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Generation-X Microcalorimeter Development & Roadmap

The Generation-X Observatory with 50 m2 Effective Area and 0.1" spatial resolution will:

Detect the first Galaxies, Stars and Black Holes Trace the Evolution of Structure, Black Holes and Galaxies and the Elements they

Produce from the Earliest Times to the Present Epoch Probe the Behavior of Matter in Extreme Environments

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The Cryogenic Microcalorimeter Technology Roadmap

1. Introduction

2. Microcalorimeter requirements for Gen-X, and requirement drivers2.1 Baseline requirements and goals 2.2 Discussion of the origin of derived requirements2.3 Strawman microcalorimeter design

3. Overview of technologies

3.1 Basic microcalorimeter detection concepts 3.2 Detector sensor technologies under study for Gen-X

4. Details of technology challenges, and some possible approaches for overcomingthem

4.1 Energy sensitivity 4.2 Count rate 4.3 Field of view and pixel size4.4 Quantum efficiency 4.5 Position sensitive detectors 4.6 Multiplexing 4.7 Heat-Sinking of arrays 4.8 Wiring of pixels within arrays 4.9 Filters

Other possible items that could be added later – non-essential currently4.10 Cryostat design and wiring 4.11 ADR mechanical design4.12 Electronics design4.13 Mass and Power4.14 Telemetry requirements4.15 Background and anticoincidence detector

5. Recommendations for development schedule, technology gates, and schedule anddistribution of funding

5.1 Development schedule and technology gates5.2 Schedule and distribution of funding

6. Acknowledgements

7. References

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1. Introduction

The Generation-X Mission is an X-ray telescope designed to study the new frontier ofastrophysics: the birth and evolution of the first stars, galaxies and black holes in theearly Universe. X-ray astronomy offers an opportunity to detect these via the activity ofthe black holes, and the supernova explosions and gamma ray burst afterglows of themassive stars. However, such objects are beyond the grasp of current missions that areoperating or even under development. The Gen-X team has conceived a mission based onan X-ray observatory with 50 m2 collecting area at 1 keV (500 times larger than Chandra)and 0.1” angular resolution (several times better than Chandra and 50 times better thanthe IXO resolution goal). Such a high-energy observatory will be capable of detecting theearliest black holes and galaxies in the Universe, and will also study the chemicalevolution of the Universe and extremes of density, gravity, magnetic fields, and kineticenergy that cannot be created in laboratories.

Below is a summary of the mission requirements based upon the science requirements :

Parameter BaselineEffective Area (@1 keV) 50 m2

Angular Resolution 0.1" HPD on axisEnergy Resolution (@1 keV) [XGS] E/dE=3,000Energy Resolution (@ 6 keV) [XMS] 2 eV FWHMBackground (0.5 – 2.0 keV) [XMS & WFI] 0.004 cts/ks/arcsec2

Energy Range [XMS & WFI] 0.2 – 10 keVField of View [WFI] 5 arcmin radiusTime Resolution [WFI & XMS] 50 msCount Rate Limit [WFI] 100 cts/sec/pixSky Availability 70%Minimum Telescope Slew Rate 1 degree per minuteCalibration [XMS, WFI, XGS] 3% absoluteLaunch Vehicle and Orbit Ares V to Sun-Earth L2Launch Date 2030+Minimum Lifetime 5 years

Table 1 : The Gen-X mission requirements. The instrument on Gen-X expected to meet a givenrequirement is given in parentheses.

The baseline configuration concept is one that has a whopping16 meter-diameterdeployable x-ray optic that has approximately 50 m2 effective area at 1 keV. Prior tolaunch it is stowed within the 10 meter diameter fairing of an Ares V. The focal length ofthe optic is 60 meters, and is held relative to the spacecraft bus and science instrumentswith an extendible optical bench. Alternative options are also under consideration, suchas one that has four 8 meter-diameter mirrors, each with an effective area of 25 m2 at 1keV. The current strategy to achieving 0.1” angular resolution is to use Piezo-electricbimorph actuators on 0.2 mm thick glass mirrors. These mirrors are of the type developedfor IXO, which then have their figure adapted to the degree necessary to achieve the

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required angular resolution. The development of the actuators are part of an existingmirror technology development program that is at the heart of what is necessary for Gen-X. It should be noted that 0.1” is close to the limit of the angular resolution that isachievable using Wolter I optics, and off-axis there will likely be a blur in the angularresolution which is a function of the off-axis angle.

Currently three different types of detectors (or “science instruments” as this mission likesto call them) are envisaged. These are a calorimeter (XMS – X-ray MicrocalorimeterSpectrometer); a wide field imager (WFI) which is currently envisaged as a large areaactive pixel sensor array; and a reflection grating spectrometer (XGS) to achieve greaterresolving powers at energies below 1 keV.

Figure 1. The Generation-X instrument layout.

One possible focal plane layout is shown in Figure 1. Here the green rectangle representsa table that would translate to bring the different instruments to the telescope optical axis,at different times. Its similarity to the current IXO concept is very evident.

Figure 2 shows a logical diagram of these three science instrument components. Thecalorimeter and active pixel array are distinct instruments, only one of which wouldoccupy the focal plane at any time. The blue central square represents a microcalorimeterarray, the 15' square show a possible active pixel imager (API) array, and the horizontalline may be either an extension of the API or a distinct detector based on similartechnology, with the purpose of reading out the reflection grating.

Fig. 2: The Gen-X focal plane layout

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While the International X-ray Observatory (IXO) already seems quite far into the futurein terms of its technology program, the microcalorimeter design is relatively advancedand close to meeting the goals for this mission. Many may think that they will likelyretire before Generation-X comes close to launching, that it is too far off forconsideration now. However it is the next decade that is the prime time for establishingthe technologies that will be necessary to turn this vision into reality. It is thereforeimportant to consider now the various options for future groundbreakingmicrocalorimeter development that will enable the next leap in capability for the X-raycommunity. The next results of the upcoming decadal survey will have a strong impact inrecommendations for technology funding in the decade to come. It is impossible to saynow, with any confidence, what the best path forward will be. To say that the XMSrequirements are challenging is major understatement. Indeed as currently stated, theymay be even be impossible to meet over the next two decades. The research program thatwe describe here will enable us to learn just how close we can get to fulfilling the dreamsof Generation-X science, and which requirements, if any, will eventually be needed to betraded off against each other to create the strongest instrument.

The purpose of this document is threefold. First, it introduces Generation-X tomicrocalorimeter scientists, and establishes the various mission requirements andpriorities focusing predominantly on the XMS instrument requirements. By keeping thisdocument up to date, we will have a traceable record of the origin, evolution and detailsof these instrument requirements. Second, it is useful for Generation-X astrophysicsscientists by clarifying the different XMS capabilities and options, including the trade-offs involved and levels of difficulty that are introduced by the various differentrequirements. Third, we discuss different challenges that need to be overcome, to meetthe science requirements that Gen-X has established. This includes some comparisons ofthe promise of different approaches, and lays out strategies and a possible roadmap fortechnology development in the next decade. We attempt to be as objective as possible,welcoming input from all members of our community as we evolve this roadmap,hopefully building a general consensus within this detector community of the mostpromising and effective paths forward. We have attempted to develop milestones andgates in ways that do not unnecessarily rule out new and alternative approaches to thosementioned.

In section 3 we will present some background information on general concepts of lowtemperature particle detection, and some of the different technologies used to read outmicrocalorimeters and other low temperature detectors. Section 4 will describe in detailsome of the most important technical developments that will be necessary to bring theGeneration-X XMS concept to TRL-6. The XMS we will need to have enoughsensitivity, dynamic range, speed, and angular resolution to meet the missionrequirements. The read-out will need to be extendable to handle hundreds of thousands ofpixels. We discuss the merits of different possible approaches for meeting these goalsbased upon our current state of understanding of different technological approaches, andwe make some recommendations as to the most promising ways forward. We will also beconsidering approaches necessary to achieve the required heat-sinking of the focal plane

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array (which is dependent upon the sensor technology) and strategies for achieving this.We will also present strawman plans for accommodating the likely large heat loads fromthe various read-out options; we will develop strategies for achieving high density wiringwithin focal plane array and between the array and the readout. We will also develop astrategy to handle the power necessary for x-ray pulse processing on board the Gen-Xspacecraft.

In Section 5 we will present a possible schedule for this development program, includingmilestones, gates, and a proposal for measures of the TRL levels with respect toGeneration-X goals. And finally we present some preliminary cost estimates for all thesetechnology developments. More information about the Gen-X science program and theGen-X mission concept can be found at : http://www.cfa.harvard.edu/hea/genx/

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2. Microcalorimeter requirements for Gen-X, and requirement drivers

2.1 Baseline requirements

DesignParameter

Requirement Goal Assumption or Comments

Pixel Pitch 0.1” 0.03” For the 60 m focal length, 0.1” pixels are 30 µmin size.

