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Photon Detection Focus on particle/astroparticle physics Christian Joram (CERN / PH)

Photon Detection Focus on particle/astroparticle physics Christian Joram (CERN / PH) 1 st EIROforum School on Instrumentation CERN 11-15 May 2009. http://www.trustedlog.com/wp-content/uploads/2007/06/northern-lights-f.jpg.

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Photon Detection Focus on particle/astroparticle physics Christian Joram (CERN / PH)

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  1. Photon Detection Focus on particle/astroparticle physics Christian Joram (CERN / PH) 1st EIROforum School on Instrumentation CERN 11-15 May 2009 http://www.trustedlog.com/wp-content/uploads/2007/06/northern-lights-f.jpg The lecture will introduce to the basic principles and design choices of photodetectors for the visible and UV range of the electromagnetic spectrum. We will also review the key factors which driving their performance. We will discuss photodetectors based on vacuum (PMT, MA-PMT), gaseous (MWPC, GEM) and solid media (photodiode, APD, G-APD), as well as hybrid devices (HPD, X-HPD).

  2. Outline extra slidenot shown • Basics of photon detection • Photoeffect • Solids, liquids, gases • Internal / external P.E. • Requirements on photodetectors • Sensitivity, Linearity, Time response (jitter), Noise … • Classes of photodetectors • Family tree • Principle, performance and typical applications of … • PMT, MAPMT • PIN / APD / G-APD • Hybrid devices • Gaseous photodetectors (CsI, TEA, TMAE)

  3. Basics of photon detection Purpose: • Convert light into detectable electronic signal • (we are not covering photographic emulsions!) Principle: • Use photoelectric effect to ‘convert’ photons (g) to photoelectrons (pe) • Details depend on the type of the photosensitive material (see below). • Photon detection involves often materials like K, Na, Rb, Cs (alkali metals) . They have the smallest electronegativity highest tendency to release electrons.

  4. Basics of photon detection Most photodetectors make use of solid or gaseous photosensitive materials. Photoeffect can also be observed from liquid materials (e.g. liquid noble gases). Solid materials (usually semiconductors) semiconductor  vacuum e- Multi-step process: absorbed g’s impart energy to electrons (e) in the material; If Eg > Eg, electrons are lifted to conductance band.  In a Si-photodiode, these electrons can create a photocurrent.  Photon detected by Internal Photoeffect. EA = electron affinity Eg = band gap Eg (Photonis) h However, if the detection method requires extraction of the electron, 2 more steps must be accomplished: energized e’s diffuse through the material, losing part of their energy (~random walk) due to electron-phonon scattering. DE ~ 0.05 eV per collision. Free path between 2 collisions lf ~ 2.5 - 5 nm  escape depth le ~ some tens of nm. only e’s reaching the surface with sufficient excess energy escape from it  External Photoeffect

  5. 0.4 Basics of photon detection Light absorption in photocathode Opaque photocathode g lA =1/a Red light (l 600 nm) a 1.5 · 105 cm-1 lA 60 nm Blue light (l 400 nm) a 4·105 cm-1 lA 25 nm substrate e- PC Semitransparent photocathode Blue light is stronger absorped than red light ! g Detector window PC  Make semitransparent photocathode just as thick as necessary! e-

  6. Frequently used photosensitive materials / photocathodes begin of arrow indicates threshold Si (1100 nm) Ultra Violet (UV) Visible Infra Red(IR) GaAs TMAE, CsI Bialkali K2CsSb Multialkali NaKCsSb TEA 12.3 4.9 3.1 2.24 1.76 1.45 E [eV] 100 250 400 550 700 850 l [nm] Remember : E[eV]  1239/l[nm] borosilicate glass NaF, MgF2, LiF, CaF2 normal window glass quartz Almost all photosensitive materials are very reactive (alkali metals). Operation only in vacuum or extremely clean gas. Exception: Silicon, CsI. Cut-off limits of window materials

  7. Basics of photon detection Requirements on photodetectors • High sensitivity, usually expressed as: • quantum efficiencyor radiant sensitivityS(mA/W), with QE can be >100% (for high energetic photons) ! • Good Linearity: Output signal  light intensity, over a large dynamic range (critical e.g. in calorimetry (energy measurment). • Fast Time response: Signal is produced instantaneously (within ns), low jitter (<ns), no afterpulses • Low intrinsic noise. A noise-free detector doesn’t exist. Thermally created photoelectrons represent the lower limit for the noise rate AoT2exp(-eWph /kT). In many detector types, noise is dominated by other sources.

