Scintillation Detectors and Applications

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Scintillation Detectors and Applications

• Seminar in Experimental Techniques for Subatomic
  Physics
• Indiana University, Bloomington
• Chuck Bower
• 21 Sept 2004

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Outline

• References
• Definition
• Brief physics of scintillation
• Types and Applications
• Organic
• Inorganic
• Photodetectors
• Photomultiplier Tubes
• Photodiodes
• Hybrids
• Practical Construction Techniques

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References

• (Bible of scintillation) Theory and Practice of
  Scintillation Counting by J.B. Birks (1964)
• (Bible of Radiation Detection) Radiation
  Detection and Measurement by Glenn F. Knoll (3rd
  ed. 1999)
• (Bible of Physical Constants) Tables of Physical
  and Chemical Constants by G.W.C. Kaye and T. H.
  Laby (16th ed. 1995)
• (Bible of Astronomical and Atmospheric Data)
  Astrophysical Quantities by C. W. Allen (4th ed.
  2000)
• (Bible of photomultiplier tubes) Photomultiplier
  Handbook by Ralph W. Engstrom (free from Burle
  Industries, Inc. other manufacturers, e.g.
  Photonis, distribute similar books)

• (Online E-loss, photon Absorption, etc. data)
  http//physics.nist.gov/PhysRefData/
• (Practical book on physics detectors)
  Experimental Techniques in High Energy Nuclear
  and Particle Physics edited by Tom Ferbel (2nd
  ed. 1992)
• (Practical book on charged particle interactions,
  etc.) High-Energy Particles by Bruno Rossi
  (1952)
• (Practical book on photon interactions, etc.)
  The Atomic Nucleus by Robley D. Evans (1955)
• (Journal of physics detectors) Nuclear
  Instruments and Methods (NIM)
• (Journal of physics detectors) IEEE Transactions
  on Nuclear Science
• (Where to find used texts) http//used.addall.com
  and choose Response Time Extremely Slow (still lt
  30 seconds, typically)

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Scintillation (definitions)

• luminescence emission of electromagnetic
  radiation in excess of thermal radiation
• fluorescence luminescent radiation which occurs
  after the source of excitation has been removed
• phosphorescence fluorescence with decay time
  greater than 1 second
• scintillation fluorescence caused by ionizing
  radiation (e.g. charged particles or high energy
  photons)
• NOTE these definitions arent rigorous

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Types of Scintillator
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Physics of Scintillation

• http//wps.prenhall.com/wps/media/objects/724/7415
  76/chapter_15.html

• Hybrid (p) orbital electrons in benzene (and its
  derivatives) are resistant to collisional
  de-excitation allowing radiative de-excitation

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Physical Process of Scintillation

• Both ionization and excitation contribute
  (roughly 50-50)
• S10 state populated by recombination and
  non-radiative de-excitation
• S10 state decays radiatively giving off UV
  (250-300 nm) photons

• (figure from Knoll, p 221.)

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Characteristics and uses of Organic Scintillator

• Fast response (nsec)
• Monotonic function of E-loss
• Sensitive to wide range of atomic number (Z)
• Long attenuation lengths (meters)
• Moderate cost
• Moderate conversion efficiency Eobs/Edep few
• Moldable/machinable into many shapes/sizes

• Time-of-Flight (ToF) msmts (cf. Mitchell)
• Charge magnitude ID
• Calorimtery (cf. Tiege)
• Position sensing with fibers (cf. Otto-Meyer)
• Triggering (cf. Smith)

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Recipe for Large Scale Organic Scintillator

• 90 Bulk vehicle
• Mineral oil for liquid scint
• Polyvinyltoluene (PVT) or polystyrene for plastic
• LONG attenuation length in visible required (few
  meters in plastic, several meters in liquid)
• 10 scintillant (benzene derivative)
• E.g. pseudocumene, xylene, toluene
• 1 waveshifter 1 (benzene derivative)
• Waveshift from 300 nm ? 350 nm (photodetector
  sensitivity region)
• E.g. PPO
• 0.01 waveshifter 2 (benzene derivative)
• Waveshift from 350 nm ? 425 nm (where bulk
  vehicle is transparent)
• E.g POPOP, bis-MSB
• NOTE if small scale, dont need bulk vehicle nor
  waveshifter 2
• NOTE waveshifters are expensive (100s to 1000
  /kg)

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Scintillating and Waveshifting Fibers

• Scintillating type
• 0.1 mm lt diameter lt few mm
• Similar performance to standard plastic
  scintillator (e.g. sheets)
• gives excellent position resolution for
  traversing charged particles
• Bendable (few cm bend radius) means easily shaped

• Wavelength shifting (WLS) type
• 0.1 mm lt diameter lt few mm
• have long (several m) attenuation lengths
• Low cost calorimtery due to minimizing
  photocathode area requirements
• Some kinds give excellent match to APD response