Field ofView

3’x3’ 3’x3’ 3’x3’ => 1800x1800 array of 0.1” pixels=> 3.24 x 106 pixels.(54 mm x 54 mm at 60 m focal length).

Energyresolution

2 eV FWHM 1 eV FWHM This is the requirement for energies up to 6 keV.This energy resolution requirement may degradeto 3 eV FWHM up to 10 keV.

Count Rate 1 cps/pixel 10 cps/pixel

Quantumefficiency 0.60 @ 6 keV 0.80 @ 6 keV

This QE is for energies above 1 keV. When onefactors in filter transmitivity, this drops at lowerenergies.

EnergyRange

200 eV – 10 keV 100 eV – 10 keV

Timingresolution

50 µs 10 µs

Calibration 0.5 eV – absolute 0.1 eV – absolute

Backgroundrate

0.004cts/ksec/arcsec2

This is the desired residual background afterantico vetoing in the 0.5-2 keV energy range.

Table 2: Microcalorimeter requirements

2.2 Discussion of the origin of derived requirements

Pixel pitch and field of view : There remains some ongoing discussion of how much thepoint spread function should be sampled. Ideally it would be over-sampled by at least afactor of 3 to avoid degrading the image quality. Certainly this level of over-sampling is arequirement for the Gen-X wide field imager (active pixel sensor), which thus satisfiesthe mission requirement. This remains a goal for the XMS but not a requirement. Wehave agreed within Gen-X that the requirement for the XMS will be pixel sizes thatmatch the size of the point spread function 1:1 in order to keep the pixel count moremanageable, and the pixel sizes practical. i.e. 0.1” pixels, while keeping the goal at 0.03”pixels. We also need to take into account the fact that the optic will attempt to have 0.1”angular resolution on axis, but is likely to degrade as a function of off-axis angle.VanSpeybroeck and Chase (1972) give an empirical fit to ray-traces for a Wolter Imirror. Taking the effective-area-weighted average at 1 keV the off-axis blur leads to ahalf-power diameter that varies with off-axis angle as shown in the estimate in the

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following figure (although there are some caveats).

Fig. 3 : The off-axis blurring of Wolter I and Wolter-Schwarzchild mirrors.

Also shown is the blur due to a Wolter-Schwarzchild mirror, which will likely be muchharder to produce than the Wolter I. Although this blurring is only an estimate, it doesserve to guide the expected scale of degradation within the XMS field of view. In order tobetter match the pixel sizes to the blurring of the optic, it might turn out that larger pixelsizes can be used for larger off-axis angle in order to reduce the number of pixels.However at this stage, for most of the strawman options considered in the next section,we consider a fixed pixel size that is 0.1” of the whole array. This is partly for simplicity,partly due to difficulties in reducing the pixel sizes when the absorbers are discrete, andpartly because on axis some of the strawman options are already not fully sampling thepoint spread function, but better sample it at larger angles. A more promising approach toimproving the effective pixel pitch is use continuous absorber position sensitivedetectors, which will be described more fully in sections 3.2 and 4.5. This should enablethe microcalorimeter array to meet the pixel pitch goal in one dimension for the one-dimensional continuous position sensitive detector (strip detector) approach, or in bothdimensions for the two-dimensional position sensitive detector approach (in strawmanoptions 3 and 4 in section 2.3). For 2-dimensional position-sensitive detectors, the goal of0.03” pixels might be achievable. It they were to cover the entire array then we wouldeffectively have 5400x5400 = 29 x 106 pixels. However, these continuous absorberapproaches have other drawbacks that may make them less attractive options, as we willdiscuss in section 4.5. Ultimately, it may be that the best option for Gen-X to have aseparate part of the array that has continuous absorbers that can be moved on-axis formeasurements that require the greatest possible angular resolution (and filling factor).Thus the possibility of including continuous absorber position sensitive detectors remainsa technology option that should be investigated further.

Energy resolution and energy range: The energy resolution of microcalorimeters isgenerally constant as a function of energy for temperature excursions within the

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calorimeter for which the signal response is a linear function of energy. This is generallythe case for most of the calorimeter thermometer options used within a range of energiesfor which the microcalorimeters are designed. The linearity microcalorimeters isgenerally improved over what might be expected since they all generally use optimalfiltering to determine the energy of a pulse as accurately as possible, and optimal filteringtends to improve linearity. With most microcalorimeter technologies, it is possible todesign the pixels to match the energy range. For Gen-X, it will be important to have thehighest possible energy resolution at 6 keV, and for all energies below 6 keV, as well asto maintain excellent energy resolution from 6-10 keV. As microcalorimeters becomenon-linear, they tend to gracefully fall off in energy resolution as a function of energy.Thus to optimize the microcalorimeter arrays, the highest energy resolution requirementof 2 eV FWHM have been set only up 6 keV, and to be 3 eV for energies between 6 keVand 10 keV. This reflects the best use of the possible microcalorimeter energy resolutionpotential, rather than require the best resolution across the entire band. Indeed, themajority of prime science for Gen-X will involve X-ray lines that are severely red-shiftedto lower energy (Z~10), and thus the key energy range for high resolution for thesemeasurements will typically be only up to ~ 0.6-1.0 keV, so options for includingmicrocalorimeter pixels designed for a linear energy responsivity only up to 1 keV maybe something to revisit in the future. The mission requirements at low energies (100-200eV) are expected to be met by the wide field imager. For the XMS, from 1 keV to 200 eVthe effective area drops according to the filter transmission. Since so many lines ofinterest in this mission will be red-shifted substantially, it will be essential to maximizethe transmission through these filters, although as yet the are no clear requirements forthe transmission at given energies. The goal of 1 eV energy resolution is primarilymotivated for energies up to 1 keV. But since the energy resolution of a calorimeter isessentially constant as a function a energy within its linear range, this goal could either beseen as one in which there is 1 eV resolution up to 6 keV and stays within its linear rangeup to this energy, or alternatively one that is designed to have 1 eV resolution at 1 keVand 2 eV at 6 keV based upon the non-linearity of the signal response.

Count rate: The original specification for the mission is 100 cps/pixel, expected for allthe pixels in the wide field imager and the XMS. This was essentially derived from aconsideration of the effective area and the most active sources one could study with Gen-X. For bright point sources there are a number of ways to overcome this, such as havingan off-axis high-count rate auxiliary array, the use of a slumped micro-channel platediffuser, or even the use of a neutral density filter. The problem of high count rates inextended sources or complexes of sources is more complex (Orion, M31 nucleus, M87core, LMC etc.). Here the use of longer observations using the neutral density filter is oneoption. Without any filter, the nominal count rate of 1 cps/pixel corresponds toapproximately 2 microCrab/pixel assuming for the Gen-X optic with an area of fiftysquare meters. An average extragalactic x-ray background point source with 0.5-10 keVflux of ~1e-14 erg/s/cm2 will have a count rate of ~1 cps given the Gen-X optic.

For position sensing detectors, in which each sensor is attached to a number of differentabsorbers (effectively pixels), high count rate per pixel is hard to achieve. However thesedetectors offer one of the most plausible routes to large areas at high angular resolution.

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Therefore, this requirement has now been reduced for the XMS main array to 1 cps/pixel,although the goal of 10 cps is highly desirable. At first sight, this seems like a fairlymodest requirement, however if we are to consider options which position sensitivemicrocalorimeters are utilized to more effectively increase the field of view, then, forexample, a 10 absorber position sensitive detector will already have a requirement ofaccommodating 10 cps and a goal of 100 cps, while maintaining an energy resolution of 2eV. Currently the use of position-sensitive detectors represents one of the most promisingways to allow the low temperature detector community to develop an instrument thatcould plausibly meet the Gen-X requirements. Since the baseline requirement has beensignificantly reduced, the potential utilization of position-sensitive detectors remains. Thenew 1 cps/pixel requirement is based upon an assumption that within the focal plane therewill be a sub-array of pixels that can accommodate a higher rate of 100 cps/pixel. Thisside array is assumed to be mostly for point sources and its size is not yet determined, butwill could well be determined by the pointing accuracy of the observatory. This pointingaccuracy is currently required to be better than 20”. The side array could be aligned to beon-axis, or left to intentionally spread high count-rates over more pixels off-axis. Thisrequirement also reflects that the primary science goals for this mission involve targetsthat are extremely faint. The first stars, galaxies, and black holes will be point sourcesand thus won't be spread over a large number of pixels, but will still be much fainter than10-14 erg/s/cm2. Similarly extended sources will mostly not generate a count-rate per pixelthat approaches 1 cps. If there is a “difficulty axis” on which Gen-X is able compromiseto some degree, it is on count rate. Of course, the count rate per pixel is not a good basisfor a requirement in which the sampling level of the point spread function is not fixed.During some initial calculations to determine a reasonable requirement it was assumedthat each pixel was 0.1”. This requirement will soon be redefined in terms count rate perunit of solid angle such as ct/s/HPD (or ct/s/80%HPD).

