Scintillation Counters and Photomultiplier Tubes

1. Scintillation Counters and Photomultiplier Tubes Learning Objectives • Understand the basic operation of CROP scintillation • counters and photomultiplier tubes (PMTs) and their • use in measuring cosmic ray air showers • Understand how light is generated in a scintillator • Understand how light is transmitted to a PMT • Understand how a PMT generates an electric signal • Be able to hook up a scintillation counter to its high • voltage and an oscilloscope for viewing signals • Be able to identify light leaks in a scintillation counter • Be able to observe scintillation counter signals using • an oscilloscope and identify cosmic ray muons • Be able to discuss scintillation counter performance • in terms of gain, efficiency and attenuation length

2. Scintillation Counters and Photomultiplier Tubes Outline • Introduction • Light Generation in Scintillators • Light Collection • Optical Interfaces and Connections • Photodetectors and photomultiplier tubes • Performance and Exercises • References

3. Scintillation Counters and Photomultiplier Tubes Introduction • Scintillation counters are multi-purpose particle detectors • used in many experimental physics applications • Used for charged particle detection (positive or negative), • but also neutral particles (photons, neutrons), • although light-generation mechanisms are different for • charged and neutral particles • Basic sequence -- light generation by particle passing • through scintillator material, light collection, • photodetector turns light into electric signal • Scintillation Counter Properties • Fast time response -- light generated almost immediately • after particle passes through scintillator, photodetectors • give fast electric signal • Can count number of particles using pulse height. • The larger the signal size, the greater the number of • particles • Position information • Based on size of active scintillator material

4. Scintillation Counters and Photomultiplier Tubes Basic principles of operation Passage of charged particle generates light in scintillator Charged particle Light guide transmits light to photodetector Photomultiplier tube (PM or PMT) generates electric signal

5. Scintillation Counters and Photomultiplier Tubes Introduction • Examples from High Energy Physics experiments • at particle accelerators • Hodoscope -- an array of several counters covering • a large area • Veto counters -- for particles you don’t want to • measure • Calorimetry -- measuring a particle’s total energy • Triggering -- a fast signal which indicates an • interesting event to record • Examples from cosmic ray experiments • CASA • KASCADE

6. Scintillation counters in High-Energy Physics Experiments Fermilab, Batavia, Illinois Protons Anti-protons CERN, Geneva, Switzerland

7. Scintillation Counters and Photomultiplier Tubes Scintillation counter hodoscope Photomultiplier tube Scintillator wedge Foil wrapping Counters arranged as pizza slices

8. Chicago Air Shower Array (CASA) Dugway Proving Grounds, Utah • University of Chicago and University of Utah collaboration • to study extended cosmic ray air showers • 1089 boxes in a rectangular grid, 15 meter spacing, each with • 4 scintillator planes and 4 photomultplier tubes • 1 low voltage and 1 high voltage supply • 1 electronics card for data triggering and data acquisition • CASA collected data in the 1990’s and is now complete • CROP will use retired scintillation counters recovered • from CASA

9. Scintillation Counters and Photomultiplier Tubes Contents of a CASA detector station Weatherproof box top Electronics card 4 scintillators and PMTs Box bottom

10. The KASCADE experiment in Karlsruhe, Germany KASCADE = KArlsruhe Shower Core and Array DEtector • 252 detector stations • Rectangular grid with 13 m spacing • Array of 200 x 200 m2

11. The KASCADE experiment

12. Scintillation Counters and Photomultiplier Tubes Introduction • Other uses of scintillation counters -- biological research, • medical applications (PET scans) • Use of scintillation counters in CROP • Several counters firing at once indicates extended air • shower -- on one school or inter-school • Pulse heights related to number of particles in shower and • energy of primary cosmic ray • Relative arrival times related to primary cosmic ray • incident direction

13. Scintillation Counters and Photomultiplier Tubes PET Scans (Positron Emission Tomography) 3-D image Scintillating crystal detector and photomultiplier Cross Section

