Other Types of Photon Detectors
AST443, Lecture 7
Stanimir Metchev
Administrative
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Project 1:
– due today
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Project 2:
– observing proposals
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1 page scientific justification
1 page technical justification, including target list
1 page for figures, tables, references
print 4 copies and bring to class
– due in class on Tuesday, Sep 29 (note Monday schedule!)
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Reading:
– chapters 4–5 of Howell: CCD data reduction and photometry
– chapters 3–5 of Wall & Jenkins: statistics, correlations, hypothesis
testing
2
Outline
• Overview of previous lecture
– CCDs
• Other kinds of photon detectors
– infrared
– x-ray, gamma-ray
– submillimeter, radio
• Basic imaging data reduction
3
Basic Concept
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electron-hole pair generation
doping:
– n-type (electrons)
– p-type (holes)
– creates additional energy levels
within band gap
– increases conductivity
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silicon (Si)
– band gap: Eg = 1.12 eV
• cut-off wavelength λc = 1.13µm
hc 1.24 µm
=
E g E g (eV)
– free-electron energy: 4 eV (3000Å)
– 1 photon -> 1 electron
"c =
!
4
Basic Concept
•
•
electron-hole pair generation
doping:
– n-type (electrons)
– p-type (holes)
– creates additional energy levels
within band gap
– increases conductivity
•
silicon (Si)
– band gap: Eg = 1.12 eV
• cut-off wavelength λc = 1.13µm
hc 1.24 µm
=
E g E g (eV)
– free-electron energy: 4 eV (3000Å)
– 1 photon -> 1 electron
"c =
!
5
Basic Concept:
A P-N Photo Diode
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depleted region
– low conductivity
– can support an E field
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•
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net positive charge (higher charge density near top)
additional E-field applied
subsequently generated electrons get trapped in potential well near top
6
CCD Charge Transfer
7
Outline
• Overview of previous lecture
– CCDs
• Other kinds of photon detectors
– infrared
– radio
– gamma-ray
– x-ray
• Basic imaging data reduction
8
Near-Infrared
• 1–5 µm
• Si sensitivity drops off at >1.1 µm
• InSb (“indium antimonide”)
–
–
–
–
hc 1.24 µm
"c =
=
E g E g (eV)
Eg ~ 0.22 eV at 77 K
λc = 5.6 µm
need low temperature (~ 30 K)
Keck IR cameras; Spitzer IRAC 3.6 and 4.5 µm
!
• Hg(1–x)CdxTe (“mercury-cadmium-telluride,
mer-cad-tel”)
– Eg = 1.55 eV for x = 1; λc = 0.8 µm
– Eg can be brought down to ~0 eV (metal)
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NearInfrared
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HgCdTe detector
signal carriers: holes
– cf. electrons in CCDs
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if l is small, hole can
diffuse to depleted
region before
recombining
hole is driven across
depleted region
producing current
pixels can be read out
photon
absorption
layer
– individually(!), in
principle
– non-destructively
(Rieke 2007)
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Near-Infrared
(x)
•
hc 1.24 µm
"c =
=
E g E g (eV)
Hg(1–x)CdxTe (“mercury-cadmium-telluride, mer-cad-tel”)
– Eg = 1.55 eV for x = 1; λc = 0.8 µm
– Eg can be brought down to ~0 eV (metal)
!
– Hg0.55Cd0.45Te has λc ~ 2.5 µm
• can operate at higher temperature (LN ~ 75 K)
• Palomar IR cameras
• HST WFC3
– can, in principle, work to very long λc
11
Mid- and Far-Infrared
• 5–40 µm
– Si:Ga (λc ~ 18 µm)
– Si:As (λc ~ 28 µm)
• Spitzer Space Telescope
– IRAC 5.8 and 8.0 µm, IRS 5–15 µm, MIPS 24 µm
• only form readily available in large format arrays
– Si:Sb (λc ~ 40 µm)
• Spitzer IRS (15–35 µm)
• 40–100 µm (bolometer arrays)
– Ge:Ga
• Spitzer MIPS (70 µm and 160 µm)
• cooled by super-fluid liquid He to 1.5 K
• ~100 pixels
– Ge:B, Ge:Sb, GaAs:Te
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Readout of IR Arrays
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can be done non-destructively
– follow up the ramp sampling
– Fowler-N sampling: read noise reduced as 1/√N
(G. Finger et al.)