Quantum efficiency and filter transmission : Typically the quantum efficiency ofmicrocalorimeter arrays for x-ray astrophysics is determined by the vertical absorptionefficiency of the calorimeters, and also by gaps left in between close-packed absorbers,with these two losses in efficiency being at a similar level. Generally, as the pixelsbecome smaller, the area filling factor will dominate this efficiency. For example, withpixels on a pitch of 30 µm, if absorber is 25 µm x 25 µm, and the gap is 5 µm in betweenneighboring absorbers, then the area filling factor will be 0.69. Achieving 5 µm gaps isalready very difficult, much higher filling factors will be extremely challenging, unlesscontinuous position-sensitive calorimeters are utilized. At low energies, the transmissionto the calorimeter array will be limited by the stack of filters in front of the array that arenecessary to minimize radiative heat load onto the calorimeter array, and generally to thelow temperature stages of the cooling system. Thus, it will be essential to maximize filtertransmission. This transmission loss below 1 keV is not currently included within thequantum efficiency requirement and goal.

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2.3 Straw-man microcalorimeter designs

In the tables below we describe some of the basic possible detector array formats andread-out that could meet the XMS instrument requirements. Variations that includespecialized sub-arrays (such as for high count-rate, low energy, high energy) andvariations of the pixel size with off-axis angle are not included in this list.

Design Parameter Design Options Pixel Size Option 1 (Single pixel sensors):

3,240,000 sensors 0.1” x 0.1” 30 x 30 µm 0.1” pixel size

Option 2 (Sensors reading out multiple discrete absorbers) 324,000 position sensitive calorimeters 0.1”x 1” (/10 via subpixel event location) 30 x 300 µm 0.1” effective pixel size

Option 3 (1-D Continuous absorber): 324,000 sensors 0.1” x 1” (/30 via virtual subpixel event location) 30 x 300 µm 0.1” x 0.03” effective pixel size

Option 4 (2-D Continuous absorber): 360,000 sensors (4 sensors per absorber) 0.6” x 0.6” (/324 via virtual subpixel event location) 180 x 180 µm 0.03” effective pixel size

Multiplexing Scheme Option 1 (For use with single pixel sensors) 3.24 x 106 sensors 80 HEMT amplifiers 1265 RF SQUIDs multiplexed on each HEMT amplifier Code division multiplexing – 32 TESs per SQUID

readoutOption 2 (Using position-sensitive detectors)

up to 3.6 x 105 sensors 8 HEMT amplifiers 1265 RF SQUIDs multiplexed on each HEMT amplifier Code division multiplexing – 32 TESs per SQUID

readout Table 3: Straw-man design options for array

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3.1 Basic microcalorimeter detection concepts

3.1.1 Measuring the energy of a photon

An x-ray photon is absorbed into a solid material and typically transfers its energy to anelectron as kinetic energy. In a short time (~ 10-100 ns), the electron thermalizes withinthe absorber through interactions between electrons and phonons, raising the averagetemperature of both systems. The energy of the X-ray is thus converted from the kineticenergy of a single electron to a macroscopically measurable quantity: temperature.

The operational principle of a microcalorimeter is depicted in Fig. 4, is to measure thetemperature rise in the sensor due to an absorbed, thermalized X-ray. In order to measurethis signal with high accuracy, the intrinsic thermal fluctuations of the device must bemuch lower than the expected signal from the X-ray. This leads to operation at cryogenictemperatures. Fig. 4 shows a schematic of an ideal microcalorimeter. An X-ray absorberwith heat capacity C is weakly coupled to a large cold bath at temperature Tb through athermal conductance G. When an X-ray is absorbed, the temperature in the absorberincreases by an amount E/C, where E is the X-ray energy, and then recovers back to thesteady state with a time constant C/G. The thermal conductance G is engineered such thatenough time is allowed for proper thermalization of the signal in the device, but fastenough such that the device has recovered back to steady state in time to absorb the nextX-ray.

3.1.2 Thermometer technologies

A thermometer is integrated into the device design to measure the temperature rise. Toensure high spectral resolution, the thermometer must be very well coupled to theabsorber, with a time constant between the two much greater than the device decay time.Here we consider two different types of microcalorimeter. Transition-edge sensors(TESs) and magnetic microcalorimeters (MMCs). There are differences in the

Fig. 4 : Cartoon of how a microcalorimeter works

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fabrication, operation, and readout for each technology, but the basic measurement is thesame.

Fig. 5 : Basic shape of a superconducting-to-normal transition in a TES.

TESs are thin superconducting films operated in the transition between theirsuperconducting and normal states. When an X-ray absorption event occurs, thetemperature increases in the sensor, which in turn increases the resistance. The device isdesigned such that it always operates in the transition for a given band pass of X-rayenergies from the telescope. The transition temperature and the sharpness of the transitionare the two most important characteristics of a TES. The transition temperature is tunedto around 100 mK, which is about a factor of two higher than the heat sink temperature.The energy resolution is proportional to temperature, so one wants to operate the devicesas cold as possible, but we are typically limited to temperatures around 100 mK due tothe cooling capability of space-flight cryogenic systems. The sharpness of the transitiondepends on the physics of the particular thermometer, but very sharp transitions of lessthan a millikelvin have been achieved. To read out the resistance, a voltage is placedacross the TES, which causes a current to flow. This current is measured with a SQUID.Since current is flowing through the TES, Joule heating takes place and a power equal toV2/R is dissipated in the TES. This quiescent power raises the temperature of the TESrelative to the cold bath, and must be taken into account when designing an array. Thepower is small, in the order of picowatts, but when considering megapixel arrays thisJoule power adds up and becomes a design driver for the focal plane instrument. Atypical arrangement for microcalorimeters with TES sensors is shown in the figurebelow:

Fig. 6 : This diagram illustrates the design of vacuum gap absorbers on top of TES array. The front-left pixel showsthe TES without an absorber and the middle front shows the part of the absorber that is in contact with the TES. In thisexample, a stem that is shaped like a “T” supports the absorber.

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MMCs utilize the temperature dependence of the magnetization of a paramagneticmaterial in a weak magnetic field to detect the temperature rise resulting from theabsorption of a photon. The absorption of an x-ray leads to a temperature rise in theabsorber and in the magnetic sensor material. Gold doped with a small amount of erbium(Au:Er) is an effective sensor [Fleischmann, 2005]. The temperature rise causes a changeof magnetization (M ∝ 1/T) of the Er spins, which is measured as a flux change in aSQUID magnetometer. The most basic geometry for constructing a magnetic calorimeteris one in which the magnetic sensor is positioned inside a loop of wire connected to theinput coil of a SQUID. An external magnetic field is required to produce a magnetizationof the paramagnetic system. When the magnetization of the sensor changes, the resultingchange in flux within the loop generates a change in current through the input coil of ahighly sensitive SQUID ammeter. A weak thermal link to a heat bath provides a meansfor the temperature of the MMC to return to its base temperature. In general MMCs haveintrinsic properties that are highly desirable for x-ray microcalorimeter arrays: No heat isdissipated in the calorimeter in measuring the magnetization with a superconductingSQUID loop. Thus, very large-format focal-plane arrays can be built without thedifficulty of removing large amounts of heat from the detector array. (The SQUIDs dodissipate power but are not located at the focal plane and hence are much less of aproblem.) A second very desirable property of MMCs is that they can be operated withsub-eV resolution. They can be made using standardized and automated vapor-depositiontechniques. The uniformity and reproducibility from one device to another is very high.

Fig. 7 : Basic concept of MMC operation and a promising MMC geometry and read-out for future MMC arrays.

A geometry suitable for an array of MMC microcalorimeters is depicted in Fig. 7. Thisdesign consists of superconducting niobium meander inductors onto which a layer ofmagnetic material is deposited. When a current is passed through the meander, amagnetic field is produced in the region of the magnetic material. When an x-ray isabsorbed, the temperature of the magnetic material changes, as does its magneticpermeability, and therefore the inductance of the meander. In the read-out circuit themeanders are in parallel with input coils of the SQUIDs. From the change in inductanceof the meander (which can also be thought of as a change in the flux it encircles), there

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will be a change of current both through the meander and through the input coil of theSQUID. The signals must be coupled from the meander to the SQUID throughsuperconducting strip lines and low inductance superconducting aluminum wire bonds tominimize stray inductance. In Fig. 8 we show pictures of actual MMCs in an array.

(a) (b)

Fig. 8: (a) Photograph of a single pixel meander on which the magnetic sensor is deposited. Also shown is the metallicheat link to the heat bath and the strip-line wiring. (b) Micrograph on an MMC array. In each pixel the central square isthe region of the meander shown in (a) with magnetic material covering it. On top of each sensor there is a absorberlarger than the sensor that is cantilevered over free space to form a “mushroom” geometry.

