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crop scintillator lesson, Notas de estudo de Engenharia Biomédica

Universidade Federal do ABC (UFABC)Engenharia Biomédica

Tubos PMT ,Detectando Cintilação

Tipologia: Notas de estudo

2012

Compartilhado em 21/11/2012

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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
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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

Photomultiplier tube (PM or PMT) generates electric signal

Light guide transmits light to photodetector

Passage of charged particle generates light in 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 counter hodoscope

Counters arranged as pizza slices

Photomultiplier tube

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

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

PET Scans

(Positron Emission Tomography)

Scintillating crystal detector and photomultiplier

3-D image

Cross Section

  • Different scintillator materials
    • Plastic scintillator -- good for large areas
    • Sodium Iodide (NaI)
    • BGO (Bi 4 Ge 2 O 12 )
    • 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

2. Light generation in scintillators

Inorganic crystals

  • 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 particle passes through here

Light transmission within scintillator