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Readout of IR Arrays
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can be done non-destructively
– follow up the ramp sampling
– Fowler-n sampling: read noise reduced as 1/√n
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smaller (InSb) arrays with individual pixel readouts
– after ADC conversion signal from each readout can be stored
– multiple non-destructive readouts can be co-added before saving
the image
– theoretically, no limitation on object brightness (no saturation)
but:
– readouts are not instantaneous (~ few ms duration)
– N co-adds increase read noise as √N
(G. Finger et al.)
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Extreme UV, X-Ray, γ-Ray
• extreme UV
– 10–100 nm [120–12 eV]
– EUVE (Extreme UV Explorer, 1992–2001)
• soft/hard x-ray
– 1–10 nm / 0.01–1 nm [0.12–120 keV]
– Einstein, RöSat, XMM-Newton, Chandra
• (soft) γ-ray
– 0.001–0.01 nm; <0.001 nm
– 0.12–1.2 MeV; >1.2 MeV
– EGRET, BATSE
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X-/ γ-ray Photon Detection:
Proportional Counters
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high-E photon removes outer-shell electrons
– x-rays: use Ar + organic gas
– γ: use solid Ge
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N (electron-ion pairs) ∝ Ephoton
– has energy resolution!
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amplification possible by setting high voltage
cathode
anode
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X-ray Photon Detection:
Scintillation Detectors
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x-ray photon removes electrons from deeper shells; is fully absorbed
higher-energy electron fills “hole”, emitting visible photon (scintillation)
strength of flash depends somewhat on Ephoton (some energy resolution)
amplification possible through a PMT
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X-Ray Photon Detection:
CCDs!
• need thicker
substrates to
ensure photon
absorption
• 1 photon → multiple
photoelectrons:
– energy resolution!
(Kitchin 2002)
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Submillimeter and Radio
• (sub)millimeter (microwave)
– 100 µm – 10 mm (3 THz – 30 GHz)
– bolometer arrays (~300 mK operation temperatures)
– telescopes: SMA (submillimeter array), JCMT, ALMA
(Atacama Large Millimete Array)
• radio
– >1 cm (<30 GHz)
– antennae
– Arecibo, VLA (Very Large Array)
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Submillimeter (Microwave)
• bolometer arrays
• thermistor =
ultra-sensitive
thermometer
– converts temperature
variations to electric
signals
(Ge:Ga)
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Submillimeter (Microwave)
• bolometer arrays
• thermistor =
ultra-sensitive
thermometer
– converts temperature
variations to electric
signals
(LABOCA bolometer on IRAM 30 m telescope)
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Radio Detection
• detect electric signal from EM radiation directly
– includes amplitude and phase information
– at MHz frequencies, use a dipole antenna placed at
telescope focus
half-wave dipole antenna
three half-wave dipole antenna
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Radio Detection
• detect electric signal from EM radiation directly
– includes amplitude and phase information
– at MHz frequencies, use a dipole antenna placed at
telescope focus
half-wave dipole antenna
Arecibo 305 m telescope (30 MHz – 10 GHz)
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The Multiwavelenth Milky Way
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Outline
• Overview of previous lecture
– CCDs
• Other kinds of photon detectors
– infrared
– x-ray, gamma-ray
– submillimeter, radio
• Basic imaging data reduction
25
Detector Calibration
(Project 1)
• bias frames
– non-zero bias voltage
– 0s integrations
• dark frames
– equal to science integrations
• flat field frames
– QE of detector pixels is non-uniform in 2-D
– QE is dependent on observing wavelength
• bad pixels
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Reduced Image vs. Raw
Images
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