talk on x-ray telescopes · ii. x-ray optical constants nni i1=+ =−+βδβ nn02⋅Θ= ⋅Θsin ...

$: Talk on X-ray telescopes · II. X-ray optical constants nni i*1=+ =−+βδβ nn02⋅Θ= ⋅Θsin * sini complex index of refraction to describe the interaction X-rays/matter δ:$
X-ray telescopes & detectorsJan 10th, 2003

Talk onTalk on XX--rayray telescopestelescopes&&

DDetectors for etectors for XX--raysraysRalf Ralf KeilKeil, MPE , MPE GarchingGarching

(held at the Advisor(held at the Advisor--Seminar Seminar AstrophysikAstrophysik,,““Astrophysical Studies fromAstrophysical Studies from RadioRadio-- to Gammato Gamma--Ray Ray EnergiesEnergies ”, ”, TUM, WS 2002/03,TUM, WS 2002/03,

AdvisorAdvisor: Dr. B. Aschenbach: Dr. B. Aschenbach))


OutlineOutline

I. First remarks

II. X-ray telescopes

III. Detection of X-rays

IV. Looking ahead: Future developments


I. First I. First remarksremarks

Information of X-ray emission has to be quantified

Quantification of four characteristic measures:• the position in the sky• the time of arrival• the energies of the X-rays• the brightness of the source

(determined by collecting all the events during the exposure time)

• polarization


II. XII. X--ray telescopesray telescopes

collimation

reflectance of X-rays / focusing optics

types of telescopes

examples of X-ray mirrors

summary of X-ray mirror performance


II. II. Generic detectors for sky obsGeneric detectors for sky obs..to avoid detection of cosmic raysshield works as second “detector“CR: signal in both devicesX-rays: signal only in detector

Distinguishing possible

to avoid light fromunwanted directionsequipped withmetallic tubes


II. II. CollimatorCollimator: : Example constructionExample construction

very high-Z materialwalls extremely thin

max. aperturebut thick enough to stopX-rays of the highestenergies

usage for very long timepossiblecheap


II. Rotation Modulation II. Rotation Modulation CollimatorCollimator

spinning satelliterotating collimator spinning transmissionbands on the sky

proportional counterbehind collimator

observation:count rate vs. timemodulation of the countrate depends on theposition of the overall FOV

locating X-ray source


II. XII. X--ray optical constantsray optical constants

* 1n n i iβ δ β= + = − +

0 2sin * sinin n⋅ Θ = ⋅ Θ

complex index of refraction to describethe interaction X-rays/matter

δ: reflectivity, changes of phaseβ: absorption ( lin. abs. coeff.)

applying Snell’s law(with n0=1 for vacuum)

Re(n*) < 1externaltotal reflection


II. II. ReflectivityReflectivity of metal of metal surfacessurfaces

The higher the energy, the smaller is the reflectivity

X-rays have to be reflected under flat angles („grazing incidence“)


II. II. Reflectivity Reflectivity on Iridium on Iridium surfacessurfaces

Iridium is partly used for the Chandra mirrors

flat angles are necessary for detecting high energy X-rays!


II. Wolter II. Wolter OpticsOptics

H. Wolter, Ann. der Physik, 1952, NY 10, 94

1951: Hans Wolter’s discovery of amirror configuration using reflective optics

(for developing an X-raymicroscope)

1960: Riccardo Giacconi & Bruno Rossi construction of an„analogous“ set of optics

(not subject to the limitations of fabrication of microscopes)


II. II. parabolic profileparabolic profile of Xof X--ray mirrorsray mirrors

2 2y p x= ⋅ ⋅

( )2p dist focus vertex= ⋅ −

known as Wolter type 0 telescope

perfect on-axis focusing

BUT: off-axis images suffer extremely from the Coma defect


II. II. parabolic profileparabolic profile of Xof X--ray mirrorsray mirrors

1 2

1 2sin sin .h h constΘ Θ= =

The intersection region of each input ray with its corresponding output ray defines the “principal or Abbe surface”

