High Dispersion XRay Spectroscopy Sep 18, 02

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High Dispersion X-Ray Spectroscopy Sep 18, 02

• Motivation for High Dispersion Spectroscopy
• Limits of Non-Diffractive Spectroscopy
• 3. High Dispersion X-ray Spectroscopy
• crystal spectroscopy Bragg condition
• transmission grating spectroscopy
• reflection grating spectroscopy

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Why High Dispersion Spectroscopy?
High dispersion spectroscopy can resolve emission
lines that may be used to study -- the
physical conditions (T,N) by comparing lines
from the same ion -- the ionization balance
(lines from different ions of the same
element) -- the chemical abundances (lines from
different elements) -- the radial velocity
and red-shift Each diagnostic requires a
characteristic resolving power, R E/?E ?/??
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Resolving Power in X-ray Spectroscopy
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Limits of Non-Diffractive Spectroscopy
Position Sensitive Proportional Counters have
very limited energy resolution RE/?E2.2
CCDs have better energy resolution, but still
limited
R7 (Exception microcalorimeter,
?E 2 eV, large R)
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Non-Diffractive vs Diffractive X-ray
Spectroscopy
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A High Resolution Spectrum of Capella
Z C 6 N 7 O 8 Mg 12 Si
14 Fe 26
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Limits of Non-Diffractive Spectroscopy
Position Sensitive Proportional Counters have
very limited energy resolution RE/?E2.2
CCDs have better energy resolution, but still
limited
R7 (Exception microcalorimeter,
?E 2 eV, large R)
The diffraction of X-rays (just as it is done in
optical wavelengths) can provide high resolving
powers -- crystal spectroscopy -- transmission
grating spectroscopy -- reflection grating
spectroscopy
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Crystal Spectroscopy
X-rays incident onto a crystal will be
preferentially reflected at certain angles.
Bragg condition n ? 2 d sin
? (diagram based on the Focal Plane Crystal
Spectrograph on board Einstein)
A resolving power up to 300 can be achieved, but
the efficiency is very small, 0.1
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Diffractive X-ray Spectroscopy Basic Principles
m? d (sin ? - sin ?)
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Diffractive X-ray Spectroscopy Basic Principles
Rowland Circle
A diverging light beam diffracted by a flat
grating cannot be focused, but ... a grating
with radius 2R will produce a focused spectrum,
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Resolving Power of Spectrometers
CCDs Gratings Calorimeter
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Low Energy Transmission Grating (LETG)
Gold mesh glued on steel structure
R2,000 good low E sensitivity
Yellow
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LETG Mounting Structure
540 gratings (1000 l/mm) mounted on a toroidal
structure
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LETG Operation Scheme
The gratings are located on a Rowland circle and
the X-rays are focused on the ACIS-S or HRC-S
detectors.
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LETG Observation of Capella
12-22 Å 0.5-1.0 keV
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High Energy Transmission Grating (HETG)
Gold gratings supported on thin polyimide films.
Good sensitivity at Egt0.8 keV
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LETG Observation of Capella
6.7 Å 1.85 keV, 19 Å 0.65 keV
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Reflection Grating Spectrometer (RGS)
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Reflection Grating Spectrometer (RGS)
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Example of RGS Observation