KEK Seminar
March 11, 2009

X-ray Calorimeter Arrays for Astrophysics
Caroline Kilbourne
NASA Goddard Space Flight Center

KEK, Tsukuba, March 11, 20091

Outline!! high-resolution astrophysical x-ray spectroscopy!! “micro”-calorimeters–! comparison with other detection schemes, including other low-temperature approaches–! ideal calorimeters and factors that determine their performance–! example implementations!! state of the art–! based on implanted silicon thermistors–! based on superconducting transition-edge sensors–! based on magnetic thermometry!! future development and deployment!! other applications

KEK, Tsukuba, March 11, 20092

What in the Universe makes x-rays?
!! Gas at temperatures of 1 - 100 million degrees.
–! Remnants of exploded stars–! Matter falling into black holes and neutron stars–! Stellar coronae–! Winds from star-forming galaxies!! Electrons accelerated in strong magnetic fields (~1012 - 1014
Gauss).
!! Electronic transitions in partially ionized atoms of atomic number
greater than or equal to 4 (Be).

KEK, Tsukuba, March 11, 20093

SO?
!! That means most of the ordinary matter in the Universe radiates x-rays.
!! And this hot ordinary matter can tell us a lot about the extraordinary stuff
that’s out there, too, because it collects in the gravitational potential
wells created by dark matter, and the relaxed gas in clusters of galaxies
can be used as a “standard candle” to study dark energy.

KEK, Tsukuba, March 11, 20094

X-ray spectroscopy probes four HUGE topics in astrophysics
Black Holes

Dark Energy & Matter-! What is the detailed structureof the inner disk around an
accreting black hole?
-! How prevalent areintermediate-mass black holes?
-! Does dark energy evolve with
red shift?

Cosmic Feedback-! How have massive black holes
affected galaxy evolution?

Life Cycles of Matter-! How do starburst galaxies
enrich the intergalactic medium?
-! What is the nature of matterthat makes neutron stars?
-! How do stellar outflows affectplanet formation?
KEK, Tsukuba, March 11, 20095

Using X-ray Spectroscopy!! Emission line ratios (e.g. within the He-like triplet) provide density andtemperature diagnostics!! Emission and absorption line energies identify ions and determine
Doppler shifts!! Line shapes can be used to study effects such as turbulence or theenvironment of a supermassive black hole!!

Black hole

Energy (keV)
KEK, Tsukuba, March 11, 20096

Chandra and XMM gratings started a new era in x-ray
astronomy, yet…

!! Gratings work by dispersing thespectrum across a position sensitive
detector, but at the expense of
confusion in spectra from spatiallyextended objects (and much of what
we want to observe is spatiallyextended).

!! Gratings have a spectral resolutionthat is a constant !", thus resolving
power degrades with increasingenergy. Fe K lines provide clean
plasma diagnostics, thus resolving
power at 6 keV is needed.
!! Thus, we need an imaging, non-dispersive x-ray spectrometer with
eV-scale resolution. We need an x-ray
camera that can distinguish tens of
thousands of x-ray colors.
KEK, Tsukuba, March 11, 20097

Non-dispersive spectroscopy – measuring energy directly
Non-equilibrium:!! Absorbed energy goes into quantized excitations.
–! Each excitation has energy much greater than kT.
–! These excitations are then counted to determine the energy.

!! Since, invariably, some of the energy goes elsewhere, such as into
heat, the ultimate energy resolution is determined by the statistics
governing the partition of energy between the system of excited statesand everything else.
!! This is the operating principle behind most photon and particle
detectors.
!! In order to improve the resolution by improving the measurement
statistics, a large number of excitation quanta is required.
!! This, in turn, requires low temperature operation.
–! Superconducting tunnel-junction devices made of small-gap
superconductors are based on this principle.

dE / E " N N

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Non-dispersive spectroscopy – measuring energy directly
Equilibrium:!! The energy is deposited in an isolated thermal mass and theresulting increase in temperature is measured.
–! At the time of the measurement, all of the deposited energy has become
heat and the sensor is in thermal equilibrium.

