energy into light light spectrum in useful range for readout short decay time of scintillation light self-transparency to own scintillation light! 5

6 Organic scintillation light generation Light generation within one molecule aromatic hydro-carbons, benzene transition of free valence electrons in π orbitals (= non-localized electrons in ring) fine structure because of vibrational modes S 1 through internal degradation T 0 through collisional de-excitation with other molecules T 0 + T 0 S 1 + S 0 + phonons Two components different wavelength and decay time fluorescence: fast (allowed transition) phosphorescence: slow (forbidden transition) Excitation by charged particles photon from the de-excitation of other molecules of base material Singlet states S 2 Triplet states T sec S sec T sec T S 0 0 6

7 Organic scintillators Application: Liquid or embedded into plastic carrier material solvent: xylene, toluene, benzene scintillators: p-terphenyle (C 18 H 14 ), PBD (C 20 H 14 N 2 O), PPO (C 15 H 11 NO) wavelength shifter: POPOP (C 24 H 16 N 2 O 2 ),... (NB: WLS = wave length shifter) Wavelength shifting through other solvent component(s) fluors (has nothing to do with the element fluor, a priori) transform primary UV spectrum into blue spectrum longer decay time through WLS Complications surface scratches (perspiration) radiation damage, temperature 7

8 Liquid noble gases light generation Energy deposition leads to scintillation: time scale 10 ns and 130 < λ < 180 nm ionisation: 20 ev/pair in calorimeters the ionisation charge is measured (dominantly) Options Argon: cheap, simple to purify Krypton: expensive, smaller radiation length Xenon: very expensive application in homogeneous and segmented calorimeters 8

9 Crystal light generation Crystals (or glass) high density, 4 8 g/ cm 3 high valence (Z) Scintillation doping or intrinsic Scintillation mechanism e excitation into conduction band e excitation into exciton band exciton = bound state of e and hole Doping with impurity elements A (for wavelength shifting) (1) h + A A + (2) e + A + A + γ (3) simultaneously (1) and (2) through exciton capture traps = losses through radiation-free transitions 9

10 Inorganic crystals: material NaI very popular scintillator light spectrum in range of bi-alkali photomultiplier tubes hygroscopic careful handling and application Bismuth germanate: BGO = Bi 4 Ge 3 O 12 high efficiency for photoelectric effect (large Z) expensive CsI with or without Thallium doping spectrum useful for Si diodes BaF 2 fastest crystal: < 1 ns (+630 ns) PbWO 4 = lead tungstate very fast decay constant small radiation damage 10

11 Comparison of scintillators Different application range energy scale: photons from Higgs boson decay or π 0 decays? radiation hardness: luminosity? decay time: bunch structure of accelerator, interaction rate readout technology: magnetic field? strength? Comparison of organic and inorganic scintillators numbers are approximate guidance values Material Plastic NaI(Tl) CsI CsI(Tl) BGO PbWO 4 density [g/cm 3 ] # photons/mev decay time [ns] f,35 s f, 15 s radiation length [cm] Molière radius [cm] emission maximum [nm] ca , radiation hardness [Gy] f = fast component, s = slow component Radiation length: longitudinal shower profile scaling variable Molière radius: transverse shower profile scaling variable 11

12 Photon Detection 12

13 Photomultiplier tubes (PMT) Primary electron photo-electric effect Photo-cathodes transmission reflection Amplification potential difference 1-3 kv secondary electrodes g 3 50 total gain g tot 10 6 Window materials photo cathode focus window HV ATLAS HCAL voltage divider dynode (vacuum) PMT sensitive to B-fields µ-metal shielding 75% Ni, 15% Fe, with Cu and Mo high magnetic permeability µ anode 13

14 Photo detectors characteristics Response characterization quantum efficiency ε Q (λ) = n p.e. n γ number of photo electrons per incoming photon (ε Q > 100% possible in Si) collection efficiency ε C (acceptance w/o photo-electron generation) gain G: number of electrons collected for each photo-electron generated Systematic issues dark current: signal without incoming photons (from thermal activity) afterpulses: by positive (rest gas) ions onto cathode ( t 1 3 µs) Energy resolution statistical term from the number of photo electrons systematic effects arising form the amplification Time resolution area of photo-cathode propagation time 14

15 PMT Variations CMS ECAL(endcap) Micro-channel plates (lead) Glass structure, ca 1-2 mm depth channels, ca d = µm continuous dynode gain: characteristics B field tolerance 1 T (axial), 0.1 T (random orientation) relatively long dead time per channel widely used for x-ray imaging, not (yet!?) in HEP Vacuum phototriode single-stage PMT mesh anode V A 800 V, V D 600 V gain: 10 B-field tolerance 10% signal reduction (4 T) radiation hard 15

16 Hybrid photo-detectors CMS HCAL LHCb RICH combination of PMT with silicon detectors large potential difference ca 20 kv quantum efficiency of ca. 30% impact ionization in Si bulk amplification ca Mapping onto silicon detector proximity focusing (diameter 5 mm) cross focusing (diameter 75 mm) Very good resolution possible 16

17 ... and other possibilities CMS ECAL Direct light measurement with photodiodes electron-hole generation by incoming photon analogous to semiconductors in tracking detectors (and solar cells) Variations, e.g. avalanche PD (APD): very high voltage induces exponential cascade gaseous PM: as in tracking detectors, e.g. MicroMegas or GEM Type λ[ nm] ε Q ε C τ[ ns] Gain HV [V] Price [$] PMT MCP HPD APD GPM (comparison per readout channel) 17