June 2 2004 X-Ray Spectroscopy with Microcalorimeters 1
X-Ray Spectrometry with Microcalorimeters
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Electromagnetic Spectrum
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Cassiopeia A in the Optical and the X-Ray Bands
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Cas A
(soft) red (medium) green (hard) blue X-rays
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1 eV
100 eV
10 eV
Energy (keV)
The need for high resolutionX-ray spectroscopy
Astrophysical Plasmas:
Simulation of the emission froma gas at T = 107 K with normalabundances of elements.
An energy resolution of ~ 10 eVis required to begin seriousX-ray spectroscopy and a resolutionof ~ 1 eV is required for completeplasma diagnostics and velocitymeasurements.
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Energy-Selected X-ray Imaging
Cassiopeia A ACIS spectrum
4-6 keV
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Cassiopeia A: Ejecta Knots
• Temperatures are comparable ~ 2 keV
• Si-rich knots have low ionization age (electron density x time)
• Fe-rich knots have ionization ages that are higher by ~50-100
SiS
Ar
Fe L Fe K
Ca
SiS
Ar
Ca
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Physical Conditions Through X-Ray SpectroscopyFe-K lines provide very clean diagnostics.
One such diagnostic: excellent density-independent temperature sensitivity in the range 107–108 Kelvin.
x
y z
w
He-like Fe “triplet”
Energy (keV)
Coun
ts
Expectedwith XRS(12 eV)
ChandraHEG(~ 60 eV)
w
y, x
zNeutral Fe
He-like Fe
H-like Fe
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The X-ray Microcalorimeter
Features high resolution, non-dispersive spectroscopy with high quantum efficiency over K- and L- atomic transition band.
Moseley, Mather and McCammon 1984
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Simple Energy Resolution Argument
• δT = E/C (temperature rise for E deposition)
• C ≈ Nk (N = # of phonons with <kT>)
• N ≈ C/k (fluctuation in N is the “noise”)
• ΔN = √N (Poisson statistics)
• R = E/(ΔE) = N/(ΔN) (resolving power)
• ΔE ≈ kT √N ≈ kT √(C/k) ≈ √(kT2C)
• More carefully, ΔE = 2.35 ζ √(kT2C)
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Spectral Resolving Power:
Depends on thermometer technology
Temperature-sensitive resistance
Resolution limited by thermal fluctuations between sensor and heat bath and Johnson noise.
Doped semiconductor
SuperconductingTransition
E 2.35 kT 2C
T = operating temperature (50-100 mK)C = heat capacity
~ 2 - 4 for doped semiconductors ~ 0.2 for transition edge sensors
For both thermometer schemes a spectral resolution of few a eV is possible!
R (
ohm
s)R
(o
hms)
Temperature
Temperature
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Types of thermometers:• resistive• capacitive• inductive• paramagnetic• electron tunneling
Basic requirements:• Low temperature• Sensitive thermometer• Thermal link weak enough that the time for restoration of the base temperature is the slowest time constant in the system yet not so weak that the device is made too slow to handle the incident flux. • Absorber with high cross section yet low heat capacity• Reproducible and efficient thermalization
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Silicon Pixel
Silicon Support Beams
X-Ray Absorber (HgTe)
Implanted Traces
Implanted Thermistor
Silicon Spacer
.
.
.
Microcalorimeter Arrays
XQC Array: 36 array of 0.5 2 mm pixels.
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X-Ray Quantum Calorimeter Dewar
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Astro-E2Astro-E2 is a powerful X-ray observatory developed jointly by the US
and Japan (Institute of Space and Astronautical Science).
High x-ray spectral resolution throughout energy band where bulk of astrophysically abundant elements exist (O - Ni)
Non-dispersive spectrometers enable imaging spectroscopy of extended sources
Large collecting area for high sensitivity
Very large simultaneous bandwidth
Complementary to Chandra and XMM-Newton X-ray Observatories
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XRT (GSFC & ISAS)XIS
(ISAS & MIT)
HXD(ISAS)
Astro-E2
GSFC/ISASXRS
Focal LengthsXRS - 4.5 m
XIS - 4.75 m
CCDResponse
He-like Fe K Z = 0.01(3000 km/sec)
X-Ray Image
Astro-E2/XRS Simulation of the Centaurus Cluster
Astro-E2 ideal for for obtaining x-ray spectra of extended sources.
Ar
S
Ca
Si
Mg
O
Fe-LNe
Ni
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Developed process to make ion-implanted Si thermistors with deeper profiles using silicon-on-insulator wafers.
Essentially eliminated 1/f noise higher resolution!
Appropriate thermal conductance achieved with thinner Si; no need to perform texturing etch to beams to make them diffusive.
DRIE to form pixels with good mechanical properties.
Ion beam
1.5 m
~ 6 times deeper thermometer
(after anneal)
New Microcalorimeter Design
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E
200
150
100
50
0C
ount
s
5890588058705860Energy [eV]
40
20
0
-20
-40
Res
idua
l
Fit Parameters FWHM: 6.40 ± 0.15 eV E_shift: -19.95 ± 0.069 eV Amplitude: 208.8 ± 4 countsy0: 0.0 ± 0 counts 2: 1.43
Energy (keV)
6.4 eV FWHM
Ion beam
1.5 m
~ 6 times deeper thermometer
(after anneal)
Deep implants using silicon-on-insulator wafers.
625 m pixels
Mn K
Mn K
GSFC
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RTS – Rotating Target Source
continuum X-ray source
X-ray continuum
X-ray lines
targets (one is open for continuum)
rotating target wheel
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target wheel
motor
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NTD Calorimeter with Sn Absorbers
SAOSilver et al.
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1480 1485 1490 1495 1500 15050
20
40
60
80
100
120
Energy (eV)
Cou
nts
per
0.25
eV
bin Instrument Resolution:
2.0 0.1 eV FWHM
Al K1,2
Al K3,4
Toward higher spectral resolution and large arrays: Transition edge microcalorimeters
4.5 eV
SRON, 4.5 eV, 100 s time constant, 30 min acquisition time
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Microcalorimeter Arrays based on Mo/Au TES
Bi
Sharp photolithography
Array of identical 150 micron devices. Soon will make these with 250 and 400 micron “mushroom” absorbers. The Bi absorbers shown are the size of the stem in the mushroom.
Array of identical devices
GSFC
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Energy (keV)
Cou
nts
Energy Resolution = 2.5 eV (FWHM)
Mo/Au TESCompact pixel design (300 m)Continuous membrane thermal isolation
Results from Compact TES Pixels
Paves the way for faster, more robust pixels for the Constellation-X Mission.
GSFC 1024 pixel absorber array