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Basic DetectionTechniques

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Detector

• More than ‘sensing device’

• Measuring– ‘Meten is weten’

• Meta information• Counting vs analog

Nrms

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Poisson

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Gaussian

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Accuracy

• Distribution: stochastic measurement process only

• ==Precision

• Accuracy -> no systematic– Hubble– Target shooting

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Statistics

• Mean• Variance• Chi-squared• Median

n i

n i

n i

xsquaredchi

xxn

var

xn

xmean

22

22 var)(sample 1

1

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Moments

Central moments

?0

0

1

3

22

1

0

dxxfxxk

k

• skewness

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Systematic errors

• Instrument environment widest sense– Coal – Parallax– Gaia

• Gal rotation• Pressure

• Model -> none

• Outliers

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Modelling

• Solve

• L2 (least-squares)

• L1 (outliers)

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(In)direct

• Direct– Raindrops– Planet directly

• Indirect– Crop size– ‘systematic’ movement of Centre of G.

• ‘Test particle’

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Measurables

• EM waves

• Neutrinos

• Matter (nuclei -> meteorites & space craft)

• Gravitational waves (<=c)

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Neutrinos

e

e

enp

epn

Weak interaction: electron

neutrinos

Strong interactionTau & muon

neutrinos

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Neutrinos 2

• Long pathlength -> memory

• 1931: Pauli – 1959: e – 1962: new muon

• Indirect

• Icecube

• Ocean

• Moon

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Neutrino 3

• Solar problem

• 1987 SN -> 19 neutrinos (water, proton decay)

• 50000 tons; 11000 PMT (50cm)

• Mass < 2.2eV

                                                                          

                                       

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Cherenkov

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GW

• 10-38 weaker than EM force

• Transparant universe• Tensor (cf vector and

potential)• Helicity +-2 (+-1)

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GW 2• Direct resonant

– Block > 1 ton Al; eigen freq. 1.5Hz– Coincident

• Direct non-resident– Michelson between 2 blocks (multiple reflections)

• Interferometer• LISA, in 2015 5Gm long 3.• Indirect: (but questioned again)

– dP/dt decay in binary pulsar.– Calculated: -2.403(0.002) 10-12 ss-1

– Observed -2.4 (0.09) 10-12 ss-1

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Matter

• Cosmic Rays (later lecture)– Pierre Auger (AR) + Northern– LOFAR

• Meteorites -> history

• Returning spacecraft

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EM radiation

• Energy == wavelength == frequency• Flux• Time variation • Spatial dependence• Polarisation:

– Only ‘directional’ measurement (magnetic field)

• Resolution in all:– Uncertainty – ‘aperture’

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EM radiation

),,,,( mltf

V

U

Q

I

•Not all simultaneous -- choose

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Spectrum

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• 21 cm = 1420 MHz [Hyperfine line, HI]• 1 cm = 30 GHz• 1 mm = 300 GHz = 1000μm• 1 μm = 1000 nm• 550 nm = 5.5 × 1014 Hz [V band centre]• 1 eV = 1.60 × 10−12 erg = 1240 nm• 13.6 eV = 91.2 nm [Lyman limit = IP of HI]• 1 keV = 1.24 nm = 2.4 × 1017 Hz• 1 PHz = 1015 Hz (petahertz)• mec2 = 511 keV

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Sensitivity

Faintest UVOIR point source detected:

• Naked eye: 5-6 mag

• Galileo telescope (1610): 8-9 mag

• Palomar 5-m (1948): 21-22 mag (pg),

• 25-26 mag (CCD)

• Keck 10-m (1992): 27-28 mag

• HST (2.4-m in space, 1990): 29-30 mag

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MeasureFlux is the energy incident per unit time per unit areawithin a defined EM band:f ≡ Ein band/A t(or power per unit area)Usually quoted at top of Earth’s atmosphere

o “Bolometric”: all frequencieso Finite bands (typically 1-20%) defined by, e.g., filters suchas U,B,V,Ko “Monochromatic”: infinitesimal band, ν → ν + dνAlso called “spectral flux density”Denoted: fν or fλ

Note conversion: since fνdν = fλdλ and ν = c/λ,→ νfν = λfλ

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Flux 21 Jy = 10−26 W m−2 Hz−1

[= 10−23 erg s−1 cm−2 Hz−1] non SI

Monochromatic Apparent Magnitudeso mλ ≡ −2.5 log10 fλ − 21.1,where fλ is in units of erg s−1 cm−2 A−1

o Normalization is chosen to coincide with the zero point of the widely-used “visual” or standard “broad-band” V magnitude system:i.e. mλ(5500 ˚A) = Vo Zero Point: fluxes at 5500 ˚A corresponding to mλ(5500˚A) = 0, are (Bessell 1998)f0

ν = 3630 Jy (janskys) or 3.63 × 10−20 erg s−1 cm−2 Hz−1

λ/hν = 1005 photons cm−2 s−1 A−1 is the corresponding photon rate per unit wavelength

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Flux 3

• Absolute Magnitudes o M ≡ m− 5 log10(D/10), where D is the distance to the source in parsec o M is the apparent magnitude the source would have if it were placed at a distance of 10 pc. o M is an intrinsic property of a source o For the Sun, MV = 4.83