!! The ultimate energy resolution is determined by how well one canmeasure this change in temperature against a background ofthermodynamically unavoidable temperature fluctuations.
!! THIS IS CALORIMETRY*.
–! Among low-temperature detectors, a calorimeter is a device that, at least
ideally, determines, through a thermal measurement, all of the energy
deposited in it in an impulse. (*I understand that in high-energy physics,
a calorimeter is something else, entirely.)!! Low temperature operation is required in order to minimize these
thermodynamic energy fluctuations.

KEK, Tsukuba, March 11, 20099

Choosing equilibrium or non-equilibrium scheme!! speed vs. resolving power (very generally)–! detectors based on non-equilbrium scheme can be made faster
before sacrificing resolving power–! detectors based on equilibrium scheme can achieve the bestenergy resolution at a given temperature in a small (low heat
capacity) device, especially at higher energies!! world astrophysics community planning several satellites thatwill place non-dispersive x-ray spectrometers behind focusing
x-ray optics; each one is based on a calorimeter array.
–! based on laboratory demonstrations of x-ray calorimeters, in avariety of implementations

KEK, Tsukuba, March 11, 200910

Microcalorimeter basics!! Thermometers can be based on: resistance, capacitance, inductance,paramagnetism, electron tunneling, thermoelectric effect!! Because the dominant noise term has the same power spectrum as
the signal, the measurement accuracy is set by the bandwidth ofmeasurement, which is typically set by other noise terms that
dominate at high frequencies and by the detector response (see next
slides…)
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due to movement ofenergy across the weak
thermal link – from10
thermodynamics
1

0.1

0.01

0.001

Signal (e.g. V)
Temperature fluctuations
in signal units (e.g. V/sqrt(Hz))

0.0001
0.001

0.01

0.1

1

10

100

1000

frequency

frequency (arbitrary scale)
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12

10

1

0.1

0.01

0.001

0.0001Signal (with thermalization time)
Temperature fluctuation noise
White noise

1 0-5
0.001

0.01

0.1

1

10

100

1000

frequency

frequency (arbitrary scale)
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Calculating expected energy resolution!! A fundamental limit on the energy resolution can be calculated for cases
in which the thermal fluctuation noise is the dominant noise term at low
frequencies, and the noise term that sets the bandwidth is also intrinsic
to the detector (thus above amplifier or environmental noise terms).
!! For an ideal resistive thermometer, the following scaling holds as longas the sensitivity is high enough that thermal noise dominates Johnson
noise at low frequencies.
–! # = dlog(R)/dlog(T)!! Because the signal of a thermistor is fundamentally a resistance change,
but is read as a voltage or current, resolution depends on the bias
power. Increasing the bias increases the sensitivity, but also heats thedevice. Thus, there is an optimal bias.
!! To set the scale: for eV-scale energy resolution, and T in the range 60 100 mK, need C ~ 0.1 pJ/K for # = 10. C can be increased to 1 pJ/K if #= 100. These values are readily achieved in designs for astronomicalcalorimeters.
KEK, Tsukuba, March 11, 200914

Resistive thermometers

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Silicon thermistor-based microcalorimeters!!

Thermistors are ion-implanted (P, B) and annealed!!

IC and MEMS techniques are well established for Si!!

Arrays are fabricated at a high level of integration–!

Presently the x-ray absorber material (HgTe) is still attached in a
separate step–!

Separate absorbers are needed for high fill factor, increased quantum
efficiency, and because the silicon of the thermistor does notthermalize well•!

energy trapped in long-lived states

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Silicon-thermistor calorimeter array of !"#/XRS (launched 2005)

Suspended thermistors sit over individual wells in the silicon frame. HgTe absorbers were attachedmanually, one at a time. The heat sink was controlled at 60 mK; the thermistors ran at ~75 mK.!
KEK, Tsukuba, March 11, 200917

No deviation fromGaussian down tobetter than 1% of peak!

