IV DETECTORS

Lit.: C.R.Kitchin: Astrophysical Techniques, 2009
C.D.Mckay: CCD’s in Astronomy, Ann.Rev. A.&A. 24, 1986
G.H.Rieke: Infrared Detector Arrays for Astronomy, Ann.Rev. A&A 45, 2007
IV DETECTORS
up to 1837: HUMAN EYE
+:
• relatively high Q.E. (~10%),
• very high dynamic range (1010!),
- :
• no integration capability (! ~ 0.1 s),
• limited spectral bandwidth (400-700 nm),
• rather low spatial resolution
• not suitable for quantitative measurements
1837 Daguerre: invention of photography
first sky object photographs (daguerrotypes):
1840 Moon
1843 Solar Spectrum
1845 Sun
J.W. Draper
“
Foucault +Fizeau
1870 invention of dry gelatine emulsions
this enabled astronomical applications !
photography dominated astronomical detection for more than a century: ~1870-1980
pro’s and con’s of
photographic plate:
Daguerrotype of the Moon,
John W. Draper
March 26, 1840
New York
+:
• ability to integrate (up to many hours)
• large detector area (up to 50x50 cm)
• enormous storage capacity (106 -108 ‘pix’/cm2)
• high resolution (grains ~ 1-10 µm)
• wavelength coverage " ~ 300-1000 nm
-:
• low Q.E. (~ 0.1– 3%, with hypersensit.: <10%)
• non-linearity (Q.E. depends on intensity)
•‘reciprocity failure’: non-linearity in exposure time
• limited dynamic range (< 104)
• emulsion properties depend on batch, processing
IV.1 PHOTOELECTRIC DETECTORS
photoelectric effect
discovery:
Hertz 1887
explanation:
Einstein 1905
first ‘photocells’: ~1910
principle:
solid absorbs photon with h# > h#0 =Elim and emits electron with Eel = h# - h#0
device to measure photon flux
#(photoelectrons) $ #(infalling photons)
electronically: photocell
electron current through resistor % voltage
astronomical application pioneered
by Stebbins & Whitford 1910-1930
! beginning of electronic revolution
in astronomical detection
problem:
currents are very small (e.g. 10-17 A)
! S/N dominated by
thermal resistance noise
‘Johnson noise’:
RMS noise voltage Vn$ (kTR)1/2
schematic of a
photomultiplier tube
solution: 1940-45 development of
photomultiplier tube ! current
amplification without resistance
gain factor 4-5 per dynode stage
! 106-107 amplification for 10 dynodes
! photon counting possible
QE 40%
photomultipliers were the prime detectors for
astronomical precision (spectro)photometry
during the period 1950-1990
20%
10%
1%
+ : • good Q.E. (up to 30%)
• wide "&range (~100-1000 nm)
• linear device
• high dynamic range (~ 106)
• suitable for photon counting
• photon-noise limited (when cooled)
- : • not usable for imaging !
a wide variety of photocathode
"-response profiles was produced by
many different cathode materials:
e.g.: Cs-Sb,
Ag-O-Cs,
Ag-Bi-O-Cs,
Na-K-Cs-Sb,
K-Cs-Sb, etc.
IMAGING PHOTO-ELECTRIC DETECTORS
although photomultipliers as ‘single pixel’ detectors are unsuitable for imaging,
imaging devices based on photoelectric detection have been developed:
1st generation image intensifiers:
• multiplication of photons (but: low gain)
• "-conversion via phosphor properties
• before ~1980: final detection by
photographic plate
Micro-Channel Plates (MCP’s), or
Multi-Anode Microchannel plate Arrays (MAMA’s)
• Q.E. up to 20%, ~106 electrons/photon
• high spatial resolution (10 µm channels)
• many ‘pixels’ (up to ~108)
• well-suited for EUV + soft X-rays !