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Flux 4

• Luminosity L (W)– Power (energy/s) radiated by source into

4π sterad

• Flux (W m-2)– f = L/4πD2 if source isotropic, no

absorption

• Brightness I (W m-2 sr-1)– f ~ IΔΩ

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Planck

1

12,

/2

3

kThec

hTB

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Planck 2

• Limiting forms:

• hν/kT << 1 → Bν(T) = 2kT /λ2 (“Rayleigh-Jeans”)

• hν/kT >> 1 → Bν(T) = 2hν3 e−hν/kT /c2 (“Wien”)

• Non-thermal– T > 1020

B

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Stars

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IR windows

μm

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Atmosphere transmission

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QEEye 10-20%

Photographic 2-10%

CCD 70-90%

PMT 20-30%

IR (HgCdTe) 30-50%

CMOS 60-80%

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QE(2)

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Spectrum

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DetectorsBolometers

• Most basic detector type: a simple absorber

• Temperature responds to total EM energy deposited by all mechanisms during thermal time-scale

• Electrical properties change with temperature

• Broad-band (unselective); slow response

• Primarily far infrared, sub-millimetre (but also high energy thermal pulse detectors)

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Bolometer

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Detectors 2Coherent Detectors

Multiparticle detection of electric field amplitude of incidentEM wave• Phase information preserved• Frequency band generally narrow but tuneable• Heterodyne technique mixes incident wave with localoscillator• Response proportional to instantaneous power collected inband• Primarily radio, millimetre wave, but some IR systems withlaser LOs

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Detectors 3

Photon Detectors• Respond to individual photon interaction with

electron(s)• Phase not preserved• Broad-band above threshold frequency• Instantaneous response proportional to

collected photon rate (not energy deposition)• Many devices are integrating (store

photoelectrons prior to readout stage)•

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Detector 4

UVOIR, X-ray, Gamma-rayo Photo excitation devices: photon absorption changesdistribution of electrons over states. E.g.: CCDs,photographyo Photoemission devices: photon absorption causesejection of photoelectron. E.g.: photocathodes anddynodes in photomultiplier tubes.o High energy cascade devices: X- or gamma-rayionization, Compton scattering, pair-production

produces multiparticle pulse. E.g. gas proportional counters, scintillators

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Detector 5

• Chemical detectors

• Eye

• Photographic plate

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Eye

• Rods (10-20%)

• Cones (1-2%) – 3 varieties

• 1ps response; 1/20s integration; 15min to revitalise

• Flashes

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Photographic

• - non-linear

• - low dynamic range

• + # pixels

• Photon excites e AgCl -> +Ag- into Ag.(defect)

• Developing == amplification

• Slow (but stroboscopic)

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PMT

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PMT-a

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PMT2

• QE 5-10%

• UV/B poor in R/IR

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MCP

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MCP2

• QE 20%

• Can be staggered (chevron)

• Up to million amplification

• 1-1000nm

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IPCS

• TV: photo electron (from Si) stored in micro-capacitors

• Scanned/recharged 25Hz -> discharge current

• High readout noise (snow)

• 1st intensifier 3 stage million gain

• Read out == photon counting digital

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Image intensifier

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CCD

• Charge Coupled Device

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CCD layout

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CCD transfer

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CCD readout

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CCD

• Workhorse up to 1.1 um -> bandgap• Dynamic range: bits; 30000:1• Linearity: same• Read-out noise 2-3 e-

• Dark current (thermal) -> cool• Shot noise: random photons• Non-uniformity -> flat fielding• Charge transfer efficiency (>.99999 has to be)• Cosmic rays: pixel error

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CCD2

• Large: 10.5 * 10.5 kpixel

• 4 stitched -> 500 million pixels

• Thinned back-illuminated: no reflection

• Thinned very expensive: fragile, but efficient

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CCD perfect?

CosmicraysHot Spots(high darkcurrent,butsometimesLEDs!)BrightColumn(charge traps)

DarkColumns

(chargetraps)

QEvariations

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CMOS

• Complementary Metal Oxide Silicon

• Direct readout

• But: 15-30 photomasks; rather than 10 for CCD

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CMOS 2

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NIR (hybrid)

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NIR

• Similar to CCD

• Non-Si layer to generate photo electrons: HgCdTe and InSb for between 0.9 and 25 um

• Hybrid Si system: well developed

• Cooled to 30-60K

• Si part: CCD or MOS capacitors: direct read-out

•Pixel cost 10* CCD (0.10-0.30 USD)

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SIS – BIB - SSPM

• Superconductor-Insulator-Superconductor tunnel junctions

• Blocked-Impurity-Band detectors

• Solid-State-PhotoMultipliers

• Josephson junctions

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Energy resolving STJ/TES

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STJ

• Fast

• Spectral resolution 1000

• UV->IR

• Cooled < 1K

• Magnetic field + Electric field

• 1 meV electron pair split (1eV for CCD!)

• More depending on energy

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Tip-tilt CCD wavefront

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Info

• C.R. Kitchin, Astrophysical Techniques

(0 7503 0946 6)• http://www.ctio.noao.edu/mailman/listinfo/ccd

-world

• Real life CCD: http://imaging.e2vtechnologies.com

• Experimental Astronomy 2006