29 hours of data –Co-added XRS array:
5.68 eV ± 0.03 eV
at 6 keV!
4 arrays completed and tested, resolution 5.3 - 6.5 eV FWHM with 1-2 outliers
per array. In-orbit performance of XRS on Suzaku: 7 eV. (Regretably, instrument
was not able make any astrophysical measurements due to premature loss of liquid helium.)
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Improved silicon thermistor-based calorimeter array for Astro-H
!!
!!
!!
!!
!!

HgCdTe from EPIR thermalizes well, but has substantially lower
specific heat
Instrument designed for lower base temperature (50 mK)
Better energy resolution despite larger pixel size (increased area
by factor of 1.7) realized in the laboratory
Demonstration of feasibility of improved heatsinking suggests
improved resolution can be maintained in orbit
Designing an 8x8 array

KEK, Tsukuba, March 11, 200919

Superconducting transition-edge sensors (TES)!! Stable operation in the transition via
electrothermal feedback (changes
in temperature cause change inresistance and a change in Joule
power that opposes the change intemperature)!! Use proximity-effect bilayers for Tc~ 0.1 K–! E.g. Mo/Cu and Mo/Au

KEK, Tsukuba, March 11, 200920

TES vs. Thermistor (Part 1)!!|#| < 10 vs. # > 100!!Increasing # increases the measurement bandwidth, improving resolution,
except, this # is only good over a small temperature range.
!!We need to increase C to stay within the linear regime. So we choose C tomatch the maximum energy. C = E/dT ~ #E/T!!For measurement of low energy quanta, full advantage of the higher
sensitivity can be made.
!!For x-rays, TES’s require larger heat capacities than the heat capacities ofuseful thermistor x-ray calorimeters, which have been designed to meetquantum-efficiency and resolution requirements. By a quirk of nature,
TES’s require heat capacities that are larger by about the same factor that# is larger. Thus, TES’s and thermistors are capable of delivering about
the same energy resolution.
!!But, the larger budget for C allows considerable design flexibility! We canchoose materials (LIKE GOLD!) that thermalize well and can be depositedas part of the detector fabrication.
KEK, Tsukuba, March 11, 200921

TES Anatomy
Normal metalfeatures toreduceexcess white
noise

Nitride thermal linkdemonstrates
ballistic transport –G depends on
perimeter but noton extent

Silicon at
50 mK
Sensor

Leads
KEK, Tsukuba, March 11, 200922

Design of reliable absorber contacts!! Touch in regions that are already normal!! Constrain contacts so that current will not be shunted through a high-conductivity absorber (high conductivity needed for uniform thermal
response across absorber)!! Provide additional contacts outside the TES for mechanical stability.

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Robust array construction

KEK, Tsukuba, March 11, 200924

Earlier test arrays using electroplated Au and Au/Bi absorbers!! Evaluated different geometries forthe absorber contact area!! Engineered to neither shuntcurrent through the absorber norchange the properties of the TES!! Robust process!! Routinely produce arrays ofdevices with better than 3 eV
resolution at 6 keV!! dTc ~ ± 0.1 mK for devices of thesame geometries!! Tc offsets between pixels ofdifferent types made common
biasing (to reduce number of wires)
impractical.
KEK, Tsukuba, March 11, 200925

Resolution with Au absorbers!! 13 pixels (“T” and “J” design) on 3 arrays have demonstrated resolution
better than 3.2 eV FWHM at 6 keV. (Bias point of each pixel not optimized.)!! Differences between results of different acquisitions on same pixel seen, due
to combination of statistics and interference from activity in the lab

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Au/Bi Absorbers (~ 1 µm Au, 4 µm Bi)