  8. (External) QE of typical semitransparent photo-cathodes Photon energy Eg (eV) 12.3 3.1 1.76 1.13 GaAsP GaAs Ag-O-Cs CsTe (solar blind) Multialkali Bialkali (Hamamatsu) Bialkali:SbKCs, SbRbCsMultialkali:SbNa2KCs (alkali metals have low work function)

  9. 50 40 30 Quantum Efficiency [%] 20 10 0 200 300 400 500 600 700 Wavelength [nm] Latest generation of high performance photocathodes QE Comparison of semitransparent bialkali QE Example Data for UBA : R7600-200 SBA : R7600-100 STD : R7600 x1.6 UBA:43% Ultra Bialkali available only for small metal chanel dynode tubes SBA:35% x1.3 Super Bialkali available for a couple of standard tubes up to 5”. STD:26%

  10. Family tree of photodetectors Photodetectors Gas External photoeffect Vacuum External photoeffect Solid state Internal photoeffect TMAE MWPC TEA + GEM CsI … • Avalanche gain • Process • Dynodes  PMT • Continuous dynode • Channeltron, MCP • Multi-Anode devices Other gain process = Hybrid tubes SiliconLuminescent anodes HPD SMART/Quasar HAPD X-HPD G-APD-HPD PIN-diode APD G-APD (SiPM) CMOS CCD Doesn’t exist yet, but was proposed by G. Barbarino et al., NIM A 594 (2008) 326–331 10

  11. (Hamamatsu) pe (http://micro.magnet.fsu.edu) Photo-multiplier tubes (PMT’s) Basic principle: • Photo-emission from photo-cathode • Secondary emission (SE) from N dynodes: • dynode gain g3-50 (function ofincoming electron energy E); • total gain M: • Example: • 10 dynodes with g=4 • M = 410  106 http://micro.magnet.fsu.edu/

  12. Gain fluctuations of PMT’s • Mainly determined by the fluctuations of the number m(d) of secondary e’s emitted from the dynodes; • Poisson distribution: • Standard deviation: •  fluctuations dominated by 1st dynode gain; GaP(Cs) dynodes EA<0 SE coefficient d (Photonis) e energy CuBe dynodes EA>0 1 pe 1 pe 2 pe SE coefficient d Counts Counts 3 pe Pedestal noise (Photonis) (Photonis) (H. Houtermanns, NIM 112 (1973) 121) e energy Pulse height Pulse height

  13. Dynode configurations of PMT’s • Traditional • Position-sensitive Mesh (Photonis) (Hamamatsu) Venetian blind Box Metal-channel(fine-machiningtechniques) (Photonis) (Hamamatsu) Linear focussing Circular cage • “Fast” PMT’s require well-designed input electron optics to limit (e) chromatic and geometric aberrations  transit time spread < 200 ps; • Compact construction (short distances between dynodes) keeps the overall transit time small (10 – 100 ns). • PMT’s are in general very sensitive to magnetic fields, even to earth field (30-60 mT). Magnetic shielding required.

