1. C. Joram CERN / PH EIROforum School on Instrumentation ESI 2009 Photon Detection 1 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 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. C. Joram CERN / PH EIROforum School on Instrumentation ESI 2009 Photon Detection 2 Outline 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) extra slide not shown

3. C. Joram CERN / PH EIROforum School on Instrumentation ESI 2009 Photon Detection 3 Purpose:  Convert light into detectable electronic signal  (we are not covering photographic emulsions!) Principle: Use photoelectric effect to ‘convert’ photons (  ) 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. Basics of photon detection

4. C. Joram CERN / PH EIROforum School on Instrumentation ESI 2009 Photon Detection 4 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) Multi-step process: 1.absorbed  ’s impart energy to electrons (e) in the material; If E  > E g, electrons are lifted to conductance band.  In a Si-photodiode, these electrons can create a photocurrent.  Photon detected by Internal Photoeffect. E A = electron affinity E g = band gap However, if the detection method requires extraction of the electron, 2 more steps must be accomplished: 2.energized e’s diffuse through the material, losing part of their energy (~random walk) due to electron-phonon scattering.  E ~ 0.05 eV per collision. Free path between 2 collisions f ~ 2.5 - 5 nm  escape depth e ~ some tens of nm. 3.only e’s reaching the surface with sufficient excess energy escape from it  External Photoeffect Basics of photon detection (Photonis) EE h e-e- semiconductor  vacuum

5. C. Joram CERN / PH EIROforum School on Instrumentation ESI 2009 Photon Detection 5 e-e-  Detector window PC  e-e- Semitransparent photocathode Opaque photocathode PC substrate A = 1/  Red light (  600 nm)   1.5 · 10 5 cm -1   60 nm Blue light (  400 nm)   4·10 5 cm -1   25 nm 0.4 Blue light is stronger absorped than red light ! Light absorption in photocathode  Make semitransparent photocathode just as thick as necessary! Basics of photon detection

6. C. Joram CERN / PH EIROforum School on Instrumentation ESI 2009 Photon Detection 6 Frequently used photosensitive materials / photocathodes 100 250 400 550 700 850 [nm] 12.3 4.9 3.1 2.24 1.76 1.45 E [eV] VisibleUltra Violet (UV) Multialkali NaKCsSb Bialkali K 2 CsSb GaAs TEA TMAE, CsI Infra Red (IR) Remember : E[eV]  1239/ [nm] NaF, MgF 2, LiF, CaF 2 Si (1100 nm) normal window glass borosilicate glassquartz Cut-off limits of window materials begin of arrow indicates threshold Almost all photosensitive materials are very reactive (alkali metals). Operation only in vacuum or extremely clean gas. Exception: Silicon, CsI.

7. C. Joram CERN / PH EIROforum School on Instrumentation ESI 2009 Photon Detection 7 Requirements on photodetectors High sensitivity, usually expressed as: quantum efficiency or radiant sensitivity S(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  A o T 2 exp(-eW ph /kT). In many detector types, noise is dominated by other sources. Basics of photon detection

8. C. Joram CERN / PH EIROforum School on Instrumentation ESI 2009 Photon Detection 8 Bialkali: SbKCs, SbRbCs Multialkali: SbNa 2 KCs (alkali metals have low work function) (Hamamatsu) (External) QE of typical semitransparent photo-cathodes GaAsP GaAs CsTe (solar blind) Multialkali Bialkali Ag-O-Cs Photon energy E  (eV) 12.3 3.1 1.76 1.13

9. C. Joram CERN / PH EIROforum School on Instrumentation ESI 2009 Photon Detection 9 0 10 20 30 40 50 200300400500600700 Wavelength [nm] Quantum Efficiency [%] Latest generation of high performance photocathodes QE Comparison of semitransparent bialkali QE Example Data for UBA : R7600-200 SBA : R7600-100 STD : R7600 UBA:43% SBA:35% STD:26% x1.3 x1.6 Ultra Bialkali available only for small metal chanel dynode tubes Super Bialkali available for a couple of standard tubes up to 5”.