Microwave kinetic inductance detectors (MKIDs) are operated in a way in which thecomponents are not in thermal equilibrium, and therefore are not really calorimeters.These detectors utilize simple, thin-film lithographed microwave resonators as photonsensors in a multiplexed readout approach [Day, 2003; Mazin, 2004]. X-ray absorption ina superconductor creates quasiparticle excitations, with number proportional to the X-rayenergy. The surface impedance of a superconductor changes with the quasiparticledensity, and if operated at T<<Tc where the quasiparticle generation-recombination noiseis low, extremely small changes in the surface impedance can be measured using the thin-film resonant circuit and microwave readout techniques. This scheme has the potential toprovide a sensitive detector with excellent energy resolution.

Fig. 9 : Schematic of the operating principle of MKIDS.

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Fig. 9 gives an overview of the operational principles of MKIDs. In (a) a photon isabsorbed in a superconducting film, breaking Cooper pairs and creating a number ofquasiparticle excitations. The efficiency of creating quasiparticles will be less that onesince some of the energy of the photon will end up as vibrations in the lattice calledphonons. In this diagram, Cooper pairs (C) are shown at the Fermi level, and the densityof states for quasiparticles, Ns(E), is plotted as the shaded area as a function ofquasiparticle energy E. Panel (b) shows the approximate equivalent electrical circuit for aquarter-wavelength resonator. The resonant circuit is depicted schematically here as aparallel LC circuit which is capacitively coupled to a through line. In reality, there willalso be a small surface-resistance component to account for the resistive loss due to theexcited quasiparticles. Changes in Ls affect the resonance frequency and changes in Rsaffect the width and depth of the resonance, which can be measured as changes of thecomplex phase and amplitude of a microwave signal transmitted through the circuit.These effects are shown in panels (c) and (d). The quasiparticles produced by anabsorbed photon move the resonance to lower frequency and make the dip broader andshallower.

This choice of circuit design, which has high transmission away from resonance, isvery well suited for frequency-domain multiplexing, since multiple resonators operatingat slightly different frequencies could all be coupled to the same through line as isdepicted for example in Fig. 10.

Fig. 10 : MKID read-out.

3.1.3 Position-Sensitive Microcalorimeters

Microcalorimeters are usually designed to thermalize as quickly as possible to avoiddegradation in energy resolution from position dependence to the pulse shapes. Eachpixel consists of an absorber and thermometer, both decoupled from the cold baththrough a weak thermal link. Each pixel requires a separate SQUID readout channel. ForGen-X, where we require millions of resolution elements, having an individual SQUIDreadout channel for each pixel becomes difficult. Furthermore, at a focal length of 60 mand an impressive 0.1” point-spread function, the Gen-X mirror requires small pixels at

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the focal plane. A Gen-X focal plane with 0.1” pixels would have pixels that are 30 µm toa side. For comparison, the IXO X-ray Microcalorimeter Spectrometer uses 300 µmpixels. Thus the area of Gen-X pixels is 100 times smaller than those for IXO.Fabricating microcalorimeter pixels that are two orders of magnitude smaller than thecurrent state of the art is a technological challenge.

Fig. 11. a) Schematic diagram of the 4-pixel Hydra design. b) Average pulse shapes for 5.89 keV photons absorbed in each Hydrapixel.

Both of these issues: number of SQUID readout channels and pixel size, can beameliorated by an order of magnitude using position-sensitive microcalorimeters. Thesedevices use one or more thermometers coupled to an absorber with engineered positiondependence. The position dependence results in different pulse shapes depending on theabsorption location. The location of the event is found from the pulse shape, and eachdevice can thus be subdivided into several resolution elements.

The absorber can be either segmented or continuous. In a segmented design, the absorberis composed of fast-thermalization pixels connected by engineered thermal conductances.The pixilation is built in to the absorber. The advantage is that the position response ofthe device is discrete, as one gets only a set number of different pulse shapescorresponding to the individual fast-thermalization pixels. The disadvantage is that therequirement for fast thermalization of the individual pixels may not always hold, and anyposition dependence inside of a fast-thermalization pixel will degrade the resolution.

In a continuous absorber design, the design uses the intrinsic thermalization time of theabsorber to determine the position. Since the absorber is continuous, much finer positionresolution is achievable than in the segmented absorber design. The tradeoff is incomputational complexity. Since we now have a continuously varying pulse shape, amore elaborate analysis is required.

The designs of position-sensitive microcalorimeters can be one- or two-dimensional.They can have one or more thermometers. The current best results are based on a 1 TESpixilated design called a Hydra [Smith et al., 2008], where one TES reads out fourseparate pixels and has achieved a resolution of 6 eV FWHM at 6 keV for all four pixels.

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For Gen-X, we envision a factor of around 10 in subpixelation, so that each SQUIDreadout channel connected to a position-sensitive microcalorimeter can read out 10resolution pixels. This would lower by a factor of 10 the number of SQUID readoutchannels required, and increase the area of each device by a factor of 10, which makesthe fabrication feasible using some of today’s technology.

One serious drawback for position-sensitive detectors is throughput. If 10 resolutionelements are being read out by one thermometer and SQUID channel, the throughput ofthis device will be 10 times slower than using 10 individual thermometers and SQUIDchannels. The dynamics of thermalization and the electrical circuits limit the speed atwhich we can operate the thermometers, and this factor of 10 drop in throughput meansthat for position-sensitive pixels, the count rate is limited to 1 to 2 counts per second perresolution element.

3.2 Detector sensor technologies under study for Gen-X

Several low temperature detector technologies have been developed over the past fewdecades that have potential for very high-energy resolution and could possibly beformatted in large arrays. These include:

1. Transition Edge Sensor Microcalorimeters (TES)2. Magnetic Microcalorimeters (MMC)3. Semi-conducting thermistors (SCT)4. Superconducting tunnel junction detectors (STJ)5. Microwave Kinetic inductance detectors (MKIDS)

Here we will describe our initial assessment of the potential for these technologies tomeet the requirements of the Gen-X.

The microcalorimeter array technology of choice for the current generation of X-rayastrophysics satellites is semi-conducting thermistor based microcalorimeters, inparticular those using implanted silicon thermistors. This technology has been used onXQC (the X-ray Quantum Calorimeter) sounding rocket program that has flown 4 times[McCammon et al, 2002]. It was also the technology used for Astro-E and Astro-E2(Suzaku) [Kelley et al., 2000]. It will has also been deployed at a new facility at theLawrence Livermore Electron Beam Ion Trap (EBIT) to carry out laboratory astrophysics[Porter et al, 2008]. This is a very well understood technology that has achieved anenergy resolution of 3.2 eV FWHM at 6 keV [Porter et al, 2006]. If pixels are designedspecifically to meet the Generation-X requirements, this technology does have thepotential to meet the energy resolution requirements. The drawback in this technology, incomparison with other options, is that there does not currently appear to be any reliableread-out technology that would allow them to have arrays of greater than a few hundredpixels. If one could be developed, then this technology could once again be reconsidered.We would recommend that any further development of semi-conducting thermistorsshould be limited initially to developing read-out concepts that have the potential to be

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the necessary very large format, which almost inevitably would mean a read-out that canbe multiplexed.

The microcalorimeter technology that is most likely to be used for IXO is transition edgesensors (TES). This technology has been strongly supported by the X-ray astrophysicscommunity for the past couple of decades. It has been chosen for an upcoming soundingrocket mission called Micro-X [Figueroa-Feliciano, 2006, Wikus et al., 2008]. The keystrengths of this technology are that it has already achieved a world record energyresolution 1.8 eV at 6 keV in an 8x8 array [Bandler et al, 2008], and that multiplexingread-outs have already been developed that have the potential to be used in a fewthousand pixel array for IXO [Kilbourne et al, 2008] and new concepts exist and areunder development that could allow the read-out of the sort of number of pixels that willbe required for Gen-X [Mates et al., 2007]. A potential drawback is the amount of heatthat is generated at the focal plane by each read-out TES. There are a number of othersignificant technology issues related to the Gen-X design that will need to be investigatedand these are outlined throughout this roadmap. TES remains a very promisingtechnology for Gen-X, with currently perhaps the best potential for meeting the missionrequirements, and as such is base-lined as one of the main technology areasrecommended for substantial development.