successfully used in Einstein, Rosat, Chandra
IV.2 SEMICONDUCTOR DETECTOR ARRAYS
creation of photo-electrons: photo-ionisation of a solid
photo-conduction:
photo-excitation in a solid
'Egap
impurity
band
• electron energy levels of atoms in a solid split and
merge into energy bands with many sub-levels
• atoms are kept together by ‘valence electrons’ in the
outer shells; combined energy levels of those shells
form the valence band
• excitation can move valence band electrons to higher
energy levels in the same band, or (if 'E> 'Egap) into
higher conduction bands
• conduction of electrons can occur in valence or higher
bands only if free E-levels are available
• case a) : E-levels in the valence band partly filled
! electrons can move easily: solid is a conductor
• case b) : E-levels in valence band filled, 'Egap is large
! electrons cannot jump to free levels: solid is insulator
• case c) : as in case b), but with small 'Egap; excitation
(thermal or photonic) can now move electrons into the
conduction band: solid is a semiconductor
• impurities cause extra E-levels in-between the bands
of pure material: impurity bands ! electrons can be
excited more easily into the conduction band
simple picture of energy levels in solids
this explains a number of facts:
• photons can excite electrons in
conductors, but these get lost in the
‘sea’ of thermally excited electrons
! no good for detectors
• 'Egap in semiconductors can be
changed by controlled ‘doping’
with impurities
• pure Si (=semicond.) has 'Egap= 1.10 eV
this corresponds to the observed
"-cutoff of 1.11 µm in CCD’s
• for Gallium-doped Si 'Egap drops to
0.07eV ! "-cutoff = 17 µm
• smaller 'Egap requires deeper cooling to
suppress thermal excitation
CHARGE-COUPLED DEVICES - CCD’s
1969 invented by W.Boyle + G.Smith (Bell Labs)
for application in computer memory
~1975 first applications as astronomical detectors
summary of main properties:
• absorption of hv (>1.10 eV) in Si ! excited electron + hole
electrodes on the chip create + potential wells that collect
the electrons, holes diffuse away into the Si lattice
high QE of this process: for " = 500-800 nm ~80% !
• pixels are created by combination of:
grid of pos. electrode strips on the Si
and ( grid of ‘channel stops’ inside the Si
(thin barriers to stop e- transfer along electrodes)
schematic
structure of a
3-phase CCD
• the + electrodes are split into multiple electrode sets
(n=3 for ‘3-phase CCD’); each set has its own voltage
• by ‘clocking’ phased voltage differences after exposure,
the accumulated pixel charges are transferred along the
columns between channel stops into output registers
chargetransfer in
a 3-phase
CCD
• similar charge transfer feeds the charges from the
output register into the output amplifier
• charge transfer efficiency: 99.9999 % !
more facts about CCD’s
• cooling is required to reduce thermal ‘dark current’
but: T) * Egap + (Si: 1.10 %1.15 eV for T 300 %100K)
this leads to reduced red response
Idark$ e-A/kT ! modest cooling is OK (typically ~200K)
• monochromatic light (especially red) will cause
interference in the Si layer (Fabry-Pérot fringing)
this can be reduced by AR-coating on Si surface
broad-band imaging: fringing not a serious problem
(exception: bright sky lines)
spectroscopy: fringing is major flat-fielding problem !
NB: FP-fringing occurs in all 2D detector arrays !
• saturated pixels (25 µm pix: ~0.5x106 e-/pix) cause
charge leakage to nearby pixels (‘blooming’) and
non-linear response
the lower sensitivity level is set by CCD readout noise
(~ few e-/pix/readout) ! together: dynamic range ~ 105
"
FIELD (IFU image slices)
detector fringing in JWST-MIRI spectrometer
• cosmic rays produce high spikes; this limits max. texp
! long texp requires shorter sub-exposures
• the CCD readout chain contains several analog
amplification stages with gain factors ,1 and an ADC
! ADC output number (ADU) , (# detected photons)
this ADU conversion factor needs to be calibrated
• electrodes cover significant part of CCD ‘front’ side
! ‘back-illuminated’ CCD’s have higher QE
• largest CCD’s: 16.8 Mp (e.g. Kodak-16801: 40962 9 µm pix)
‘blooming’ of saturated star image
IV.3 INFRARED DETECTORS
•
most modern IR detector arrays are based
on photoconductivity in semiconductors
•
many different materials are used
•
from semiconductor bandgap
properties we can understand:
Near-IR
% pure materials can be used
Mid/Far-IR % doped materials are needed
•
the "-cutoffs in the table are reflected in the
detectors of the Spitzer instruments:
•
‘BIB’ (blocked impurity band) detectors use heavily doped Si
which would normally lead to high dark current;
to suppress this there is an extra blocking layer of pure Si
•
present maximum sizes of IR arrays:
NIR: 20482; ‘short-wave MIR’: 10242; ‘long-wave MIR’: 2562
•
unlike CCD’s, IR-arrays don’t use charge transfer, but are
read pixel-by-pixel by means of complex readout circuits
and MOSFET amplifiers in an underlying multiplexer (‘MUX’)
! allows non-destructive/multiple readouts
•
‘bolometers’ are sensitive
thermometers based on
change in electrical resistance
after conversion of absorbed
radiation into heat
in the NIR/MIR they have been
replaced by photoconductor
arrays, but semiconductor
bolometers are still used in the
far IR (" > 100 µm)
overview of photoconductor detector arrays
for the NIR/MIR (G.H.Rieke 2007):
-
- IBC = BIB