1 Adam Woodcraft SUPA, University of Edinburgh Instrumentation for sub-mm astronomy.
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Transcript of 1 Adam Woodcraft SUPA, University of Edinburgh Instrumentation for sub-mm astronomy.
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Adam Woodcrafthttp://woodcraft.lowtemp.orgSUPA, University of Edinburgh
Instrumentation forsub-mm astronomy
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Introduction
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Astronomy at sub-mm wavelengths
Between infrared and millimetreNo strict definition: usually from ~ 200 µm to ~ few mm
Sub-mm astronomy
CSO and JCMT,Mauna Kea,Hawaii
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Example: Eagle Nebula in visible light (Hubble Space Telescope):
Example: sub-mm (850 μm) contours overlaid (SCUBA)
• Sub-mm emission usually “optically thin”; so we see the interior rather than just the surface of objects
Why do sub-mm astronomy?It lets us see cold things - peak in a 10 K blackbody is at 300 μm
Cold things are interesting: usually objects in formation (galaxies, stars, planets…)
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Dominant detector type for photometry (as opposed to spectroscopy) is the bolometer
Bolometers
Absorber
Heat sink
Radiation
Thermometer
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Semiconductorbolometers
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The first bolometer
Bolometer invented by S. P. Langley in 1880 for infra-red astronomy (and luminous insects)
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Cryogenic bolometers
81 years later, F. Low developed the cryogenic (4 K) bolometer using doped germanium as the thermistor
Low temperature operation:• Reduces blackbody background radiation• Increases sensitivity:
• heat capacity is reduced• doped semiconductors can have very large dR/dT
The original application was not astronomy, but soon adopted (along with the inventor) for IR astronomy
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Cryogenic bolometers
Now replaced by photodetectors in IR, but the detector of choice for photometry in the sub-mm:
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Cryogenic bolometers
To get sufficiently good performance, operate at 300 mK or lower
• Makes instruments complex (and expensive)• Much lower than needed in most areas of astronomy
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Cryogenic bolometers
Bolometers are broad-band devices: they respond equally to all absorbed wavelengths
• Have to filter out unwanted wavelengths• Metal mesh filters can be produced with precisely defined bandpasses
Filters Bolometer
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Cryogenic bolometers
For high resolution spectroscopy, astronomers use coherent (heterodyne) systems, as in radio astronomy
• Outside the scope of this review• Also operate at low temperatures and challenging to build
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Cryogenic bolometers
Unlike at optical and infra-red wavelengths, historically few commercial and military applications in sub-mm
Development largely in universities and government labs rather than industry
Cost $2000/pixel c.f.$0.12 for infrared,$0.01 for optical
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Cryogenic bolometers
• Composite bolometer reduces heat capacity further by separating absorber and thermometer
Composite bolometers introduced in 1970s • Reducing thermal conductance increases sensitivity• But also increases time constant• Reduce again by reducing heat capacity• This is the main reason for such low temperatures
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Cryogenic bolometers
Early instruments contained a single pixel
UKT14 (ROE, Edinburgh)
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Bolometer arrays
Arrays appeared in the 1980’s, making better use of telescopes
SCUBA (ROE, Edinburgh)
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SCUBALargest of the early arrays
• 131 pixels• Composite bolometers (sapphire substrate, brass wire thermal link)• Hand assembled from individual pixels
• Arrays (and in particular SCUBA) revolutionised the field
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NTD germanium
Sensitive and uniform behaviour requires uniform doping
• SCUBA and other modern germanium bolometers use Neutron Transmutation Doping (NTD)• Converts 70Ge to 71Ga (acceptor) and 74Ge to 75As (donor)• Since germanium isotopes are uniformly distributed, result is uniform doping and simple behaviour
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Early instruments
IRAM- MPIfR 1.3mm array
CSO-SHARC 350 m array
JCMT-SCUBA 350/450 &
750/850m JCMT-UKT14 350m-2mm
1 20
91
37
37
91
Also 19 pixel 2 mm array at 0.1 K
1986-19961996-
1997-
1998-
.1K.3K
.3K.3K
.1K
.3K
Number of pixels
Operating temperature
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Modern bolometers
Modern bolometers built by micromachining
• Silicon nitride deposited on silicon wafer• Silicon etched to form SiN membranes• Form absorber and supports• Metallisation defines absorber and weak thermal link• “Spiderweb” shape reduces heat capacity and exposure to ionizing radiation
JPL spiderweb bolometers
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Modern bolometers
Either break out into individual detectors, or leave to form an array
But still have to stick germanium chips individually on each pixel
HFI bolometers (JPL/Cardiff)
Spiderweb array wafer (JPL)
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Modern bolometers
Alternative: make thermistors from the silicon itself by ion implantation
• Initial problems with excess noise, but recently discovered it could be removed by using thicker implants
SHARC-II (GSFC/Caltech)
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Modern bolometers
Difficult to multiplex germanium or silicon bolometers without introducing too much noise
