View
217
Download
0
Category
Preview:
Citation preview
8/10/2019 L03_detectx
1/27
Detection of X-Rays
Detector characteristics
Proportional counters
Microchannel plates
Solid state detectors
Microcalorimeters
8/10/2019 L03_detectx
2/27
Detector Characteristics
Sensitivity
Quantum efficiency Energy resolution
Time resolution
Position resolution
8/10/2019 L03_detectx
3/27
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
8/10/2019 L03_detectx
4/27
Sensitivity
Signal to noise ratio of source detection
Limiting sensitivity
tABtB
SAtn
21
AtBABS n
21min
/
8/10/2019 L03_detectx
5/27
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
8/10/2019 L03_detectx
6/27
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
8/10/2019 L03_detectx
7/27
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
8/10/2019 L03_detectx
8/27
SXRP Proportional Counter
8/10/2019 L03_detectx
9/27
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)
8/10/2019 L03_detectx
10/27
Efficiency versus Energy
8/10/2019 L03_detectx
11/27
Microchannel Plates
8/10/2019 L03_detectx
12/27
Microchannel Plates
8/10/2019 L03_detectx
13/27
Solid State X-ray Detectors
X-ray interacts in material to produce photoelectrons
which are collected by applying a drift field
8/10/2019 L03_detectx
14/27
Charge Coupled Devices
8/10/2019 L03_detectx
15/27
Charge Coupled Devices
8/10/2019 L03_detectx
16/27
1
2
3
+5V
0V
-5V
+5V
0V
-5V
+5V
0V
-5V
Time-slice shown in diagram
1
2
3
Charge Transfer in CCDs
8/10/2019 L03_detectx
17/27
Frame Store CCD
8/10/2019 L03_detectx
18/27
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
8/10/2019 L03_detectx
19/27
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
8/10/2019 L03_detectx
20/27
Microcalorimeters
E = 6 eV
@ 6 keV
8/10/2019 L03_detectx
21/27
X-Ray Reflectivity
8/10/2019 L03_detectx
22/27
Grazing Incidence Optics
8/10/2019 L03_detectx
23/27
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
8/10/2019 L03_detectx
24/27
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
8/10/2019 L03_detectx
25/27
Gratings
http://space.mit.edu/CSR/hetgs_diagram.gif8/10/2019 L03_detectx
26/27
Chandra
8/10/2019 L03_detectx
27/27
Reading
Longair: 6.4, 6.5, 7.1, 7.3
Recommended