Scintillation Light and PhotonDetectorsUrs Langenegger

(Paul Scherrer Institute)

Fall 2016

• Inorganic scintillators

• Organic scintillators

• Photon detectors

References (and sources)• Books

Title Author RemarksTeilchendetektoren 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 calorimetryTechniques for Nuclear and Particle W. Leo ‘hands-on’

Physics ExperimentsReview of Particle Physics PDG 2016 chapters 30, 31, and 32The PMT Handbook Hamatsu Co. ‘Photon is our Business’Particle detector briefbook R. Bock and A. Vasilescu online version is gone

• Review articles and other resourcesTitle Author ReferenceAdvances in Hadron Calorimetry Wigmans AR, 41, 133 (1991)Calorimetry for HEP Fabjan and Gianotti RMP, 75, 001243 (2001)Advances in Calorimetry Brau, Jaros, and Ma AR, 60, 615 (2010)Extensive Air Showers and Hadronic Interactions . . . Engel, Heck, and Pierog AR,61, 467Photodetection in the LHC experiments Joram NIM, A695, 13 (2012)Status and New Ideas Regarding Liquid Argon Detectors Marchionni AR, 63, 269 (2013)Photodetectors in Particle Physics Experiments Križan and Korpar AR, 63, 329 (2013)

RMP = Reviews of Modern PhysicsAR = Annual Reviews of Nucl. and Part. SciencesNIM = Nuclear Instruments and Methods in Physics Research

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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

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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

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Scintillators• 2 categories and 3 scintillation mechanisms

Category Form Scintillation mechanisms

inorganic crystal excitons in latticeliquid 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!

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Organic scintillation light generation

T1

T0

10−8sec

10−4sec

S0

S1

S2

T210−14

Singlet states

Triplet states

sec

• 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. S1 through ‘internal degradation’. T0 through collisional de-excitation

with other moleculesT0 + T0 → S1 + S0+ phonons

→ Franck-Condon effect

⇒ Two components. different wavelength and decay time• fluorescence: fast (allowed transition)• phosphorescence: slow (forbidden transition)

• Excitation by. charged particles. (radiationless) excitation by base material

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Organic scintillators in reality• Application: Liquid or embedded into plastic carrier (< 10%)

. solvent: xylene, toluene, benzene

. scintillators: p-terphenyle (C18H14), PBD (C20H14N2O), PPO (C15H11NO)

. wavelength shifter: POPOP (C24H16N2O2), . . .(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

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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 (1% of air), ‘simple’ to purify, but: 39A with τ1/2 = 269 y

. Krypton: expensive, smaller radiation length (used in NA48 calorimeter)

. Xenon: very expensive (used in many low-background experiments)

⇒ application in homogeneous and segmented calorimeters

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Crystal light generation• Crystals (or glass)

. high density, 4− 8 g/ cm3

. high valence (Z)high sensitivity to e.m. interactionsand compact calorimeters (see lecture 2)

• 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

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Inorganic crystals: material• NaI

. very popular scintillator

. light spectrum in range of bi-alkali photomultiplier tubes

. hygroscopic→ careful handling and application

• Bismuth germanate: BGO = Bi4Ge3O12

. high efficiency for photoelectric effect (large Z)

. expensive

• CsI. with or without Thallium doping. spectrum useful for Si diodes

• BaF2

. fastest crystal: < 1 ns (+630 ns)

• PbWO4 = lead tungstate. very fast decay constant. small radiation damage

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Comparison of scintillators• Different application range

. energy scale: photons from Higgs boson decay or π0 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 PbWO4

density [g/cm3] 1.03 3.7 4.5 4.5 7.1 8.3# photons/MeV 10000 40000 2000 50000 8000 200decay time [ns] 2-5 250 10f,35s 1000 300 5f, 15sradiation length [cm] 2.6 1.9 1.9 1.1 0.9Molière radius [cm] 4.5 3.8 3.8 2.4 2.2emission maximum [nm] ca.400 410 305,480 565 410 440radiation hardness [Gy] - 1 103 10 1 105

f = fast component, s = slow component

Radiation length: longitudinal shower profile scaling variableMolière radius: transverse shower profile scaling variable

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Photon Detection

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Photomultiplier tubes (PMT)photo−cathode

voltage divider

anode

window

focus dynode

−HV

(vacuum)

ATLAS HCAL

• Primary electron. photo-electric effect

• Photo-cathodes. transmission. reflection

• Amplificationpotential difference 1-3 kVsecondary electrodes g ≈ 3− 50total gain gtot ≈ 106

• Characteristics

• PMT sensitive to B-fields. µ-metal shielding

75% Ni, 15% Fe, with Cu and Mo→ high magnetic permeability µ

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Photo detectors characteristics• Response characterization

. quantum efficiency εQ(λ) =np.e.nγ

number of photo electrons per incoming photon (εQ > 100% possible in Si)

. collection efficiency εC (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− 3µs)

• Energy resolution. statistical term from the number of photo electrons. systematic effects arising from the amplification

• Time resolution. area of photo-cathode. propagation time

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Vacuum photodetectorsCMS ECAL (endcap)

• Micro-channel plates. (lead) glass structure, ca 1-2mm depth. channels, ca d = 10− 100µm• continuous dynode• gain: 104 − 107

. characteristics• B field tolerance

1 T (axial), 0.1 T (random orientation)• relatively long dead time per channel

. also in ‘chevron’ configuration with two MCP combined:

• Vacuum phototriode. single-stage PMT• mesh anode• VA ≈ 800 V, VD ≈ 600 V• gain: ≈ 10

. B-field tolerance• 10% signal reduction (4 T)

. radiation hard

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Solid state detectorsCMS ECAL

[APD S8148 Hamamatsu (CMS)]

• Direct light measurement with photodiodes. electron-hole generation by incoming photon. analogous to semiconductors in tracking

detectors (and solar cells). P-N junction, reverse biased. additional bias voltage: higher efficiency, less noise. often PIN (p-type, intrinsic, n-type) diodes→ ionisation charge produces current

• Variations, e.g.. avalanche PD (APD): very high voltage

induces exponential cascade. silion photomultiplier (SiPM)

geiger-mode APD: add quench resistorarray of small-scale APDs

. gaseous PM: as in tracking detectors,e.g. MicroMegas or GEM

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Hybrid photo-detectorsCMS HCALLHCb RICH

cross talk sources (LHCb RICH)

• Combine (large-area) PMT with (high-resolution) silicon detectors. large potential difference ca 20 kV. quantum efficiency of ca. 30%. impact ionization in Si bulk. amplification ca. 5× 103

• Mapping onto silicon detector. ‘proximity focusing’ (diameter ≈ 5mm). ‘cross focusing’ (diameter ≈ 75mm, demagnify by ≈ 5)

• Very good resolution possible

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Summary• Technical aspects

Type λ[ nm] εQεC τ [ ns] Gain HV [V] Price [$]

PMT 100− 1100 0.2 1 103 − 107 2× 103 1000MCP 100− 600 0.05 0.2 103 − 107 2× 103 1000HPD 100− 800 0.3 7 103 − 104 2× 104 600APD 300− 1500 0.7 1 10− 108 400− 1400 100GPM 100− 500 0.2 0.1 103 − 106 1× 103 10

(comparison per readout channel)

• Photon detectors in use at the LHC experiments

Detector CMS ATLAS LHCb ALICEECAL APD (barrel) -- PMT APD

VPT (endcap) -- -- --HCAL HPD (HCAL) PMT PMT --RICH/PID -- -- HPD (3m2) CsI on MWPC(11m2)

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