Magnetic microcalorimeters (MMC) is a technology that should be able to meet theenergy resolution requirements for Gen-X [Fleischmann et al., 2007]. They have beenunder development for the past decade, but at a far lower level of support compared withother technologies. They have already achieved an energy resolution that is comparableto TESs and SCTs of 2.7 eV at 6 keV [Fleischmann et al., 2009]. MMCs have threesignificant potential advantages over other technologies. They have the potential toachieve higher energy resolution than either technology; sub-eV energy resolution maybe possible. They are dissipationless in their read-out within the focal plane array,meaning that scaling up arrays in size and number is potentially far easier than for othertechnologies. And they can be directly connected to a metallic heat-bath without affectingthe way in which they are read-out. This unique property is very important forminimizing thermal cross-talk and heat-sinking amidst the array. The read-out for MMCsuses low noise SQUID amplifiers. This read-out is compatible with the read-out of TESarrays, and thus is a technology alternative that would have relatively little impact on thereference design if both TESs and MMCs were developed. This technology does havesome potential drawbacks beyond it being currently less developed. This technologyrequires completely superconducting contacts between the detectors and the read-outSQUIDs. It also requires somewhat more demanding SQUID read-out properties thatmay reduce the number of channels that can be multiplexed in comparison with TESs andMKIDs.

Superconducting tunnel junctions (STJ) is a fourth technology that one might consider forthe Gen-X high-resolution detector. These detectors have the intrinsic advantage that theyare very fast and can accommodate extremely high counting rates. Having intrinsic rise-times of 2-3 µs, and decay times around 20 µs decay, reasonably high energy-resolution(<10 eV) has been demonstrated at 277 eV for count rates up to 10,000 cps [Frank et al,

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1998]. However, they have not achieved a competitive energy resolution such as isneeded by Gen-X for energies in the 1-10 keV band-pass. The best energy resolution in asingle STJ at 6 keV is 12 eV FWHM [Angloher et al, 2000]. In a position sensitive stripdetector (a 20 µm x 100 µm region of a 200 µm x200 µm Ta absorber), 13 eV FWHMhas been achieved at 6 keV [Li et al, 2001], although this performance is not uniformacross the whole absorber. Most development has been done with Ta and Nb absorbers.Not only is the measured performance not good enough for Gen-X, but also thetheoretical limits for these materials do not meet the requirements at 6 keV, Actualperformance, though good, has only come close to the theoretical limits at energies below1 keV Although STJs are producing exciting results in soft x-ray and UV/opticalspectroscopy, and are extremely useful in applications requiring extremely high countrates and an energy resolution that is not <10 eV at 6 keV, they are not presentlycompetitive with microcalorimeters for high resolution spectroscopy [Stahle et al, 1999].STJ devices using superconductors with smaller energy gaps, such as Ti or Hf [Kraft etal, 1998], have been proposed to overcome the theoretical limits, but no experimentalresults have yet been reported. The small energy gap requires, naturally, a lower heatsink temperature. For Hf, <20 mK is necessary, which would have a strong impact on thedesign of the cooler.

Another drawback to tunnel junctions for this application is that there appears to no goodmultiplexing technology for their read-out. While the use of radio frequency singleelectron transistors (RFSETs) [Schoelkopf et al, 1988]) has been proposed as a read-outfor STJs in the past, which could then potentially be multiplexed at low temperatures,programs to develop RFSETs are currently dormant, and there appears to be fundamentalbarriers to this approach. With STJs it may be possible to use a direct read-out at roomtemperature for each STJ using ASIC (application specific integrated circuits) chips atroom temperature [Friedrich, 2008]. But, in this scenario, the number of wires runningfrom room temperature could potentially be prohibitive for an application like Gen-X,where potentially hundreds of thousands of wires would then be needed. Although STJscould have a niche for future applications that will require only moderate energyresolution and the need to accommodate very high counting rates, we are not currentlyconsidering STJs as a seriously option for Gen-X.

Microwave Kinetic Inductance Devices (MKIDs) is the fifth technology that mightconceivably be used for Gen-X. This technology has the inherent major advantage in thatits read-out can probably most easily be adapted to microwave multiplexing of thousandsof channels per read-out amplifier. If MKIDs for X-ray detection continue to develop,and current sources of excess noise are removed, they have every chance of progressingto the point where can achieve a sensitivity level that would allow an energy resolution of2 eV energy resolution at 6 keV. One of the biggest challenges with this approach will bein demonstrating the same energy resolution as the sensitivity would predict for reasonsthat are almost the same as why STJs have fundamentally limit resolving power. Thisdetector is one that essentially counts quasi-particles produced in a superconductingabsorber by trapping them at the sensitive element of a resonator. The challenge arises inensuring that the same number of particles is collected near the resonator for each photon

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of identical energy. This is because the energy of the x-ray is divided between quasi-particles and phonons, and this division needs to be the same for each x-ray absorbed.The energy resolution is fundamentally Fano-noise-limited such that an energy resolutionof less than 2 eV at 6 keV is impossible, except perhaps when using some very lowenergy gap superconducting absorbers such as Re (1.7 eV), Mo (1.3 eV) or Hf (1.1 eV).Again, just like with STJs, the potential use of such absorbers would necessitate the useof even lower energy gap superconductors for trapping quasi-particles, necessitatinglower temperatures of operation.

In addition to the problems presented by Fano-noise there are other potential processesthat could potentially degrade the energy resolution. The quasi-particles can also becometrapped, either temporarily of permanently, at surfaces, defect sights, or impurity sights.For calorimeters in which thermal equilibrium are established, this is not a significantproblem as electrons are thermalized in metals and zero band-gap semiconductors onvery fast timescales. Since MKIDs and STJs are operated in a way in which thecomponents are not in thermal equilibrium, they are not calorimeters. The potential highsensitivity of MKIDs is more easily utilized in infrared bolometers, where high resolvingpowers are not required. As X-ray detectors, the most important next step for thistechnology would be to overcome the inherent challenge of non-equilibrium detectors bydemonstrating a high resolving power. If this happens, then this technology becomes avery attractive option for Gen-X.

To summarize, we feel that currently the two technologies currently with the mostpotential for meeting the Gen-X requirements are TESs and MMCs, and consequentlymuch of the emphasis in the rest of this document will tend to concentrate on these twotechnologies. If the challenge of high resolving powers are ever overcome in MKIDs,then this option could also become one of leading technology options for Gen-X, itsinherent ease of multiplexability being extremely advantageous . If a suitablemultiplexing technology can be developed to read out semi-conducting sensors, thenSCTs should also be considered a candidate.

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4 Details of technology challenges, and some possible approaches forovercoming them

4.1 Energy Resolution

The required energy resolution for Gen-X is 2 eV FWHM for energies up to 6 keV, and 3eV FHWM up to 10 keV. In addition, there is a goal of 1 eV FWHM for energies up to 1keV. 95% of the individual pixels in the array should have energy resolutions that arewithin 0.3 eV of the requirements. The net energy resolution from summing pixels acrossany 1”x1” region should meet the required energy resolutions, which serve as upperbounds.

The progress of different microcalorimeter thermometer technology’s energy resolutionat 6 keV is plotted below as a function of time in Fig. 12.

Fig. 12. Time evolution of energy resolution at 6 keV as a function of time for three microcalorimetertechnologies.

From this figure it is noticeable that for all technologies there is a period of relative rapidimprovement in energy resolution performance to get under 10 eV. To progress furthertowards the ultimate limits is generally more difficult and takes more time. Ultimately thedifferent microcalorimeter technologies have different limits to how far this can progress,and different technological hurdles they need to overcome. Not shown on this plot areMKIDs or STJs. So far there is one published result for energy resolution for of MKIDsfor 6 keV x-rays [Mazin, 2006]; this result gave an energy resolution of 62 eV FWHM.

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In general the energy resolution of microcalorimeters are independent of energy withinthe energy range in which they have a linear responsivity. Therefore typically an energyresolution requirement and goal at lower energies translates into a detector that has asimilar energy resolution for all energies. However for Gen-X, the size of pixels, andeven the size of position sensitive pixels, is so small that the performance of themicrocalorimeter may indeed be a non-linear function of energy. Therefore theperformance of the microcalorimeter may indeed match the non-uniform missionrequirements, which are motivated by the energy resolution of scientific observations.

For TESs, the range of energies over which the response is a linear function of energy isgenerally fairly limited due to the narrow width of the superconducting to normaltransition. For traditional microcalorimeters for X-ray astrophysics, the required size hasbeen much larger than is desired for Gen-X. Thus, efforts were made to keep the heatcapacity of the absorber low enough to optimize the energy resolution for a fixed energyrange. For Gen-X sized pixels, where the intrinsic heat capacity is naturally muchsmaller, the opposite may be true, and it may be necessary to purposefully add heatcapacity to the absorbers to keep their response linear over the energy range of interest.Overall, it should be expected that roughly the same energy resolution should beachievable in Gen-X sized pixels as is currently achieved in IXO sized pixels (300 µm x300 µm). This is because since the energy range is the same, then so should the heatcapacity also be very similar. Since energy resolution of these devices generally scales asthe (the square root of) heat capacity, then the expected energy resolution should also besimilar. An energy resolution as low as ~ 1.2 eV may ultimately be possible in singlepixel TESs. So far the best energy resolution achieved is 1.8 eV FWHM at 6 keV, whichis shown in the Fig. 13.