14. Scintillation Counters and Photomultiplier Tubes 2. Light generation in scintillators • Different scintillator materials • Plastic scintillator -- good for large areas • Sodium Iodide (NaI) • BGO (Bi4Ge2O12) • Lead Tungstanate (PbWO4) • Focus on plastic scintillator • Composition • Polystyrene (plexiglass) • Doped with small admixture of a fluor • Fluor is organic macro-molecule like • POPOP: 1,4-Bis-[2-(5-phenyloxazolyl)]-benzene • C24H16N2O2 • Light generated by fluorescence process • One of energy loss mechanisms when charged • particles pass through matter • Similar to television screen or computer monitor • Quantum mechanical process • Light (photons) emitted isotropically • Emission spectrum from typical scintillator • Relation to visible light spectrum Inorganic crystals

15. Energy absorption and emission diagram Electrons excited to higher energy levels when a charged particle passes, absorbing part of its energy Electron ground state Electrons drop back to ground state, emitting fluorescence or scintillation light

16. Scintillation Counters and Photomultiplier Tubes Typical plastic scintillator emission spectrum Wavelength of emitted light • 1 nm = 1 nanometer = 1  10-9 meter • For reference, 1 nm = 10 Angstroms, • where 1 Angstrom is approximate size of an atom • Maximum emission at about 425 nm

17. Scintillation Counters and Photomultiplier Tubes The wavelengths of visible light

18. Scintillation Counters and Photomultiplier Tubes

19. Scintillation Counters and Photomultiplier Tubes 3. Light Collection • Purpose -- Direct as much generated light as possible to • the photodetector • Need for making counters light tight • Light transmission within scintillator • Reflections from surfaces, total internal reflection • Transmission through surfaces • Critical angle • Importance of smooth polished surfaces • Use of reflective coverings • (foil, white paint, white paper, black paper) • Multiple bounces (many!) • Ray-tracing simulation programs • Attenuation of light in scintillator

20. Scintillation Counters and Photomultiplier Tubes Light transmission within scintillator Charged particle passes through here Scintillator Light rays Photomultiplier tubes

21. Scintillation Counters and Photomultiplier Tubes Reflection and transmission at surfaces Air Scintillator material Light totally internally reflected for incident angle greater than critical which depends on optical properties of scintillator and air Scintillator Air Refraction (i.e. transmission) outside scintillator for incident angle less than critical

22. Scintillation Counters and Photomultiplier Tubes 3. Light Collection • Different light collection schemes • Different types of plastic light guides • Air light guides (KASCADE) • CASA scheme • Not optimal, PMT glued onto surface • Wavelength-shifting side bars • Embedded wavelength-shifting optical fibers • Connected to clear optical fibers • Can transport light over long distance • Other fiber optics applications • Laproscopic surgery • Telecommunications

23. Scintillation Counters and Photomultiplier Tubes Laproscopic surgery • Optical fibers transmit image to surgeon • Less instrusive technique

24. Light collection in the KASCADE experiment Electron and photon detector Photomultiplier 33 kg of liquid scintillator Argon-filled space (better light transmission than air) Light emitted from scintillator is guided by conical reflecting surfaces to photomultiplier tube above

25. Light collection in the KASCADE experiment Muon detector Wavelength-shifting bars around perimeter of planes guide light to photomultiplier tubes 4 plastic scintillator planes

26. Scintillation Counters and Photomultiplier Tubes Optical Fibers • Fiber core and cladding optimized to • prevent leakage of light out of the fiber • 95% transmission over 1 km • If this were true for ocean water, you could • clearly see ocean bottom Transmission modes within optical fibers

27. Scintillation Counters and Photomultiplier Tubes What’s wrong with this picture?

28. Scintillation Counters and Photomultiplier Tubes Several scintillators tied together optically with optical fibers To photo-detector Wavelehgth-shifting optical fiber Scintillator planes

29. Scintillation Counters and Photomultiplier Tubes • Advantages and limitations of each type of light • read-out scheme • Definition of efficiency of light collection • Number of photons arriving at the photo-detector • Number of photons generated by charged particle • About 10% for light guide attached to side • A few percent for CASA counters