10. C. Joram CERN / PH EIROforum School on Instrumentation ESI 2009 Photon Detection 10 Photodetectors Vacuum External photoeffect Gas External photoeffect Solid state Internal photoeffect Avalanche gain Process Dynodes  PMT Continuous dynode  Channeltron, MCP Multi-Anode devices Other gain process = Hybrid tubes SiliconLuminescent anodes HPD SMART/Quasar HAPDX-HPD G-APD-HPD TMAE MWPC TEA + GEM CsI … PIN-diode APD G-APD (SiPM) CMOS CCD 10 Family tree of photodetectors Doesn’t exist yet, but was proposed by G. Barbarino et al., NIM A 594 (2008) 326–331

11. C. Joram CERN / PH EIROforum School on Instrumentation ESI 2009 Photon Detection 11 Basic principle: Photo-emission from photo-cathode  Secondary emission (SE) from N dynodes: - dynode gain g  3-50 (function of incoming electron energy E); - total gain M:  Example: - 10 dynodes with g=4 - M = 4 10  10 6 Photo-multiplier tubes (PMT’s) http://micro.magnet.fsu.edu/ pe (http://micro.magnet.fsu.edu) (Hamamatsu)

12. C. Joram CERN / PH EIROforum School on Instrumentation ESI 2009 Photon Detection 12 Mainly determined by the fluctuations of the number m(  ) of secondary e’s emitted from the dynodes; Poisson distribution: Standard deviation:  fluctuations dominated by 1 st dynode gain; Pulse height Counts (H. Houtermanns, NIM 112 (1973) 121) Gain fluctuations of PMT’s (Photonis) 1 pe Pedestal noise CuBe dynodes E A >0 GaP(Cs) dynodes E A <0 SE coefficient  e energy (Photonis) Pulse height Counts SE coefficient  e energy 1 pe 2 pe 3 pe (Photonis)

13. C. Joram CERN / PH EIROforum School on Instrumentation ESI 2009 Photon Detection 13 Position-sensitive (Photonis) Traditional “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. Dynode configurations of PMT’s (Photonis) (Hamamatsu) Venetian blindBox Linear focussingCircular cage Mesh Metal-channel (fine-machining techniques)

14. C. Joram CERN / PH EIROforum School on Instrumentation ESI 2009 Photon Detection 14 Multi-anode (Hamamatsu H7546) Up to 8  8 channels (2  2 mm 2 each); Size: 28  28 mm 2 ; Active area 18.1  18.1 mm 2 (41%); Bialkali PC: QE  25 - 45% @ max = 400 nm; Gain  3 10 5 ; 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%) Multi-anode and flat-panel PMT’s 50 mm (Hamamatsu) Cherenkov rings from 3 GeV/c  – through aerogel (T. Matsumoto et al., NIMA 521 (2004) 367)

15. C. Joram CERN / PH EIROforum School on Instrumentation ESI 2009 Photon Detection 15 Micro Channel Plate (MCP) based PMTs Window/Faceplate Photocathode Dual MCP Anode Gain ~ 10 6 Photoelectron  V ~ 200V  V~ 2000V photon MCP-OUT Pulse Typical secondary yield is 2 For 40:1 L:D there are typically 10 strikes (2 10 ~ 10 3 gain single plate) Pore sizes range from <10 to 25  m. Small distances  small TTS and good immunity to B-field Anode & Pins Dual MCP Ceramic Insulators Gain stage and detection are decoupled  lots of potential and freedom for MA-PMTs: Anode can be easily segmented in application specific way. Available with up to 1024 (32 x 32) channels (1.6 x 1.6 mm 2 ) 50 mm

16. C. Joram CERN / PH EIROforum School on Instrumentation ESI 2009 Photon Detection 16 Light absorption in Silicon (http://pdg.ge.infn.it/~deg/ccd.html) At long, temperature effects dominate

17. C. Joram CERN / PH EIROforum School on Instrumentation ESI 2009 Photon Detection 17 (Si) - Photodiodes:  P(I)N type  p layer very thin (<1  m), as visible light is rapidly absorbed by silicon (see next slide);  High QE (80% @  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 (>10 5 V/cm) leads to avalanche multiplication;  High gain: typ. 100-1000;  Rel. high gain fluctuations (excess noise) Solid-state photon detectors Avalanche  (http://micro.magnet.fsu.edu) e h p + i(n) n + 

18. C. Joram CERN / PH EIROforum School on Instrumentation ESI 2009 Photon Detection 18 Electric field strength Traditional ‘Reach-through’ structure (long wavelengths) Electric field strength Reverse structure (short wavelength) Solid state … Avalanche Photodiode (APD) Used in CMS ECAL;

19. C. Joram CERN / PH EIROforum School on Instrumentation ESI 2009 Photon Detection 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

20. C. Joram CERN / PH EIROforum School on Instrumentation ESI 2009 Photon Detection 20 Solid-state … Geiger mode Avalanche Photodiode (G-APD) IDID  = R Q C D 10s of ns  = R S C D (sub – ns) I max ~(V BIAS -V BD )/R Q Gain = Q / e = I max ·  e = (V BIAS -V BD ) C D / e G ~ 10 5 -10 6 at reasonable bias voltage (<100 V) J. Haba, RICH2007 Sample of 3 G-APDs

21. C. Joram CERN / PH EIROforum School on Instrumentation ESI 2009 Photon Detection 21 GM-APD Quench resistor 1mm 100 – several 1000 pix / mm 2 Bias bus Multi pixel G-APD, called G-APD, MPPC, SiPM, … Musienko @PD07 Quasi-analog detector allows photon counting with a clearly quantized signal 20 x 20 pix  V bias  GM-APD   Q Q 2Q Quench resistor 1  2  3  Sizes up to 5×5 mm2 now standard. Only part of surface is photosensitive!