Fig. 13. Best energy resolution achieved in a microcalorimeter at 6 keV. The spectrum is from Mn Kα x-rays and is consistent with a TES energy resolution of 1.8 eV FWHM.

For quite some time the TES community has been exploring a so-called “excess” broadband noise that was for a long time not well understood. In recent years, in devices thathave yielded the best performance, this noise appears to be consistent with three physical

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effects: (1) Johnson noise of a non-linear resistor near equilibrium [Irwin et. al, 2006;Iyomoto et al, 2008]; (2) internal thermal fluctuation noise in relatively high resistanceTESs [Hoevers et al, 2008]; and (3) noise due to hanging heat capacities in relatively lowheat capacity devices, such as due to two level systems in the supporting membrane[Rostem, 2008; Kenyon, 2006]. Considering these noise terms, it now appears difficultfor TES microcalorimeters to achieve resolutions below ~1.2 eV at 6 keV. Until recentlyit had been thought that the energy resolution of TESs was independent of the thermaldecay time. It is now apparent that there is some theoretical dependence due to thedependence of energy resolution on non-linear Johnson noise on β (=∂log R/∂log I), thedimensionless logarithmic sensitivity of TES resistance on current. Since β is currentdependent, increasing as the current increases, there is some weak dependence of energyresolution as a function of the decay time [Smith et al., 2009]. The energy resolutionimproves for longer thermal decay times.

When position sensitive TESs is considered, there is an additional noise contribution dueto the internal thermal fluctuation noise as heat is conducted between different absorbers.The level of this noise can vary according to the design and the number of differentelements, and it will be a technological challenge to achieve 2 eV resolution in this typeof calorimeter.

For MMCs, the best energy resolution achieved so far in a single pixel is 2.7 eV FWHM[Fleischmann et al., 2009]. Existing multi-pixel array designs have been modeled thatinclude all the main known noise sources, including the SQUID and thermodynamicnoise (which dominate the performance), and it has been shown that an energy resolutionas low as 0.5 eV FWHM at 6 keV might be possible [Bandler et al, 2008], although thislevel of resolution is yet to be demonstrated. Like TESs, theoretically the energyresolution depends upon the thermal decay time, albeit with a rather weak dependence ~τ-1/4. Fig. 14 below shows an example of the main noise contributions for MMCscurrently under development, and on the right hand side shows how the energy resolutionis expected to vary with thermal decay time.

Fig. 14. (a) The modeled main noise contributions in an MMC with a decay time of 3 ms. (b) The predicted energyresolution of MMCs as a function of the thermal decay time.

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MMC sensors remain very sensitive for relatively large temperature rises. Thus theresponse is less non-linear for larger temperature excursions, and therefore the energyresolution should reduce more slowly as a function of energy than for TESs. For bothTESs and MMCs it may be possible to use algorithms to reduce the effects of non-linearity of the response. Effective utilization of such techniques with pulse processingthat does not overly increase the pulse processing electronics could be very important toGen-X.

Although the energy resolution of microcalorimeter technologies is already approachingthe level required for IXO, it is likely that when large arrays of multiplexed pixels aredeveloped, it is likely that reaching the same energy resolution across a large array willbe substantially more difficult. Many different mechanisms can lead to degradation ofperformance, such as through multiplexing, through thermal and electrical cross-talk, andthrough non-uniformity of the operating environment leading to non-uniformresponsivity. Thus it will likely be necessary to develop microcalorimeters that haveenergy resolution significantly better than the requirements to achieve an instrument thatultimately meets the requirements. Thus great emphasis should be placed on very highenergy resolution, perhaps less than 1 eV (the Gen-X goal) if it possible.

The energy resolution results described so far, for the most part, have been for discretepixel sensors with dimensions far larger than is going to be needed for Gen-X.Reproducing these kinds of energy resolution for pixel sizes which are close to 100 timessmaller will be a huge challenge, especially in large format, high fill-factor geometries.The use of position sensitive detectors with many different absorbers may reduce thischallenge in some ways, but at the potential penalty of reduced energy resolution as wediscuss in section 4.5. The challenge of maintaining sufficient energy resolution insmaller pixels is described in 4.3.

4.2 Count rate

Practical factors that make 1-10 cts/s/pix extremely hard to realize in a mega-pixel arrayinclude the following:

• Energy resolution• Heat sinking• Readout bandwidth per channel and its effect on MUXing• Pileup in position-sensitive detectors at high count rates

4.2.1 Energy resolution versus count-rate

For TES microcalorimeters, as the required count rate increases, it is necessary toincrease the thermal conductance G to the heat bath. This is necessary to keep theproportion of pulses with long record lengths high, in terms of how many τ0 (=C/G) is ineach record length. To determine the energy of each x-ray pulse we typically use of anoptimal filter. With this filter, the energy resolution is ideally given by the followingequation where S and N are the signal and noise levels at frequencies f, respectively:

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However for practical reasons, the “DC” or “zero” frequency bin for the optimal filter isextremely difficult to determine and is generally ignored. Thus the integral usually startsat a frequency f0 = 1/T, where T is the record length used. In a TES detector, the intrinsicdecay time of a pulse is shortened by the effect of the electro-thermal feedback (typicallyby 5-10 times). In the absence of any SQUID/readout noise the resolution is independentof the feedback (since it changes both the signal and the noise in the same way).Consequently the loss in energy resolution due to a suppressed zero frequency bin ofarbitrary width will also be independent of feedback. This means that, irrespective of thefeedback, (and hence the apparent length of the pulse), the required record length in termsof τ0 is always the same. Fig. 15 shows an example of how the record length used in theoptimal filter can affect the energy resolution where τ0=2ms, τeff=0.4 ms (after electro-thermal feedback), and τcrit = 0.15 ms (after critical damping with a Nyquist inductor).

Fig. 15. Modeled energy resolution of a TES as a function of the record length, in units of thermal decaytime (tau0).

Since longer record lengths will generally lead greater energy resolution in practice weTypically we assign grades to different pulses according to how long the record lengthcan be while avoid any pulse pile-up. Eg. For records longer than 10 τ0 we could assignpulses to be “high” resolution, between 1 and 10 τ0 we could assign a “mid” resolution,and for records shorter than 1 τ0 we could assign pulses to be low resolution. With anassignment such as this, the relative throughput in terms of fraction of each event grade,as a function of count rate per pixel, is shown in the Fig. 16.

€

ΔErms =4 S( f ) 2

N( f ) 2df

0

∞

∫

−1/ 2

=4

NEP( f )20

∞

∫ df

−1/ 2

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Fig. 16. The relative throughput in terms of fraction of each event grade, as a function of count rate perpixel

Thus for TESs, there is a trade off between count rate and energy resolution from thiseffect. For magnetic calorimeters there is a similar trade off between energy resolutionand record length, such as is shown in the modeled examples shown in Fig. 17 for twodecay times of 0.4 and 1.0 ms.

Fig. 17. Modeled energy resolution of a MMCs as a function of the record length, in units of thermal decaytime (tau). Models for two decay times of 0.4 ms and 1.0 ms are shown.

In terms of degradation percentage as a function of record length, this is very similar tothe TES case. It is clear from this graph and figure X that an appropriate comparison ofenergy resolution verses count rate, between MMCs and TESs, can only be made bycomparing the energy resolutions for the same thermal decay times, prior to any feedbackeffects. Currently, this trade-off between energy resolution and count-rate is based purely

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upon models, and assumes that it is impossible to determine the zero frequency bin n theoptimal filter. Over the next decade it will be important to verify whether this is anappropriate trade-off.

For TESs and MMCs, as the count rate requirement increases, then necessarily G mustalso increase. However, as G increases, the requirement on how short the thermalizationtime needs to be in the calorimeter becomes shorter. This is to avoid energy resolutiondegradation due to position dependence ie. a signal response that will depend upon wherethe X-ray photon hits the absorber, since different locations would lead to differentresponses.

4.2.2 Heat sinking

For TES-based microcalorimeter arrays there is also an important trade-off betweenstrength of thermal conductance (ie. due to count-rate). As G increases, then the powerneeded to bias each of the pixels increases as well. As we discuss in section 4.7,sufficient heat sinking of all the pixels amidst the array, and from the array out to the heatbath, is extremely challenging. It is imperative for the TES approach, that the heat fromthe bias power within the array does not alter the heat sink temperature seen by eachpixel, so that the bias point (and hence performance) is non-uniform. Since MMCs aredissipationless, this is not a relevant trade-off for this technology.

A second issue relating to the size of G (and thus dependent upon the count rate), is thatof thermal cross-talk. It is interesting that cross-talk in TESs and MMCs depend on Gvery differently. For TESs, as G increases, it is necessary to increase the heat-sinking ofthe substrate for all the pixels to the heat bath. For MMCs, the heat bath is metallic and Gto the heat bath is predominantly a weak metal link. As long as the metallic heat bath isheat-sunk well enough, the thermal cross-talk will only be due to the proportion of G dueto phonon conduction, which will decrease as G of the metal link increases. It should benoted that thermal cross-talk is diminished in TESs due to electro-thermal feedback, anddiminished in MMCs due to the proportion of heat conducting to neighboring pixelsthrough phonons is small due to this G being much smaller than the G of the dominatingmetal link.