30. Scintillation Counters and Photomultiplier Tubes 4. Optical Interfaces and Connections • Purpose -- transmit light with high efficiency, • sometimes provide mechanical stability of detector • as well (should decouple the two tasks if possible) • Interface between scintillator material and • Light guide • Optical fiber • Wavelength-shifting bar • Interface between light guide or fiber and • photodetector • Commonly used • Optical cements and epoxies • Optical grease • Air gap

31. Scintillation Counters and Photomultiplier Tubes 5. Photodetectors and Photomultiplier Tubes • Purpose -- transform light into electric signal for • further processing of particle information • Examples • Photomultiplier tube (CROP focus) • Photodiode • Charged-coupled device • Avalanche photodiode (APD) • Visible Light Photon Counter (cryogenics) • Photomultiplier tube details • Entrance window • Must be transparent for light wavelengths which • need to enter tube • Common: glass • Fused silicate -- transmits ultraviolet as well

32. Scintillation Counters and Photomultiplier Tubes Schematic drawing of a photomultiplier tube (from scintillator) Photocathode Photons eject electrons via photoelectric effect Each incident electron ejects about 4 new electrons at each dynode stage Vacuum inside tube An applied voltage difference between dynodes makes electrons accelerate from stage to stage “Multiplied” signal comes out here

33. Scintillation Counters and Photomultiplier Tubes Different types of dynode chain geometries

34. Scintillation Counters and Photomultiplier Tubes • Definition of Photomultiplier Tube Gain •  = average number of electrons generated at each dynode • stage • Typically,  = 4 , but this depends on dynode material • and the voltage difference between dynodes • n = number of multiplication stages • Photomultiplier tube gain =  n • For n = 10 stages and  = 4 , gain = 410 = 1  107 • This means that one electron emitted from the • photocathode (these are called “photoelectrons”) • yields 1  107 electrons at the signal output

35. Scintillation Counters and Photomultiplier Tubes The Photocathode • Incoming photons expel electrons from the metallic • surface of the photocathode via the photoelectric effect. • The effect was discovered by • Heinrich Hertz in 1887 and • explained by Albert Einstein • in 1905. • According to Einstein's theory, • light is composed of discrete • particles of energy, or quanta, • called PHOTONS. When photons with enough energy • strike the photocathode, they liberate electrons that have • a kinetic energy equal to the energy of the photons less • the “work function” (the energy required to free the • electrons from a particular material). • Einstein received the Nobel Prize for his 1905 paper • explaining the photoelectric effect. What were the other • two famous Einstein papers from 1905? • Theory of special relativity • Explanation of Brownian motion

36. Scintillation Counters and Photomultiplier Tubes The Photocathode • Photocathode composition • Semiconductor material made of antimony (Sb) and • one or more alkalai metals (Cs, Na, K) • Thin, so ejected electrons can escape • Definition of photocathode quantum efficiency, h(l) • Number of photoelectrons released • h(l) = • Number of incident photons (l) on cathode • Typical photocathode quantum efficiency is 10 - 30% • Photocathode response spectrum • Need for matching scintillator light output spectrum with • photocathode response spectrum

37. Scintillation Counters and Photomultiplier Tubes Typical photocathode response curve 200 nm 700 nm Wavelength of light 1 nm = 1 nanometer = 1  10-9 meter Note: Quantum efficiency > 20% in range 300 - 475 nm Peak response for light wavelengths near 400 nm

38. Scintillation Counters and Photomultiplier Tubes • The dynode chain • High voltage applied to dynodes creates electric fields • which guide electrons between from stage to stage • Process of secondary emission yields more electrons • at each stage • This is the “multiplication” in “photomultiplier” • Process is similar to photoelectric effect, with incident • photon replaced by incident electron • Composition of dynodes • Ag - Mg • Cu - Be Deposited in thin layer on • Cs - Sb conducting support • Sensitivity to earth’s magnetic field • Earth’s magnetic field is typically 0.5 - 1.0 Gauss • Trajectories of charged particles moving in a magnetic • field will curve, depending on field orientation • Can cause photoelectrons and secondary-emitted • electrons not to reach next stage • First few stages, when there are few electrons, • most vulnerable • Use of magnetic shields • Should extend shield beyond front of tube

