Lecture 2-Building a Detector
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Transcript of Lecture 2-Building a Detector
Lecture 2-Building a Detector
George K. Parks
Space Sciences Laboratory
UC Berkeley, Berkeley, CA
Brief summary of Lecture 1
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.
Schematic of Earth’s Magnetosphere
Density of Major Constituents in Earth’s atmosphere
Differential Energy Fluxes
Typical Oxygen spectra in the heliosphere
Measurement Requirements
Requirements
Detectors and Components
Detectors for Space
Measurement and Instrument Requirements
A Simple Detector for Photon Measurement
Imaging DetectorCollimator
Scintillators
Common Inorganic Scintillators
Light transmission
• Scintillators must be ableto transmit the light it generates.
• Generally not a problem withmost scintillators.
CsI Scintillator
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.
Absorption in material
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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)
Temperature Dependence of NaI(Tl)
Entrance Window Material
• NaI(Tl) is hydroscopic, sealed in vacuum.
• Transmission of X-rays through various material in front of sealed NaI (Tl).
X-ray Absorption in NaI(Tl)
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I = Ioe−μx
• 2 mm70% @ 100 keV
• 1/4 in (6.35 mm)~95% @ 100 keV
X-ray Absorption in CsI(Tl)
• Density = 4.51 g/cm3
• 2 mm83% @ 100 keV
• ¼ in (6.35 mm)~100% at 100 keV
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I = Ioe−μx
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
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
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I = Ioe−μx
Properties of Scintillators (Room T)
Maximize photon collection
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+
Light emission of Inorganic Scintillators
Desired Properties of Scintillators
Conversion Efficiency Calculation (cont’d)
More Worries!
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
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A=Aoe−t /τ
Summary of important factors
Reminder-A simple Photon Detector
Reminder-Photomultiplier Tube
PMTs
Operating principle of PMTs
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?
Reminder-Buiding Detectors