4.2.3 Readout bandwidth per channel and its effect on MUXing

As G increases, so does the bandwidth that is needed to read out each pixel beingmultiplexed, thus making multiplexing harder. The degree of multiplexing that will bepossible will likely scale inversely with G.

4.2.4 Pileup in position-sensitive detectors at high count rates

Here we simply restate one of the trade-offs in determining the number of differentabsorbers (or effective “pixels”) that when using position sensitive detectors to increasethe field of view. As the number of absorbers increases, the count rate that the sensor

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needs to accommodate increases by the same amount. Thus, for instance, a positionsensor with 10 absorbers attached would need to accommodate 100 cps in order for eachpixel to accommodate 10 cps. Based upon the trade-off of total count rate and energyresolution described in 4.2.1, it is likely that a maximum total count rate for each sensorwhile maintaining high energy resolution will be in the range of 10-100 cps.

4.3 Field of view and pixel size

The Gen-X field of view requirement of 3’x3’ is not by itself an intimidating challenge.For comparison, the inner “core” array of the IXO calorimeter instrument is 2’x2’, andthe outer array extends to 5.5’x5.5’. The huge challenge arises due to the dramaticallyreduced angular scale. IXO is planning 3” pixels, Gen-X requires 0.1”, which is 900times smaller in angular scale by area (and the goal is for even smaller pixels). There issome relief in reducing the pixel size due to the long focal length expected for Gen-X of60 m, which is 3 times longer than IXO, and therefore the pixel sizes are a rather easy toremember 100 times smaller by area, 30 µm x30 µm. With such a small pixel size, thenumber of them need to fill the 3’x3’ field of view is huge, 1800x1800, over 3 millionpixels in total. The greatest challenges are thus two-fold, reducing the pixel size whilemaintaining energy resolution, and increasing the number of pixels from a few kilo-pixelsto a few mega-pixels. In this section we describe the issue of reducing the pixel sizewhile maintaining sufficient energy resolution and how great a challenge this is for TESsand MMCs.

4.3.1 Small TES microcalorimeters

If the area is reduced a factor of 100 without changing other aspects of the design, theheat capacity would be reduced by a factor of 100, and a 6 keV x-ray would cause acorrespondingly larger temperature change in the TES. Because a TES is a sensitivethermometer only over a restricted temperature range, the larger temperature changes willresult in saturated signals for a large portion of the energy band of interest. While thereare computational techniques [Fixsen, 2002] to recover spectral information from a TESthat goes into saturation, these are computationally intensive. Furthermore, a TESoperated in saturation at high count rates is vulnerable to undetectable pile-up. Therefore,it is more ideal to design TES sensors so that the temperature rise will remain well withinthe TES transition for photons in the energy band of 0.2 to 10 keV.

In TES microcalorimeters developed for IXO that have demonstrated 2-eV resolution, a6-keV photon produce a temperature rise that causes the TES to traverse ~40% of thetransition (starting from a bias point about 20% up the transition). These have a heatcapacity of approximately 0.4 pJ/K. This design results in a relatively linear response. Forthe smaller physical scale of Gen-X, the likely best approach will be to maintain thisresponse level. There are two techniques that can be used to accomplish this, with theoptimum solution likely involving both of these to some degree. First, the absorber willbe made of relatively thick pure gold. Current 0.2-10 keV detectors with all-goldabsorbers have absorbers that are 4 to 5 microns thick, which provides sufficient quantum

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efficiency. Thicknesses up to ~12 µm of gold, however, are potentially obtainable. For aposition sensitive detector 10 separate absorbers attached (Straw-man option 2), thiswould be enough thickness to produce the necessary heat capacity to match the heatcapacity of existing pixels with 2-eV energy resolution. Second, the width of the TESsuperconducting transition can be broadened. It has been demonstrated by severalresearch groups that horizontal normal features placed atop the TES thermometer resultin a wider superconducting transition. Or third, the heat capacity could be increased usinga material with a higher specific heat capacity than gold.

Another technical hurdle that will need a research effort to overcome is the probably needto miniaturize TES sensors in order to have a high fill-factor array. In most geometriesone can imagine, some form of miniaturization will be necessary, although the approachof using continuous two dimensional position sensors that will be discussed in section 4.5could make this hurdle significantly easier to overcome. Simply making the TES smalleris non trivial, as it is likely to dramatically change the properties of the TES. This isbecause recent measurements seem to indicate the importance of the lateral proximityeffect at short distance scales (<10 µm), both in terms of the proximity tosuperconducting contact leads, and the proximity to normal metal features [Sadleir et al.,2008]. The figure below shows photographs of some TES arrays in which the TES sizewas varied between 12 µm and 300 µm.

The analysis of data is ongoing, but the plots in Fig. 18 give an indication of howimportant the size of TES is. These transitions were characterized at very low biascurrent. Both Tc and ΔTc scale with TES size (or superconducting lead separation) to the -2 power.

(a) (b)

Fig. 18. Study of transition temperature and width for TES sizes from 300 µm to 12 µm.(a) Transition temperature, Tc, defined at the 10, 50, and 90% the normal state resistance, versus TES size,L, for devices with no zebra stripes. Fits are made to the data with the coefficients in units of µm and mKfinding L-2 scaling. (b) Transition width versus TES size, L, for devices with no zebra stripes. Again eachpart of the transition, follows a L-2 power law scaling. The curves fitting the transition width havecoefficients in units of µm and mK.

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Similar effects of transition shifting and changing width have also been observed due tothe addition of normal metal features. As smaller TESs are needed for Gen-X, furtherresearch will be necessary to fully understand the physical effects and the various trade-offs.

Another important technical challenge as pixels become smaller will be to maintain theimportant design feature of having all the pixels supported on thin membranes (typicallysilicon nitride). This is important for most potential detector technologies, as membranesare important for inhibiting athermal phonon loss to the heat bath, which can lead toenergy resolution degradation from the variation of this loss due from X-ray events atdifferent locations in the absorber. Typically TES sensor fabrication involves the backetching of small areas of silicon underneath just the TES itself. The back-etching of holeson the size scale of Gen-X will be challenging, and alternative approaches to fabricatingmembranes may be necessary.

4.3.1 Small MMC microcalorimeters

The MMC meander pixel geometry that is currently being developed has sensors that aretypically 100 µm x 100 µm, on a meaner pitch that is 5 µm (i.e. 20 meander lines long).In principle it should be possible to make these pixels significantly smaller by bothreducing the number of meander lines and the pitch. The use of electron beamlithography would allow this possibility to be explored. Since the dependence of thesignal size is very well understood as a function of the geometry [Fleischmann, 2005], itis reasonable to assume that the fundamental properties of this sensor technology will notchange as it size is scaled down to sizes necessary for Gen-X. One advantage of reducingthe pixel pitch is that the dc current passing through the meander to produce the magneticfield in the sensor would need to be less. The challenge for this approach from reducingthe sensor size will be to reduce the effects of stray inductance, which can reduce thesensitivity if the stray inductance approaches the inductance of the sensor. This can bemitigated by the use of a finer meander pitch, and possibly also from the use oftransformers, although it will be challenging to design transformers with sufficientcoupling efficiency that will be small enough to be integrated with high density withinthe large format Gen-X array.

4.4 Quantum efficiency

It will be very challenging to meet the goal of total quantum efficiency being greater than0.8 at 6 keV, although the goal of 0.6 should be achievable. The attenuation length ofgold at 6 keV is 1.17 µm. A thickness of 5 µm will have a vertical quantum efficiency ofgreater than 98%. The challenge for large format pixilated arrays, as the absorbers forpixels become smaller, is to achieve a high area filling factor for the absorbers. Forexample, with pixels on a pitch of 30 µm, if the absorber is 25 µm x 25 µm and the gap is5 µm in between neighboring absorbers, then the area filling factor will be 0.69.Achieving gaps of less than 5 µm between absorbers than are 4-5 microns thick will bevery challenging, and will require technology development. At present, ion milling isused to delineate absorbers. Side-wall angles of 20o are typical, which leads to the typical

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gaps that are typically greater than 5 µm. Achieving much higher filling factors will bevery challenging, unless continuous position sensitive calorimeters or MKIDs areutilized.

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4.5 Position-Sensitive Detectors

One promising route to attain a megapixel array of microcalorimeters is through the useof position-sensitive detectors. A position-sensitive detector is consists of a macro-pixel(which may be continuous or made of interconnected discrete elements) read out by oneor more thermometers. The device is designed such that the shape of the temperaturepulse depends on the location of absorption on the macro-pixel. This inherent positionsensitivity of the signal is then exploited to determine the location of the event on themacro-pixel, effectively yielding one device with may sub-pixels.