39. Scintillation Counters and Photomultiplier Tubes • The phototube base and high voltage supply • Purpose -- provide an electric field between • photocathode and first dynode • successive dynodes • to accelerate electrons from stage to stage • About 100 V voltage difference needed between stages • Chain of resistors forms voltage divider to split up • high voltage into small steps • Capacitors store readily-available charge for electron • multiplication • Typical base draws 1 - 2 milliamperes of current

40. Scintillation Counters and Photomultiplier Tubes The electric field between successive dynodes A simplified view Represents a dynode - - - - - - - Electric field between plates 100 Volts + + + + + + + Represents the next dynode An electron (negative charge) released from the negative plate will be accelerated toward the positive plate

41. Scintillation Counters and Photomultiplier Tubes Typical phototube base schematic Output signal to oscilloscope Photocathode Dynodes Tube body Ground High voltage supply Positive Current flows through resistor chain for voltage division Capacitors (which store charge) needed for final stages when there are many electrons Output signal flows out of tube

42. Scintillation Counters and Photomultiplier Tubes A simple voltage divider Greek omega for resistance unit, Ohms Current, I (amperes) 4 W = R1 Battery Vbatt = 9 Volts a + - Voltmeter here 2 W = R2 b Measured with voltmeter between points (a) and (b) You have “divided” the 9 Volt battery: 3 Volts and 6 Volts are now accessible with this circuit.

43. Scintillation Counters and Photomultiplier Tubes • Vacuum inside tube body • Purpose -- minimize collisions of electrons with gas • molecules during transit • Requires strong tube body • Pins for electrical connections pierce through glass • at bottom of tube (leak-tight) • Damage to tube by helium or hydrogen • “Small” gas molecules can leak into tube, even • through glass

44. Scintillation Counters and Photomultiplier Tubes • Variation of PMT gain with high voltage • Increasing high voltage increases electron transmission • efficiency from stage to stage • Especially important in first 1-2 dynodes • Increasing high voltage increases kinetic energy of • electrons impacting dynodes • Increases amplification factor d

45. Plastic scintillator 10 nsec / division Inorganic crystal, NaI 5000 nsec / division (Longer time scale for fluorescence to occur) Scintillation Counters and Photomultiplier Tubes Oscilloscope traces from scintillation counters

46. Scintillation Counters and Photomultiplier Tubes Close-up of photoelectron trajectories to first dynode

47. Scintillation Counters and Photomultiplier Tubes References 1. Introduction to Experimental Particle Physics by Richard Fernow, Cambridge University Press, 1986, ISBN 0-521-30170-7 (paperback), Chapter 7, pages 148-177 (includes exercises) 2. Photomultiplier Manual, Technical Series PT-61, 1970, RCA Corporation 3. Techniques for Nuclear and Particle Physics by W. R. Leo, Springer-Verlag, Germany, 1994, ISBN 3-540-57280-5, Chapters 7-9, pages 157-214 4. Radiation Detection and Measurement, 3rd Edition, by Glenn F.Knoll, Wiley 2000, ISBN 0-417-07338-5, Chapters 8-10, pages 219 - 306

48. Scintillation Counters and Photomultiplier Tubes Light transmission through entrance wnidow Different window materials Percent of light which passes Wavelength of light 200 nm 700 nm • Observe: • 20% transmission typical for 400 nm light • Fused silica extends transmission into lower wavelengths • Less than 400 nm is ultraviolet light

49. Scintillation Counters and Photomultiplier Tubes

50. Scintillation Counters and Photomultiplier Tubes 6. Performance and exercises Signal shape, pulse height and duration Pulse height distributions Linearity Attenuation length Oscilloscope examples and exercises with changing high voltage, radioactive source, attenuation length