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Detection of X-Rays

Detector characteristics

Proportional counters

Microchannel plates

Solid state detectors

Microcalorimeters

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Detector Characteristics

Sensitivity

Quantum efficiency Energy resolution

Time resolution

Position resolution

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Sensitivity Fluctuations in background signal:

B1is particle background

is detector solid angle

Ais detector effective area

AB2is rate of X-ray background tis integration time

Sis source flux (counts cm-2s-1)

21 ABBtN

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Sensitivity

Signal to noise ratio of source detection

Limiting sensitivity

tABtB

SAtn

21

AtBABS n

21min

/

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Proportional Counter

X-ray enters counter, interacts with gas emitting photoelectrons

which drift toward anode

E field near anode is high, electrons are accelerated and ionized

additional atoms, original charge is multiplied

Output is one electrical pulse per interacting X-ray

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Energy ResolutionNumber of initial photoelectronsN=E/w, whereE= energy of X-

ray, w= average ionization energy (26.2 eV for Ar, 21.5 eV for Xe)

Creation of photoelectrons is a random process, number fluctuates

Variance of N: N2=FN, whereFis the Fano factor, fluctuations

are lower than expected from Poisson statistics (F

= 0.17 for Ar, Xe)Energy resolution (FWHM) is

E

wF

NE

E N

35.235.2

Energy resolution is usually worse because of fluctuations

in multiplication

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Position Sensing

Need to have drift E field which is parallel

Readout anodes or cathodes are segmemted or crossed

wires are used

Resolution is limited by diffusion of electron cloud

Time resolution is limited by drift time

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SXRP Proportional Counter

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Quantum Efficiency

To be detected, X-ray must pass through window

without being absorbed and then be absorbed in gas

gw

w dtTQ

exp1exp

Twis geometric open fraction of window, tis windowthickness, dis gas depth, s are absorption length for

window/gas (energy dependent)

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Efficiency versus Energy

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Microchannel Plates

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Microchannel Plates

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Solid State X-ray Detectors

X-ray interacts in material to produce photoelectrons

which are collected by applying a drift field

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Charge Coupled Devices

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Charge Coupled Devices

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1

2

3

+5V

0V

-5V

+5V

0V

-5V

+5V

0V

-5V

Time-slice shown in diagram

1

2

3

Charge Transfer in CCDs

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Frame Store CCD

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Pixelated Detectors

CCDs have small pixel sizes,good energy resolution, and a

single readout electronics

channel, but are slow, thin (< 300

microns), and only made in Si.Pixelated detectors have larger

pixel sizes, require many

electronics channels, but are fast

and can be made thick and ofvarious materialstherefore can

be efficient up to higher energies

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Energy ResolutionEnergy resolution obeys same equation as for proportional counters,

but average ionization energy is much smaller than for gases

Material w (eV) Fano

factor

E@

6 keV (eV)

Ar 26.2 0.17 600-1200

Xe 21.5 0.17 600-1200

Si 3.62 0.115 120-250

Ge 2.96 0.13 112

CdTe 4.4 0.11 130-2000

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Microcalorimeters

E = 6 eV

@ 6 keV

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X-Ray Reflectivity

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Grazing Incidence Optics

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Scientific Gains from Imaging

Increase S/N and thus sensitivity

Reduce source area and thus the associated background

Allow more accurate background estimation

Take background events from the immediate vicinity of asource

Enable the study of extended objects

Structures of SNR, clusters of galaxies, galaxies, diffuseemission, jets,

Minimize source confusion E.g., source distribution in galaxies

Provide precise positions of sources

Identify counterparts at other wavelengths

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Gratings

d

m sinsin

= incidence angle, = diffraction angle, = wavelength, m =

diffraction order (1, 2, ), d= groove spacing

For X-ray diffraction needd~ 0.11 m

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Gratings

http://space.mit.edu/CSR/hetgs_diagram.gif

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Chandra

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Reading

Longair: 6.4, 6.5, 7.1, 7.3