22. C. Joram CERN / PH EIROforum School on Instrumentation ESI 2009 Photon Detection 22 Uozumi@VCI2007 Multi pixel G-APD = G-APD, MPPC, SiPM, … You cannot get "something for nothing” G-APD show dark noise rate in the O(100 kHz – MHz / mm 2 ) range. The gain is temperature dependent O(10% /°K) The signal linearity is limited The price is (still too) high Hamamatsu catalog ~10 producers are now in the market. Expect improvement in technology and performance.

23. C. Joram CERN / PH EIROforum School on Instrumentation ESI 2009 Photon Detection 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 keV pe’s in solid-state detector anode; this results in low gain fluctuations; Output: direct electronic signal; Encapsulation of Si-sensor in the tube implies: o compatibility with high vacuum technology (low outgassing, high T° bake-out cycles); o internal (for speed and fine segmentation) or external connectivity to read-out electronics; o heat dissipation issues; Energy loss eV th in (thin) ohmic contact W Si = 3.6 eV e-h  V = 20 kV  M ~ 5000 F = Fano factor F Si ~ 0.1

24. C. Joram CERN / PH EIROforum School on Instrumentation ESI 2009 Photon Detection 24 1 p.e. pulse height (ADC counts) 2 p.e. 3 p.e. 4 p.e. 5 p.e. pulse height signals of 1 Si pad HV HPD = 26 kV Pedestal cut 10-inches (25.4 cm) 10-inch prototype HPD (CERN) for Air Shower Telescope CLUE. Photon counting. Continuum due to electron back scattering. Hybrid Photon Detectors (HPD’s)

25. C. Joram CERN / PH EIROforum School on Instrumentation ESI 2009 Photon Detection 25 Cross-focused electron optics pixel array sensor bump-bonded to binary electronic chip, developed at CERN 8192 pixels of 50 × 400  m. specially developed high T° bump-bonding; Flip-chip assembly, tube encapsulation (multi- alkali PC) performed in industry (VTT, Photonis/DEP) Pixel-HPD’s for LHCb RICH detectors 50mm Pixel-HPD anode 72mm  active During commissioning: illumination of 144 tubes by beamer. In total : 484 tubes. T. Gys, NIM A 567 (2006) 176-179

26. C. Joram CERN / PH EIROforum School on Instrumentation ESI 2009 Photon Detection 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 (d  ~ 3  ) Possibility of anode segmentation  imaging capability (limited!) Sensitivity gain through ‘Double-cathode effect’  QE max ~ 50% observed. T ~ 0.4 QE ) A. Braem et al., NIM A 602, (2009), 193-196 26

27. C. Joram CERN / PH EIROforum School on Instrumentation ESI 2009 Photon Detection X-HPD project (CERN / Photonis) X-HPD (PC120) - 20 kV - 0 kV

28. C. Joram CERN / PH EIROforum School on Instrumentation ESI 2009 Photon Detection Gaseous Photodetectors 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 e.g. CH 4 + TEA Thin CsI coating on cathode pads 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. Usual issues: How to achieve high gain (10 5 ) ? How to control ion feedback and light emisson from avalanche? How to purify gas and keep it clean? How to control aging ?

29. C. Joram CERN / PH EIROforum School on Instrumentation ESI 2009 Photon Detection Gaeous photodetectors: A few implementations... CsI on readout pads photocathode HV Proven technology: Cherenkov detectors in ALICE, HADES, COMPASS, J-LAB…. Many m 2 of CsI photocathodes Built, just starting up: HBD (RICH) of PHENIX. R&D: Thick GEM structures Visible PC (bialkali) Sealed gaseous devices CsI on multi-GEM structure Sealed gaseous photodetector with bialkali PC. (Weizmann Inst., Israel)

30. C. Joram CERN / PH EIROforum School on Instrumentation ESI 2009 Photon Detection 30 Literature / Acknowledements Non-exhaustive list: www.hamamatsu.com www.photonis.com: “Photomultiplier tubes, principles and applications” www.photonis.com (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.