Slide 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). Slide 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 Slide 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 Slide 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 es 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 es reaching the surface with sufficient excess energy escape from it External Photoeffect Basics of photon detection (Photonis) EE h e-e- semiconductor vacuum Slide 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) 410 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 Slide 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. Slide 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 ( C. Joram CERN / PH EIROforum School on Instrumentation ESI 2009 Photon Detection 12 Mainly determined by the fluctuations of the number m( ) of secondary es 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 PMTs (Photonis) 1 pe Pedestal noise CuBe dynodes E A >0 GaP(Cs) dynodes E A