We made data cuts based on the DC level of thesignal channel prior to a pulse, which isrepresentative of the TES temperature justbefore the x-ray is absorbed. Such a cut
removes sensitivity to slight variations in the
TES bias point that may be due to any of anumber of systems issues. Thus, 1.8 eVshould represent the intrinsic resolution of thisdetector, for which we measured 2.1-eV
resolution without the data cuts.
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!! Choice of absorber contact

Uniform array!! No significant difference in best
resolution achieved in “J” and“T”.
!! “T” design requires expanding
the membrane on one side of
the TES (whereas “J” requires it
on both sides), thus leaving
more of the thick siliconbetween pixels!! Choice of absorber composition

250 µm
KEK, Tsukuba, March 11, 2009!! Can use Au/Bi composition to
trade-off heat capacity andthermalization!! Both layers electroplated!! 2.5 µm Au / 3 µm Bi chosen foruniform arrays used for array-scale read-out demonstration28

Array-scale read-out using NIST time-division multiplexing(Irwin, Doriese)!! 2 x 2 array is shown as
example of N-row by M
-column array

Eachcoloredblock is 1
pixel!! TDM operation:–! each TES coupled to its
own SQ1–! TESs stay on all the time–! rows of SQ1s turned on
and off sequentially–! wait for transients tosettle, sample ITES, moveon–! SQUIDs are nonlinear
amplifiers, so use digital
feedback–! Ver sampled, VFB stored
for next visit to pixel
KEK, Tsukuba, March 11, 2009–! each column: interleaved
data stream of pixels

29

Why multiplex?
Multiplexing advantage for 32 x 32 array (1024 pixels):!! Not multiplexed:–! 7 wire pairs per pixel–! 7168 pairs total–! 1024 electronics channels and FB loops!! Multiplexed:–! 1 pair per row–! 6 pairs per column–! 224 pairs total–! More manageable electronics–! Lower heat load on cryogenic stages
KEK, Tsukuba, March 11, 200930

Demonstration of 2x8 multiplexing of high-resolution TES pixels

KEK, Tsukuba, March 11, 200931

Single-TES, multi-absorber devices!! 4 or more absorbers connected to single
TES through different thermal links;
absorber is identified by pulse shape!! achieved 6 eV resolution at 6 keV in a 4-pixel device

view from back throughnitride membrane
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International X-ray Observatory (~2020)!! Reference design for IXO XMS instrument includes a >3000-pixel arrayusing superconducting transition-edge sensor (TES) microcalorimeterswith multiplexed SQUID readout (40x40 core with 2.5 eV resolution,
surrounded by multi-absorber devices)–! we’ve begun testing a 32x32 array at GSFC now!! TES vs. thermistor (part 2): Despite the in-flight proof of silicon-thermistor technology and its planned used for Astro-H,TES
technology is necessary for larger arrays of higher-resolution elements
for the following principal reasons:–! the already demonstrated capability of SQUIDs to be multiplexed–! the higher heat capacity, and thus greater design flexibility, afforded by theirhigher sensitivity–! already demonstrated superior resolution (though just barely)–! the ability to optimize for high resolution at somewhat faster speeds
KEK, Tsukuba, March 11, 200933

Another promising approach– the metallic magnetic calorimeter (MMC)
X-ray

Au:Er
paramagnetic
sensordc SQUID

international MMC collaboration
led by S. Bandler (GSFC)
includes Brown, Heidelberg,
NIST/Boulder and PTB/Berlin

weak thermal linkbath!! No heat dissipated-! real advantage over resistive calorimeters for large arrays-! pixels in TES arrays presented dissipated 4 – 8 pW each!! Sub-eV resolution possible at 6 keV
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Best single-pixel MMC resolution - hand constructed in Heidelberg(definitely in the same league as best Si-thermistor and TES-based devices)
SQUID
junction