  14. 50 mm (Hamamatsu) Multi-anode and flat-panel PMT’s • Multi-anode (Hamamatsu H7546) • Up to 8  8 channels (2  2 mm2 each); • Size: 28  28 mm2; • Active area 18.1  18.1 mm2 (41%); • Bialkali PC: QE  25 - 45% @ lmax = 400 nm; • Gain  3 105; • Gain uniformity typ. 1 : 2.5; • Cross-talk typ. 2% • Flat-panel (Hamamatsu H8500): • 8 x 8 channels (5.8 x 5.8 mm2 each) • Excellent surface coverage (89%) (Hamamatsu) Cherenkov rings from 3 GeV/c p– through aerogel (T. Matsumoto et al., NIMA 521 (2004) 367)

  15. Micro Channel Plate (MCP) based PMTs photon Window/Faceplate Photocathode • Typical secondary yield is 2 • For 40:1 L:D there are typically 10 strikes (210 ~ 103 gain single plate) • Pore sizes range from <10 to 25 mm. • Small distances  small TTS and good immunity to B-field DV ~ 200V Photoelectron Dual MCP DV~ 2000V Gain ~ 106 MCP-OUT Pulse DV ~ 200V Anode Gain stage and detection are decoupled  lots of potential and freedom for MA-PMTs: Anode can be easily segmented in application specific way. Ceramic Insulators Dual MCP Available with up to 1024 (32 x 32) channels (1.6 x 1.6 mm2) Anode & Pins 50 mm

  16. Light absorption in Silicon At long l, temperature effects dominate (http://pdg.ge.infn.it/~deg/ccd.html)

  17. p+ i(n) n+ e h g g Avalanche Solid-state photon detectors (Si) - Photodiodes: • P(I)N type • p layer very thin (<1 mm), as visible light is rapidly absorbed by silicon (see next slide); • High QE (80% @ l  700nm); • No gain: cannot be used for single photon detection; Avalanche photodiode: • High reverse bias voltage: typ. 100-200 V  due to doping profile, high internal field (>105 V/cm)leads to avalanche multiplication; • High gain: typ. 100-1000; • Rel. high gain fluctuations (excess noise) (http://micro.magnet.fsu.edu)

  18. Solid state … Avalanche Photodiode (APD) Traditional ‘Reach-through’ structure (long wavelengths) Reverse structure (short wavelength) Electric field strength Electric field strength Used in CMS ECAL;

  19. Solid-state … Geiger mode Avalanche Photodiode (G-APD) How to obtain higher gain (= single photon detection) without suffering from excessive noise ? Operate APD cell in Geiger mode (= full discharge), however with a (passive) quenching. Photon conversion + avalanche short-circuits the diode. J. Haba, RICH2007 J. Haba, RICH2007 J. Haba, RICH2007

  20. Solid-state … Geiger mode Avalanche Photodiode (G-APD) Imax~(VBIAS-VBD)/RQ t = RSCD (sub – ns) Gain = Q / e = Imax·t / e = (VBIAS-VBD)CD / e G ~ 105 -106 at reasonable bias voltage (<100 V) ID • t = RQCD • 10s of ns Sample of 3 G-APDs Sample of 3 G-APDs J. Haba, RICH2007 J. Haba, RICH2007

  21. g -Vbias   GM-APD Multi pixel G-APD, called G-APD, MPPC, SiPM, … 1mm 100 – several 1000 pix / mm2 GM-APD Only part of surface is photosensitive! Bias bus Quench resistor Sizes up to 5×5 mm2 now standard. Quench resistor 1 g g 2 g 20 x 20 pix 3 g Q Q Musienko @PD07 2Q Quasi-analog detector allows photon counting with a clearly quantized signal

  22. Multi pixel G-APD = G-APD, MPPC, SiPM, … ~10 producers are now in the market. Expect improvement in technology and performance. You cannot get "something for nothing” • G-APD show dark noise rate in the O(100 kHz – MHz / mm2) range. • The gain is temperature dependent O(10% /°K) • The signal linearity is limited • The price is (still too) high Uozumi@VCI2007 Hamamatsu catalog

  23. Hybrid Photon Detectors (HPD’s) Basic principle: • Combination of vacuum photon detectors and solid-state technology; • Input: collection lens, (active) optical window, photo-cathode; • Gain: achieved in one step by energy dissipation of keVpe’s in solid-state detector anode; this results in low gain fluctuations; • Output: direct electronic signal; • Encapsulation of Si-sensor in the tube implies: • compatibility with high vacuum technology (low outgassing, high T° bake-out cycles); • internal (for speed and fine segmentation) or external connectivity to read-out electronics; • heat dissipation issues; e-h WSi = 3.6 eV Energy loss eVth in (thin) ohmic contact DV = 20 kV  M ~ 5000 F = Fano factor FSi ~ 0.1