This must be a sphere around the image

Coma:off-axis aberration caused by the different magnification of rays,reflected from various positions of the mirror surface

to get Coma - free focusing mirrorssatisfaction of the Abbe sine condition


II. II. parabolic profileparabolic profile of Xof X--ray mirrorsray mirrorsobeys approximately the Abbe sine condition

• only near the vertex• NOT for grazing incidence angles

the parabolic configuration is ‘useless’ for X-ray telescopes


II. II. TypesTypes of Wolter of Wolter telescopestelescopessolution to the X-ray imaging

Wolter type I, II, and III telescopescombination of a paraboloid and ahyperboloid/ellipsoid mirrormounted confocally and coaxially

the type I and II telescopes can bebuilt very compact(example: UV telescope on ROSAT)the type II telescope resembles theoptical Cassegrain system,the type III is even more compact, but so far it has never been used


II. Wolter II. Wolter OpticsOptics

R. Giacconi & B. Rossi, JGR, 1960, 65, 773

1951: Hans Wolter’s discovery of amirror configuration usingreflective optics

(for developing an X-ray microscope)

1960: Riccardo Giacconi & Bruno Rossiconstruction of an„analogous“ set of optics

(not subject to the limitations of fabrication of microscopes)


The Nobel Prize in Physics 2002

„ for pioneering contributions to astrophysics, which have led to the

discovery of cosmic X-ray sources “

Riccardo Giacconi


minimal focal length Ffor a given aperture Y0 with

Y0 = aperture radiusθ = on-axis incidence

angle

geometric area (per shell) rathersmall because

• just one ring• central X-rays won‘t be reflected loss

solution: nesting many confocalmirror shells !!

II. II. profileprofile of of thethe Wolter Wolter typetype II

( )0 / tan 4F Y θ=


2 28 .effA F L Reflπ θ= ⋅ ⋅ ⋅ ⋅

II. Wolter II. Wolter typetype I I telescopestelescopes

LL

effective area Aeff (per shell) isgiven by

solution: nesting many confocalmirror shells can maximise theeffective (collecting) area !!


II. II. ExampleExample: : TheThe ROSAT ROSAT mirrorsmirrors

black slots: total collecting area = 1140 cm2

focal length = 240 cm

83 cm


segmented thin mirrorsmirror material: Nickel – coating with Au3 moduls with 58(!) shells/moduleshell thickness: 0.5 mm and 1 mm (very close!)focal length = 750 cm, max. diameter = 70 cmeffective Area @ 1 keV = 1475 cm2 /module

II. II. ExampleExample: : TheThe XMMXMM--Newton Newton mirrorsmirrors


II. II. ExampleExample: : The ChandraThe Chandra MirrorsMirrors

full thick shell mirrors – coating with Irextremely high angular resolution of 1.5´´shell‘s thickness: ~ 2 - 3 cmmade of Zerodur(= glass with an expansion coefficient = 0)


II. II. Effective AreaEffective Area

effective Area= total geometric Area * reflectance

XMM-Newton: 5 * Chandra

~ 2.2 keV:gold edge (of reflectivity) clear visible

~ 10 keV:much longer focal lengths needed


II. XII. X--ray mirror performanceray mirror performance

or the difficulty toproduce high-qualitymirrors

to reach higherresolutions:large increase in theMass/Geom. Area rationecessary

better

better

~ an

gula

r res

olut

ion

„stiffness“


II. II. Multilayer mirrorsMultilayer mirrorsStack of alternating layers (materials of low and high Z + index ofrefraction)

operating on the basis of Bragg reflectiond = bi-layer thickness

wide range of thicknessbroadband reflectance

2 sinn dλ θ=


III. III. Detection Detection of Xof X--rays rays

XMMXMM--Newton Newton payloadpayload


III. Detection of XIII. Detection of X--raysrays

Scintillation detectorsGeiger-Müller-counterProportional counters / gas detectorsMicrochannel plate detectors (MCP) Charge-Coupled Devices (CCD)Spectroscopic devices: gratingsX-ray bolometers / calorimeters


III. III. TypesTypes of of detectorsdetectors//spectrometersspectrometers

Detector physical Energy of R=E/∆EInteraction ionization Eion @ 1 keV

Scintillator photo-electron, keV -inner electron

Proportional Counter photo-electron, noble gas 30 eV 2.5

Semiconductor Detector Silicon, electron-hole pairs 3 eV 8

Bolometer/Calorimeter phonons, Cooper pairs few meV 1000

Transition Edge Sensor heat 1000

Gratings diffraction > 1000


III. III. ScintillatorsScintillators & & PhotomultiplierPhotomultiplier

converting X-ray energy into visible light

organic scintillators (plastics)almost exclusively used as anti-coincidence shields

inorganic scintillators (crystals: NaI, CsI, BGO ...)

