Scintillation Counters and Photomultiplier Tubes Learning Objectives Understand the basic operation...

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


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


Photomultiplier tube (PM or PMT) generates electric signal

Light guide transmits light

to photodetector

Passage ofcharged particle generates lightin scintillator

Charged particle

Basic principles of operation


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-EnergyPhysics Experiments

Fermilab, Batavia, Illinois

CERN, Geneva, Switzerland

Protons

Anti-protons


Scintillation counter hodoscope

Counters arrangedas pizza slices

Photomultipliertube

Scintillator wedge

Foil wrapping

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

Box bottom

4 scintillators and PMTs

Electronics card

The KASCADE experimentin Karlsruhe, Germany

KASCADE = KArlsruhe Shower Core and Array DEtector

• 252 detector stations• Rectangular grid with 13 m spacing• Array of 200 x 200 m2

The KASCADE experiment


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

Introduction


PET Scans(Positron Emission Tomography)

Scintillating crystal detectorand photomultiplier

3-D image

CrossSection


• 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

2. Light generation in scintillators

Inorganic crystals

Energy absorption and emission diagram

Electron groundstate

Electrons drop backto ground state,emitting fluorescence or scintillation light

Electrons excitedto higher energylevels when acharged particlepasses, absorbingpart of its energy


• 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

Typical plastic scintillator emission spectrum

Wavelength of emitted light


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


Photomultiplier tubes

Light rays

Scintillator

Charged particlepasses throughhere

Light transmission within scintillator


Reflection and transmission at surfaces

Scintillator material

Air

Light totally internally reflected for incident angle greater than critical which depends on opticalproperties of scintillator and air

Refraction (i.e. transmission) outside scintillator for incident angle less than critical

Air

Scintillator


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

Argon-filled space (better light transmission than air)

33 kg of liquid scintillator

Light emitted from scintillator is guided by conical reflectingsurfaces to photomultiplier tube above

Photomultiplier

Muon detector

Light collection in the KASCADE experiment

4 plastic scintillator planesWavelength-shifting bars around perimeter of planes guide light tophotomultiplier tubes


Optical Fibers

Transmission modes within 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


What’s wrong withthis picture?


Several scintillators tied togetheroptically with optical fibers

Wavelehgth-shiftingoptical fiber

Scintillator planes

To photo-detector


• 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 detectoras 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

Photons eject electrons via photoelectric effect

Photocathode

(from scintillator)

Each incidentelectron ejectsabout 4 newelectrons at eachdynode stage

Vacuum insidetube

“Multiplied” signalcomes out here

An applied voltagedifference betweendynodes makeselectrons acceleratefrom stage to stage


Different types of dynode chain geometries


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


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


• 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,

Number of photoelectrons released= Number of incident photons () on cathode

• Typical photocathode quantum efficiency is 10 - 30%

• Photocathode response spectrum

• Need for matching scintillator light output spectrum with photocathode response spectrum

The Photocathode


200 nm 700 nmWavelength of light

Typical photocathode response curve

1 nm = 1 nanometer = 1 10-9 meterNote: 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 dynodesA simplified view

100 Volts

+

-

Represents a dynode

Electric field between plates

+ + + + + +

- - - - - -

Represents the next dynode

An electron (negative charge) released from the negativeplate will be accelerated toward the positive plate


High voltagesupply

Typical phototube base schematic

Ground

Positive

Dynodes

Tube body

Photocathode

Output signal flows out of tube

Output signalto oscilloscope

Capacitors(which storecharge) neededfor final stageswhen there aremany electrons

Current flowsthrough resistorchain for voltagedivision


A simple voltage divider

BatteryVbatt = 9 Volts

2 R

4 R

a

b

Current, I(amperes)

+- Voltmeter

here

Greek omega forresistance unit, Ohms

Volts 3) 2)(Amps 5.1(V

Amps 5.1 6

Volts 9 :circuitin Current

or :law sOhm'

2 across

21

batt

2

RIRR

VI

R

VIRIV

R

You have “divided” the 9 Volt battery: 3 Volts and 6 Volts are now accessible with this circuit.

Measured with voltmeter between points (a) and (b)


Vacuum inside tube body

Purpose -- minimize collisions of electrons with gasmolecules 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


Oscilloscope traces from scintillation counters

Plastic scintillator

5000 nsec / division(Longer time scale forfluorescence to occur)

Inorganic crystal, NaI

10 nsec / division


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

200 nm 700 nmWavelength of light

Differentwindowmaterials

Per

cent

of

ligh

t w

hic

h p

asse

s

• 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 changinghigh 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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Scintillation Counters and Photomultiplier Tubes Learning Objectives Understand the basic operation...

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