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
Outline Introduction Light Generation in Scintillators Light Collection Optical Interfaces and Connections Photodetectors and photomultiplier tubes Performance and Exercises References
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
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
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
Scintillation counters in High-Energy Physics Experiments Fermilab, Batavia, Illinois Protons Anti-protons 1m CERN, Geneva, Switzerland
Scintillation counter hodoscope Photomultiplier tube Scintillator wedge Foil wrapping Counters arranged as pizza slices
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
Contents of a CASA detector station Weatherproof box top Electronics card 4 scintillators and PMTs Box bottom
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 m 2
The KASCADE experiment
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
PET Scans (Positron Emission Tomography) Scintillating crystal detector and photomultiplier 3-D image Cross Section
2. Light generation in scintillators Different scintillator materials Plastic scintillator -- good for large areas Sodium Iodide (NaI) BGO (Bi 4 Ge 2 O 12 ) Inorganic crystals Lead Tungstanate (PbWO 4 ) 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 C 24 H 16 N 2 O 2 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
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
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
The wavelengths of visible light
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
Light transmission within scintillator Charged particle passes through here Scintillator Light rays Photomultiplier tubes
Reflection and transmission at surfaces Air Scintillator material Light totally internally reflected for incident angle greater than Θ critical which depends on optical properties of scintillator and air Scintillator Air Refraction (i.e. transmission) outside scintillator for incident angle less than Θ critical
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
Laproscopic surgery Optical fibers transmit image to surgeon Less instrusive technique
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
Light collection in the KASCADE experiment Muon detector Wavelength-shifting bars around perimeter of planes guide light to photomultiplier tubes 4 plastic scintillator planes
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
What s wrong with this picture? Scintillation Counters and Photomultiplier Tubes
Several scintillators tied together optically with optical fibers Wavelehgth-shifting optical fiber To photo-detector Scintillator planes
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
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
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
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 Multiplied signal comes out here An applied voltage difference between dynodes makes electrons accelerate from stage to stage
Different types of dynode chain geometries
Definition of Photomultiplier Tube Gain δ = average number of electrons generated at each dynode stage Typically, δ = 4, but this depends on dynode material and the voltage difference between dynodes n = number of multiplication stages Photomultiplier tube gain = δ n For n = 10 stages and δ = 4, gain = 4 10 = 1 10 7 This means that one electron emitted from the photocathode (these are called photoelectrons ) yields 1 10 7 electrons at the signal output
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
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, η(λ) η(λ) = Number of photoelectrons released Number of incident photons (λ) on cathode Typical photocathode quantum efficiency is 10-30% Photocathode response spectrum Need for matching scintillator light output spectrum with photocathode response spectrum
Typical photocathode response curve 200 nm Wavelength of light 700 nm 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
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
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
The electric field between successive dynodes A simplified view Represents a dynode 100 Volts - + --- --- + + + + + + Electric field between plates Represents the next dynode An electron (negative charge) released from the negative plate will be accelerated toward the positive plate
Typical phototube base schematic Output signal to oscilloscope Photocathode Dynodes Tube body Ground Positive High voltage supply Capacitors (which store charge) needed for final stages when there are many electrons Output signal flows out of tube Current flows through resistor chain for voltage division
A simple voltage divider Current, I (amperes) 4 Ω = R 1 Greek omega for resistance unit, Ohms Battery V batt = 9 Volts + - 2 Ω = R 2 a Voltmeter here b V Ohm's law : V = I R or I = R Vbatt 9 Volts Current in circuit : I = = = 1.5 Amps R1 + R2 6 Ω Vacross R 2 = I R2 = (1.5 Amps)(2 Ω) = 3 Volts 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.
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
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 δ
Oscilloscope traces from scintillation counters Plastic scintillator 10 nsec / division Inorganic crystal, NaI 5000 nsec / division (Longer time scale for fluorescence to occur)
Close-up of photoelectron trajectories to first dynode
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
Light transmission through entrance wnidow Percent of light which passes Different window materials 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
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
Development Questions Request permission to use figures now Specific figures or general release? What format to aim for this summer? Powerpoint presentation (with embedded figures?) Accompanying text Accessibility on the web, with more detail here links Curriculum & Instruction check for level-appropriateness Format for field-testing in schools
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