Alkali halides: NaI, CsI• can be made into large area crystals: NaI(Tl), CsI(Na)• good X-ray stopping power• efficient light producers

photo-electron creates scintillation pulse withdifferent decay times for different materials

total light output proportional to energy input


III. GeigerIII. Geiger--Müller Müller CounterCounter

windowed gas cell filled with a mixture of noble gases Ar, Xe, ...(low ionization potential)detection of X-rays via the photoionization of the counter gasionisation energy Eion ~ 30 eVphoto-electrons are multiplied by high voltage accelerationevents are counted by electric charge pulses at the output amplifiercontaining information on:

• the energies• arrival times• interaction positions of the X-ray photons


III. III. TypicalTypical Proportional Proportional CountersCountersBackground rejection methods

Energy selectionreducing the raw background ratesby a factor 100

Rise-time discriminationless effective as the X-ray energyincreases (@ 6 keV: factor 30)

Anti-coincidence with asub-divided gas cellreducing the raw background ratesby a factor 100

Intrinsic timing resolution limited by

anode-cathode spacingpositive ion mobility

microsecond level

~ µm thin


III. III. TheThe ROSAT PSPCROSAT PSPC~ µm thin plastic entrance window

coated with Al to keep optical light outsupported with thin wires

Gas suppliers


III. III. MicrochannelplateMicrochannelplate detectorsdetectorsmade of microbores

chargecloud

compact electron multipliers of high gainconsists typically of ~ 107 closed packedchannels of common diameter (~ 10 µm)each channel acts as an independent,continous dynode photomultiplierusage: distortionless imaging with very

high spatial resolution

Physical principleX-rays are being converted into photo-electronsacceleration and multiplication through avoltage gradient in a channelfinally, electrons hit a phosphor plate,which then emits visible photons

position-measure of the incident photonenergy resolution = 0 (∆E/E = 1 @ 1 keV)present on EINSTEIN(HRI), ROSAT(HRI)

very goodimaging(0.5 arcsec)

III. III. Microchannel plate detectorsMicrochannel plate detectors


III. III. The ChandraThe Chandra HRCsHRCs2 microchannelplate detectors (0.1 – 10.0 keV)

HRC-I• one 90 mm square detector• effective area: 225 cm2 @ 1 keV• ~ 0.5 arcsec spatial resolution

HRC-S• one 20x300 mm rectangular detector• optimized for use with the LETG

to avoidscattering light


III. III. SemiconductorSemiconductor detectorsdetectors --ChargeCharge--Coupled Devices Coupled Devices ((pnpn--CCDsCCDs) )

Si energy band gap = 1.1 eVaverage efficiency => 3.5 eV/reaction

solid-state imager

higher quantum efficiency thancounters, but more noisy (cooling)

high purity Si doped on both sideswith n-carriers and p-carriers

incoming photon excites electronsinto the conduction band

number of sensitive material stripscorresponding to number of read-out electrodes („pixel“)

accumulation of charge in the pixels

transfer to successive electrodesand read-out by separate rowamplifiers


III. III. pnpn--CCD CCD operating principleoperating principle

view from inside the CCD, from the n-side

X-rays

applying an electric field in the Sisensitive regiontransfer of the created chargesto the gatecycling the voltages of the electrodes

move of the charges to the read-out

e--production


III. XMMIII. XMM--Newton EPIC Newton EPIC pnpn--CCDCCD

6 cm

64 pixel

6 cm

, 2 x

200

pix

el

Electronic board with 64 amplifiers

Front side

Cooling device(T ~ -80 to -90 K)