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Inorganic Scintillator Composition

• Typically salt crystals w/one element from column
  IA or IIA and other from column VIIA
• Desire at least one (preferably both) elements
  have Z gt 50
• Figure www.pbs.org/wgbh/ nova/kaboom/elemental/

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Characteristics and Applications of Inorganic
Scintillators

• () High Z materials ? high X-ray, g-ray cross
  sections
• () High conversion efficiency 4x organic
  scintillator
• (-) slow (100s ms) decay times (exceptions
  BaF2,CsF)
• (-) crystals are grown ? shape/size limitations
• (-) moderately expensive
• (-) fragile
• (-) many are hygroscopic (absorb water ?
  clouding)

• High Energy photon detection, e.g. radioactive
  decays, cosmic g-rays

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Photomultiplier How it works

• (Figure from old Thorn/EMI catalog, company now
  called Electron Tubes)
• Voltage Divider (figure from Knoll, p. 284)

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Quantum Efficiency(QE)

• Definition Quantum Efficiency is the probability
  that a photon incident upon a photodetectors
  active surface results in a detectable electrical
  signal
• QE f(l)
• for standard photomultiplier, small l are
  absorbed by glass window large l cant produce
  electrons which excede the photocathode work
  function
• (figure from Thorn/EMI catalog)

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Magnetic Field Effects

• Lorentz force on electrons causes deflection ?
  loss
• Some PMs sensitive at earth field level!
• Align PM with direction of B-field to minimize
  effect
• Ferromagnetic materials formed (usually
  cylindrical) to shield
• (figure from Thorn/EMI catalog)

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Resolution (general)

• (definition) resolution is the ability to resolve
  or separate events with different input
  parameters (e.g. energy, time, mass, momentum,
  light intensity)
• Signal (H0) n (number of carriers, e.g.
  photons)
• Full Width at Half Maximum (FWHM) n½
• Resolution FWHM/H0 n½/n 1/n½
• ? better (smaller) resolution results from larger
  n
• (figure from Knoll, p. 115)

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Photomultiplier Resolution

• Electron charge is quantized
• Resolution depends upon number of electrons in
  signal, dominated by photoelectrons (pe) from
  cathode and secondary electrons at first dynode
• larger light pulse leads to better resolution
• Higher gain at first dynode leads to better
  resolution
• Dynode gain (d) 5 (typical) smears pe peaks
• Dynode gain 50 (e.g. from GaP) separates pe
  peaks
• (figures from Knoll, pp. 273-274)

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Silicon Photodiodes (PD) as photon detectors for
scintillator

• () Less energy required to convert photon to
  electron
• () No work function to overcome
• () electron/hole mobility ? thicker active layer
  than PM photocathode leads to higher QE (above
  80!!!)
• () very low magnetic field susc.
• (-) more susceptible to thermal electrons (i.e.
  noise)
• Cooling to -20 C alleviates
• (-) low gain (electron multipication) means
  downstream electronics (and associated noise) are
  required
• 105 lt typ. PM gain lt 109
• 1 lt typ. PD gain lt 103 (Avalanche PhotoDiode
  APD)

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Hybrid Photomultiplier (HPMT)

• also called hybrid photodiode (HPD)
• QE better than PM, not as good as APD (50)
• Gain (few thousand) better than APD, not as good
  as PM
• Typically multianode ? position sensitive
• Resolution better than either PM or APD
• (figures from U. Minn., U. Regina)

• (figure below from Knoll, p. 299, and from NIM
  A345, 279, 1994)

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Lightpipes/lightguides

• For transporting photons from scintillator to
  photodetector
• Various types
• fishtail (top right figure)
• Simplest (least costly)
• Inefficient
• Adiabatic ? area at photodetector end area at
  scintillator end (bottom right)
• Efficient (Liouvilles Theorem)
• Twisted strips
• Adiabatic
• More uniform photon propogation times ? best
  timing resolutions

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More Construction Techniques

• Optical coupling of scintillator/lightpipe to
  photodetector with similar refractive index
  material eliminates/minimizes reflection losses
  at interface
• Optical grease (silicone gel), very easily
  reversible
• Optical RTV (curable rubber, similar to some
  bathtub caulks), moderately difficult to reverse
• Optical epoxies, nearly impossible to reverse
• Reflective materials adjacent to scintillator and
  lightpipe to increase light yield
• Specular (mirror) type (e.g. aluminum foil)
  around scintillator
• Diffuse (matte) type (e.g. teflon tape) around
  lightpipes
• Light tight wrapping (e.g. black Tyvek, black
  vinyl tape) essential because room light is many
  orders of magnitude higher intensity than
  scintillation signal