  24. Hybrid Photon Detectors (HPD’s) 10-inches (25.4 cm) 1 p.e. pulse height signals of 1 Si pad HVHPD = 26 kV 2 p.e. 3 p.e. Pedestal cut 4 p.e. 5 p.e. pulse height (ADC counts) 10-inch prototype HPD (CERN) for Air Shower Telescope CLUE. Photon counting. Continuum due to electron back scattering.

  25. Pixel-HPD’s for LHCb RICH detectors 50mm 72mm  active • Cross-focused electron optics • pixel array sensor bump-bonded to binary electronic chip, developed at CERN • 8192 pixels of 50 × 400 mm. • specially developed high T° bump-bonding; • Flip-chip assembly, tube encapsulation (multi-alkali PC) performed in industry (VTT, Photonis/DEP) T. Gys, NIM A 567 (2006) 176-179 Pixel-HPD anode During commissioning: illumination of 144 tubes by beamer. In total : 484 tubes.

  26. T ~ 0.4 QE QE X-HPD project (CERN / Photonis) ) Concept of a large spherical tube with central spacial scintillation crystal (X-tal) anode = modern implementation of Philips Smart / Lake Baikal concept. • Accelerate photoelectron hits scintillator and generates scintillation light: ~ 25 photons/keV. • Detect scint light with small external photodetector (e.g. PMT, G-APD). 1 photon = 30-50 detected photoelectrons. • Radial electric field • negligible transit time spread • ~100% collection efficiency • no magnetic shielding required • Large viewing angle (dW ~ 3p) • Possibility of anode segmentation • imaging capability (limited!) • Sensitivity gain through • ‘Double-cathode effect’  QEmax ~ 50% observed. A. Braem et al., NIM A 602, (2009), 193-196 26

  27. X-HPD project (CERN / Photonis) X-HPD (PC120) - 20 kV - 0 kV

  28. Gaseous Photodetectors e.g. CH4 + TEA • Principle: (A) Ionize photosensitive molecules, admixed to the counter gas (TMAE, TEA); or (B) release photoelectron from a solid photocathode (CsI, bialkali...); Then use free p.e. to trigger a Townsend avalanche  Gain TEA, TMAE, CsI work only in deep UV region. Bialkali works in visible domain, however requires VERY clean gases. Long term operation in a real detector not yet demonstrated. Thin CsI coating on cathode pads • Usual issues: How to achieve high gain (105) ? How to control ion feedback and light emisson from avalanche? How to purify gas and keep it clean? How to control aging ?

  29. HV photocathode CsI on readout pads Gaeous photodetectors: A few implementations... Built, just starting up: HBD (RICH) of PHENIX. Proven technology: Cherenkov detectors in ALICE, HADES, COMPASS, J-LAB…. Many m2 of CsI photocathodes • R&D: • Thick GEM structures • Visible PC (bialkali) • Sealed gaseous devices Sealed gaseous photodetector with bialkali PC. (Weizmann Inst., Israel) CsI on multi-GEM structure

  30. Literature / Acknowledements Non-exhaustive list: • www.hamamatsu.com • www.photonis.com: “Photomultiplier tubes, principles and applications” • (Photonis stops PMT activity (summer 2009), however keeps night vision and HPD). • A.H. Sommer, ”Photoemissive materials”, J. Wiley & Sons (1968); • H. Bruining, “Physics and Applications of Secondary Electron Emission”, Pergamon Press (1954); • I. P. Csorba, “Image Tubes”, Sams (1985); • Proceedings of the ‘Beaune Conferences’ (1996-1999-2002-2005-2008) on “New Developments in Photo-detection”, published in NIM A 387, A 442, A 504, A567, A xxx Thanks to T. Gys and J. Haba for some plots and drawings.

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