European Photon Imaging Camera (EPIC)array of 12 back-illum. CCDs (0.1 – 15.0 keV)E/∆E = 20 – 50field of view: 27.5 arcmin diameterpixel size: 150 x 150 µm2 (4 arcsec)2


III. XMMIII. XMM--Newton EPIC MOSNewton EPIC MOS--CCDCCDEuropean Photon Imaging Camera (EPIC),made of Metal-Oxide-Silicon2 x Array of 7 CCDs (0.1 – 10.0 keV)each CCD consists of 600 x 600 pixelsE/∆E = 20 – 50 pixel size: 40 x 40 µm2 (1.1 arcsec)2

field of view: 30 arcmin diameter


III. III. Grating SpectroscopyGrating Spectroscopy

Reflection grating

Rowland Circle

Transmission grating

Reflection at grazing incidence anglesby a grating

Transmission spectrometers

= circles, each with a little transmission gratingmounted on a Wolter telescope


III. III. ChandraChandra Transmission Transmission GratingsGratings


III. III. ChandraChandra and XMMand XMM--NewtonNewton

HRMA: 4 nested mirrorsLETG

• 540 grating elements• energy range: 0.07 – 10.0 keV• spectral E/∆E = 30 – 2000• grating facets alignment: 30 arcmin

HRC-S • readout element for the LETG

telescope: 2 x 58 nested mirrorsRGA - Reflection Grating Array

• 182 grating elements• energy range: 0.35 – 2.5 keV• spectral E/∆E = 200 – 800

RFC - RGS Focal Camera• 2 x 9 BI CCD detectors

RFC

RGA

mirrors

HRMALETG

HRC-S


III. III. ChandraChandra Transmission Transmission GratingGratingLETGLETG

grating material: free standing gold wires (0.5 x 0.5 µm2) on a supporting mesh

diameter of each grating facet: 1.6 cmfacet frame material: stainless steel540 single grating facets mounted on a ring shape frame


IV. Looking ahead: IV. Looking ahead: Future developmentsFuture developments

Bolometer/calorimeterthermal X-ray detectors

Transition Edge Sensors (TES)

The DEPFET principle


IV. IV. bolometersbolometers//calorimeterscalorimeters

energy of an incoming photon directlyconverted into thermal agitation of a crystallattice or of an electron gasamplitude of the thermal impulseproportional to its energy

measure of the temperature variation

for arrival times of photons larger than thecharacteristic time constant of the bolometer

discrimination of individual events

system must be cooled to very lowtemperatures ( ~ 1 mK., mech. refrigerator)

thermal X-ray detectors


IV. IV. TransitionTransition--Edge Sensors (TES)Edge Sensors (TES)

bolometer = absorber of radiation + thermometer (with a cooling device)

one type of thermometer: TESTES = strips of superconducting material~ 96 mK: strong increase in resistivity

decrease in current (for a given voltage)

Physical principleabsorption of an photonsudden increase in temperatureincrease in resistivitymeasure of an electric pulse


IV. IV. TheThe DEPFET DEPFET principle principle DEPFET = Fully depleted active field-

effect transistora new kind of CCD, where every pixel isread-out

very short read-out time

View through one pixel of an DEPFET structure DEPFET matrix layout

ring-like shapethe gate, drain, and source ofeach of the pixels are visible~ 106 amplifiers are present


IV. Modern detectorsIV. Modern detectors

Detector WFI NFI1 NFI2Type DEPFET STJ TES/BolometerNo. of Pixels 1000 x 1000 50 x 50 32 x 32Pixel Size 0.3 arcsec 0.6 arcsec 1 arcsecField of view 5 x 5 arcmin2 1 x 1 arcmin2 1 x 1 arcmin2

Temperature 200 K 350 mK 50 mKResolution 60 eV @ 1keV 1 eV @ 1keV 5 eV @ 8keV

WFI = Wide Field ImagerNFI = Narrow Field ImagerSTJ = Superconducting Tunnel JunctionTES = Transition Edge Sensor

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talk on x-ray telescopes · ii. x-ray optical constants nni i1=+ =−+βδβ nn02⋅Θ= ⋅Θsin ...