50 µm

Au absorbers spot-welded
within pickup coils:
180 µm x 180 µm x 5 µm
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MMC arrays!! magnetic-thermometer-in-SQUID approach cannot easily be extended toarrays–! yield higher if SQUIDs and sensors fabricated separately–! magnetic crosstalk a problem for close packed arrays–! design requires a magnetic field applied to the sensor material at each pixel!! for close-packed arrays, meander geometries are more promsing–! arrays of superconducting Nb meanders onto each of which a layer ofmagnetic material (Au:Er) is deposited–! when a current passes through the meander, a magnetic field is produced in
the magnetic material. No additional applied field required.
–! when an x-ray is absorbed, the heating changes the magnetic permeability,
and, thus, the inductance of the meander–! meanders placed in parallel with input coils of read-out SQUIDs, thus achange in inductance of the meander changes the current both through themeander and through the input coil of the SQUID
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Read-out of MMC meander geometry

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Geometry of MMC pixel with meander!! 2d geometry with high filling-factor!! every part of sensor is in close
proximity to a wire that connects to
the SQUID pick-up coil!! No external field coils needed!! Negligible magnetic cross-talkbetween pixels - ~10-5!! Reduced pickup of magnetic
Johnson noise!! Thinner films needed - easier tofabricate

KEK, Tsukuba, March 11, 200938

Experimental MMC array layout is similar to TES array!! thermal link is metallic; membrane used to impede loss of energetic
phonons prior to thermalization, not as weak link
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MMC arrays fabricated at GSFC

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Best results in MMC meander!
FWHM = 5.4 eV +/- 0.2 eV!! limited by oxidation of Er, which resulted in lower temperaturesensitivity than designed!! can also improve through lowering absorber heat capacity
KEK, Tsukuba, March 11, 200941

Optimization of MMC pixels – fundamental and practical!! fundamental band-limiting noise in an MMC comes from thermodynamic
exchange of energy between the spin and electron systems, leading to:!! however, for realistic thermometer sensitivities, the SQUID noise is higher
than the internal thermal fluctuation noise.

!! Nonetheless, for Cabsorber = 0.2 pJ/K at 40 mK, still predict energy resolution= 0.53 eV (FWHM) for $1 = 3 ms (= 0.91 eV (FWHM) for $1 = 300 µs)
KEK, Tsukuba, March 11, 200942

X-ray calorimeter arrays for astrophysics – scale vs. time!! Astro-H–! 2013–! 32 – 64 pixels–! silicon-thermistors and HgCdTe absorbers!! IXO–! 2020?
–! kilopixels–! TES with normal-metal absorbers!! Gen-X–! 2030?
–! megapixels–! MMCs?
–! readouts need to move in step with detector arrays!! hopefully there will be opportunities for smaller, targeted missions(outside of these observatory class satellites)–! MIT-led micro-X sounding-rocket payload will have 128 TES channels
scheduled for first flight in 2011
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Other applications?
!! ground-based x-ray spectroscopy–! arrays not generally needed for imaging, but to increase the area of high-resolution sensor–! materials analysis–! plasma physics•! GSFC has facility calorimeter instrument at LLNL EBIT!! other energy quanta–! other photon bands–! particle spectroscopy–! ….

!! versatile concept–! whether feasible for any given application depends on the requirementsof the experiment and practical considerations

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GSFC x-ray calorimeter team!! Joe Adams!! Wen-Ting Hsieh!! Simon Bandler!! Rich Kelley!! Regis Brekosky!! Caroline Kilbourne!! Ari-David Brown!! Scott Porter!! Jay Chervenak!! Jack Sadleir!! Megan Eckart!! Stephen Smith!! Fred Finkbeiner!! Thomas Stevenson

on TES and MMC systems, collaborating closely with!! NIST – Boulder: Randy Doriese, Gene Hilton, Kent Irwin,
Carl Reintsema, Joel Ullom!! PTB: Joern Beyer!! MIT: Enectali Figueroa
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