Download - Fall 2007 - X-rays and detectors lecture
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X-Ray Astronomy Lab
• X-rays• Why look for X-rays?
– High temperatures– Atomic lines– Non-thermal processes
• X-ray detectors• X-ray telescopes• The Lab
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X-rays
• Measure X-ray energies in energy units (eV or keV) or wavelength units (Angstroms)
• Soft X-rays = 0.1-2 keV• Medium (“standard”) X-rays = 2-10 keV• Hard X-rays 20-200 keV
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PhotonsEnergy of photon is set by frequency/wavelength
λν hchE ==
)Angstroms(
4.12)keV(
λ=E
Unit is electon-volt (eV or keV)
1 eV = 1.6×10-19 J = 1.6×10-12 erg
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Thermal Radiation
Thermal spectrum peaks at 2.7 kT, falls off sharply at higher and lower energies.
Wien’s Law:
Peak of radiation
= 2.9×107 Å/ T(K)
= (0.43 keV) ×(T/106 K)
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Black holes make X-rays• BH of 10 solar masses can have a luminosity of
100,000 times the Sun’s emitted from a region ~ 200 km in radius
• Use Stefan-Boltzman law to find temperature, L = 4πR2σT4
10001
000,100
000,700
1004/12/14/12/1
=
=
=
−−
B
A
B
A
B
A
L
L
R
R
T
T
TA = 1000 × 5700 K ~ 6,000,000 K
Peak at 4.8 Å = 2.6 keV
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Atomic lines
Link to tables of line energies
Photons emitted from transitions to inner electron shells are in the X-ray band
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Non-thermal processes
Particle acceleration in magnetic fields
• Supernova remnants
• Corona of black hole accretion disks
• Radiation from pulsars
• Jet acceleration by black holes
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X-Ray Detectors
• Usually detect each individual photon
• Wish to measure photon properties– Energy
– Number– Time of arrival– Position– Polarization
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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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Energy ResolutionNumber of initial photoelectrons N = E/w, where E = energy of X-ray, w = average ionization energy (3.62 eV for Si)
Creation of photoelectrons is a random process, number fluctuates
Variance of N: σN2 = FN, where F is 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 ==∆ σ
For silicon, F = 0.115, w = 3.62 eV. Energy resolution is often degraded by electronic noise.
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Quantum Efficiency
To be detected, X-ray must pass through window without being absorbed and then be absorbed in gas
−−
−=
gww
dtTQ
λλexp1exp
Tw is geometric open fraction of window, t is window thickness, d is gas depth, λ’s are absorption length for window/gas (energy dependent)
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Charge Coupled Devices
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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 of various materials – therefore can be efficient up to higher energies
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X-Ray Reflectivity
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Grazing Incidence Optics
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The Lab
1. Shine X-rays on sample
2. Measure energies of fluorescent X-rays
3. Determine elements in sample
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Silicon X-Ray Detector
X-Ray Generator
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Setup
Preamp Multichannel analyzer
X-ray source
Target
Si
X
X e-
1. Calibrate MCA eV/channel:Measure spectra of known targets
2. Determine composition of unknown target:
Measure spectrum and identify lines.