• Limits array sizes• “CCD-like” CMOS multiplexed silicon arrays have been produced using very high thermistor resistances to increase signals to partially overcome multiplexer noise
PACS arrays (CEA/LETI)
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Current instruments
Facility instruments on telescopes now
Doesn’t include dedicated PI instruments or CMB instruments
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AzTEC (JPL)144 pixels LABOCA (MPIfR)
295 pixels
NTD germanium arrays
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Current instruments
Facility instruments on telescopes now
Doesn’t include dedicated PI instruments or CMB instruments
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Silicon arrays
SHARC-II (GSFC/Caltech)384 pixels
PACS arrays(CEA/LETI)2560 pixels
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Superconductingbolometers
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Superconducting bolometers
Even without multiplexing, fundamental noise limits reached Solution: superconductors (transition edge sensor; TES)
• Very large dR/dT at transition• But have to keep on transition• Key to use in astronomy was realisation (K. Irwin, 1995) that voltage bias keeps them automatically on transition
0.02
0.04
0.06
0
95.8 96 96.2
Temperature (mK)
Res
ista
nce
(ž)
Copper Molybdenum
Substrate
WarmingResistance
increaseJoule heating
decreasesCooling
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Superconducting bolometers
Has taken ~ 10 years to find and eliminate excess noise sources to make TES arrays practical
Advantages:• Low fundamental noise limits• Can be constructed on an array scale by thin-film deposition and lithography• Can be multiplexed with minimal noise penalty by superconducting electronics
New generation of instruments using TES arrays now in construction and on telescopes
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SCUBA-2
• Eight arrays; 1280 pixels each• Constructed from detector and multiplexer silicon wafer, indium bump bonded together like an infrared array
Detector wafer Indium bump bonds
Multiplexer wafer
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SCUBA-2
Somewhat bigger than predecessors
Detector wafer Indium bump bonds
Multiplexer wafer
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Current instruments
Facility instruments on telescopes now
Doesn’t include dedicated PI instruments or CMB instruments
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MUSTANG
(64 pixels)
SCUBA-2
(1280 pixels installed here)
Facility instruments
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Other arraysOther arrays already on telescope dedicated to CMB work
APEX-SZ 55 x 6 pixels
(on telescope)
MBAC 1024 pixels
(on telescope)
And many more to come…
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BUT
• Multiplexer fabrication is complex, especially for large arrays
• Increasing array sizes further will be very difficult
• Too much power sends detector above transition; no response
• Worrying for a space mission where background unknown, and can’t fix problems
• Semiconductor bolometers work in high background with reduced sensitivity
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The future
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KIDsAlternative technology: Kinetic Inductance Device
• Use superconductor below transition• Radiation breaks Cooper pairs
• like electron-hole pair creation in semiconductor, but with smaller energy gap
• Detect by change in AC inductance• Advantage: can read out many devices with single coax
• Simple detector fabrication• No complex multiplexer to make
• Still need ultra low temperatures though• Looks very promising
Prototype KID camera (Caltech/JPL)
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STJs
Superconducting tunnel junctions use similar principle
• Pair breaking detected by current flowing through tunnel junction which blocks Cooper pairs• Like semiconductor photoconductor• BUT: currently no practical way to multiplex
STJ array (ESTEC/ESA)
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STJ with RF-SET multiplexer
Other technologies
Hot-spot superconducting detectorsQuantum dot devices
QWIPs (quantum wells)
SQPT photoconductor
Cold Electron Bolometers + quasiparticle amplifier
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Antenna coupling
Another area being developed is antenna coupled detectors• Radiation detected by planar antenna• Transmitted to detector by waveguide• Can filter wavelengths electrically rather than optically• One antenna can feed several pixels for different wavelengths
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X and gamma detection
All these technologies can also be used to detect X and gamma rays
• Detect energy pulse from individual photons
• Therefore have energy/wavelength resolution
• Appealing for X-ray astronomy (and industrial applications)
• High resolution and efficiency justify complication of cooling to under 100 mK
• Useful since can share development with sub-mm community
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Optical/IR
They can even be operated at optical/IR wavelengths
• Detect heat from absorption of single photon, and use to determine wavelength!• Unique combination of spatial and spectral measurement along with accurate timing information• Used to measure rapidly varying spectrum e.g. Crab Nebula
Optical TES array (Stanford)
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Conclusions
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ConclusionsThe next few years will be very interesting:
• Many new instruments coming on line
• Not clear which technologies will dominate for the next generation of instruments
One current goal is to produce detectors for a space mission with a cold (5 K) mirror
• Will have to be considerably more sensitive than current detectors
• Different groups developing TES, KID, CMOS multiplexed silicon arrays and many more…
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The End
19831880
2008