lecture 2-building a detector
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Lecture 2-Building a Detector. George K. Parks Space Sciences Laboratory UC Berkeley, Berkeley, CA. Brief summary of Lecture 1. Operations of Detectors: • Detection of particles and photons relies how particles and photons interact with matter. - PowerPoint PPT PresentationTRANSCRIPT
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Lecture 2-Building a Detector
George K. Parks
Space Sciences Laboratory
UC Berkeley, Berkeley, CA
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Brief summary of Lecture 1
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Brief summary of Lecture 1 (cont’d)
• A detector is a device that converts incident particles and photons into signals without distorting the original information.
• Two major physics discoveries led to important development of detectors: photoelectric effect and that secondary electrons can be produced.
• Detector components include Photomultiplier Tubes (PMT) and Channel Electron Multipliers (CEM).
- PMTs multiply the number of electrons by discreet dynodes whereas CEMs multiply electrons continuously.
• Assemble a million of CEMs in a geometrical array and form Micro Channel Plates (MCP).
- Each channel is a pixel, so MCPs can form Images.
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Schematic of Earth’s Magnetosphere
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Density of Major Constituents in Earth’s atmosphere
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Differential Energy Fluxes
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Typical Oxygen spectra in the heliosphere
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Measurement Requirements
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Requirements
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Detectors and Components
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Detectors for Space
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Measurement and Instrument Requirements
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A Simple Detector for Photon Measurement
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Imaging DetectorCollimator
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Scintillators
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Common Inorganic Scintillators
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Light transmission
• Scintillators must be ableto transmit the light it generates.
• Generally not a problem withmost scintillators.
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CsI Scintillator
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Emission Spectrum of scintillators
• Scintillators produce different amount of light.
• NaI (Tl) more efficient than CsI (Na)
• It’s better if there is more light.
• Why? Directly affects the energy resolution of the detection system.
• How? Affects Statistics.
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Absorption in material
€
I = Ioe−μx
Io = # incident h through xx = thickness = attenuation coefficient
• X-and gamma rays are penetrating.• Need high Z material to stop them.• Inorganic scintillators have higher density that organic scintillators.
NaI(Tl)
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Temperature Dependence of NaI(Tl)
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Entrance Window Material
• NaI(Tl) is hydroscopic, sealed in vacuum.
• Transmission of X-rays through various material in front of sealed NaI (Tl).
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X-ray Absorption in NaI(Tl)
€
I = Ioe−μx
• 2 mm70% @ 100 keV
• 1/4 in (6.35 mm)~95% @ 100 keV
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X-ray Absorption in CsI(Tl)
• Density = 4.51 g/cm3
• 2 mm83% @ 100 keV
• ¼ in (6.35 mm)~100% at 100 keV
€
I = Ioe−μx
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X-ray Absorption in BGO
•Density = 7.13 g/cm3
• % of incident X-rays stopped in BGO. €
I = Ioe−μx
• 1 mm95% @ 100 keV
• 1.5 mm~100% @ 100 keV
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X-ray Absorption in Plastic
• Density = 1.03 g/cm3
• Plastic scintillator often used in anti-conincidence part of an experiment to reduce cosmic ray contribution.
• 10 mm20% @ 20 keV• 130 mm82% @ 100 keV98% @ 20 keV
€
I = Ioe−μx
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Properties of Scintillators (Room T)
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Maximize photon collection
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Plastic Scintillator (NE 102)
• Light emission by various particles• Sufficient for A/C application
• Range of various particles• Few mm to stop 2 MeV p+
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Light emission of Inorganic Scintillators
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Desired Properties of Scintillators
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Conversion Efficiency Calculation (cont’d)
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More Worries!
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Conversion Efficiency Calculation
• To compute DE for different energies, use
different radioactive sources.
• Half-life of Sources. How to correct?
where A = activity level now
Ao = original activity level
t = time interval since the source calibrated
= mean half-life of the source
1 Curie = 3.7x1010 dps
€
A=Aoe−t /τ
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Summary of important factors
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Reminder-A simple Photon Detector
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Reminder-Photomultiplier Tube
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PMTs
Operating principle of PMTs
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Photomultiplier tubes (PMTs)
• Hamamatsu listsmore than 300 different types
of PMTs.
• Different shapes, size, gain, etc..
• So many different parameters!
• What do they mean?
• How does one choose which PMTs to use?
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Reminder-Buiding Detectors