The main advantage of such devices is that they reduce the total number of electronicchannels required to read out a given number of pixels. This eases the requirements onthe readout system, but increases the complexity of the analysis electronics as they nowhave the added task of determining the sub-pixel event location. Since the complexity ofthe readout system is one of the main limiting factors to the size of the array, the use ofposition-sensitive detectors can yield significant gains in the final array size.

For Gen-X, the 3 to 36 million-pixel array (requirement and goal) is a substantial leapfrom the current state of the art of 16 multiplexed pixels demonstrating sub-3 eV energyresolution for IXO (Kilbourne et al. 2008). The Gen-X goal of a 36 million-pixel arraywith 10 µm pixel pitch is extremely hard to imagine without some form of position-sensitive devices.

4.5.1 Position sensitive detector drawbacks

The extra sub-pixel information does not come for free. We now detail several designtradeoffs inherent in position sensitive devices.

To obtain the position, pulse shape differences must exist for different absorptionlocations, and these in turn lead to different signal-to-noise characteristics for each sub-pixel. The theoretical energy resolution of each sub-pixel is then slightly different, and ingeneral the resolution of a position-sensitive device will always be worse than theresolution of an equivalent set of single pixel detectors, although this difference can beminimized through careful design. The larger the number of sub-pixels, the larger thenegative effect on energy resolution. Tradeoffs in the design of the device must be madeto compromise between energy resolution and the number of sub-pixels on a givendevice.

The other drawback of position-sensitive detectors is pileup. An argument that is made isthat since several sub-pixels are read out by the same thermometer(s), pileuprequirements mean that the maximum count rate on a position-sensitive device with Nsub-pixels be N times smaller than the single pixel requirement for devices with the samedecay time. This last statement assumes a uniform illumination of the array, which ishardly ever the case. The actual pileup rate on a position-sensitive device depends onboth its physical design and on the science targets one uses it for, with a worst-casescenario of an N times smaller maximum count rate. One might try to get around this by

34

designing faster decay time position sensitive devices, but faster decay times mean largerbandwidths, which impact the readout system, and faster equilibration times, whichimpact the energy resolution. Thus all of these factors need to be taken into account in thedesign.

After careful consideration of all the issues, one realizes that one does not get somethingfor nothing, and position-sensitive devices only make sense if one is willing to give up alittle energy resolution and a lot of maximum count rate for an increase in the number ofpixels for a given number of readout channels.

For Gen-X, the small plate scale (30 µm for 0.1” pixels) is actually beneficial. For bothmicrocalorimeters and MKIDs, building 0.1” individual pixels is very challenging, and0.03” pixels practically unfeasible. For either 1 or 2 dimensional position-sensitivedetectors, the macro pixels would have sizes closer to the dimensions currently underdesign for IXO, and using the position sensitivity on continuous absorbers makes itpossible to lay out a path to attaining 0.03” pixels.

4.5.1 TES Position sensitive detectors

Figure 19. (a) Hydra concept being studied for the IXO mission that reads out 16 pixels with one TES. 4absorbers with different thermal links are connected to the TES. Each of these inner absorbers is connectedto three others in such a way that each absorber has a different thermal path back to the TES. Thesedifferences in the thermal path lead to different thermalization times of the device for X-rays hitting eachabsorber. (b) Simulated response from this device where the 16 different pixels can be identified by theirpulse shape.

Position-sensitive microcalorimeters have been demonstrated using TES thermometersand have achieved a resolution of 5-6 eV FWHM on a 4-pixel device read out by a singleTES called a Hydra [Smith et al., 2008]. These devices are being developed for the IXOmission with a goal of sub 10 eV resolution. For microcalorimeters, the pulse shapedifferentiation is achieved by controlling the thermal diffusion constant (in the case of acontinuous absorber) or engineering thermal bottlenecks in the absorber (in the case of apixilated absorber), which lead to a loss in energy resolution with respect to an equivalenthigh-diffusion-constant design. Furthermore, the energy resolution depends on the totalheat capacity of the device, so a larger macro-pixel will have a lower energy resolution

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than a smaller one. The large plate scale of the IXO design means that many-pixel Hydrashave a large heat capacity, and significantly degrade the theoretical energy resolution ofsuch a device.

For Gen-X, the small plate scale means that a device such as depicted in Fig. 19(a) with16 absorbers may be developed into a viable design. The phase space of small pixelposition-sensitive devices has not been explored in detail at this time, but we expect thatdue to the tight tolerances required to make individual absorbers in the 10-30 µm size,continuous absorbers may be preferred. In these designs, the diffusion time of theabsorber is what determines the position sensitivity, and a careful design of the absorberto obtain the desired heat capacity, X-ray stopping power, and diffusion time will berequired. The current IXO absorber design consists of a layer of gold for thermalizationand a layer of bismuth for extra stopping power. In general, gold thermalizes too quicklyfor a continuous absorber design. A material with a slower diffusion constant is required.Furthermore, the small plate scale will push to increase the heat capacity per unit area tomuch higher numbers than currently used in IXO designs. In Rausch et al. 2008(doi:10.1117/12.790246), small palladium position-sensitive devices for solar work aredemonstrated with 30 eV resolution and 10-15 µm position resolution. The use ofpalladium absorbers is one possible route toward making Gen-X style devices.

To increase the number of pixels read out per channel, 2-dimensional position-sensitivedetectors may also be competitive for Gen-X.

4.5.2 MMC position sensitive detectors

Fig. 20. (a) This diagram shows the layout of an MMC position sensitive detector that has been fabricated and tested.(b) This figure shows temporal revolution of the pulse-shapes that were recorded for the first MMC position sensitivedetectors to be fabricated. The inset shows an expanded view of the rise-time profiles, where the different pulse shapescan easily be distinguished.

4.5.3 MKID position sensitive detectors

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Fig. 21. The left hand figure shows a diagram of the layout of the an MKID position sensitive detector. The right handside shows a plot of the signal height of the left hand MKID versus the signal height of the right hand MKID for x-rayfrom a Mn source. The inset (b) shows the resulting spectrum.

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4.6 Multiplexing

Fig. 22. This figure illustrates the time evolution of the multiplexed signals for time division multiplexing,frequency division multiplexing, and code division multiplexing.

Currently the use of time-division multiplexing is a promising approach for arrays ofmedium size, such as IXO. This approach uses classic SQUID multiplexer circuits thatare switched by turning on a series of SQUIDs in sequence, either by applying a SQUIDbias current [Reintsema et al., 2003, Doriese et al., 2004], or shunting with flux actuatedswitches [Beyer, 2008]. This is similar to the TDMA technology used in cell phones. Analternative approach to multiplexing uses frequency domain multiplexing, which is usedin FDMA cell phones. The ability to scale both of these approaches to much largerarrays, such as is needed for Gen-X, is limited by both power dissipation and the tens ofmegahertz bandwidth of dc SQUIDs. In contrast, read-out consisting of a microwavemultiplexer offers a path to meeting the Gen-X requirements. Several different versionsof this type of multiplexer exist, depending upon whether it is adapted for the read-out ofTESs, MMCs or MKIDs. For TESs and MMCs, unshunted, non-hysteretic rf SQUIDs areincorporated into the read-out that have negligible power dissipation even for extremelylarge arrays [Mates, 2008]. Furthermore, rf SQUIDs can be coupled to high-Q microwaveresonant circuits fabricated from superconducting coplanar waveguides with resonantfrequencies of several gigahertz. In this approach, the bandwidth of eachmicrocalorimeter is limited by the resonant circuit after amplification by the rf SQUID. Asingle high electron mobility transistor (HEMT) amplifier has the bandwidth anddynamic range to read out many hundreds of rf SQUIDs operated in superconductingmicroresonators tuned to different frequencies, all coupled to the same coplanar-waveguide feedline. These devices also do not need feedback, since they are constantlymodulated through their response curve with flux-ramp modulation. At NIST, microwaveSQUID multiplexers are being developed with funding from NASA for long wavelengthapplications, and the success of further development of this effort is essential to enablefuture mission concepts such as Gen-X. NIST have already demonstrated thatmicrowave SQUID multiplexers work well, with high Q (5,000-20,000) and excellentnoise performance. They have also demonstrated the flux-ramp modulation technique,which eliminates the need to feed back to every pixel.

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Fig. 23. Schematic of the microwave multiplexing readout of an MMC.

The detector technology that can perhaps mostly easily be implemented with microwaveread-out are KIDs, since in this case no SQUIDs or modulation techniques are necessary.While the essential components of this technology are the same for TES sensors andMMCs, their optimization is slightly different. The most important difference betweenthe read-out of TESs and MMCs is the adjustment of the strength of the couplingbetween the SQUID and the resonator. In the case of TESs, which do not need the bestpossible SQUID noise performance, the coupling between the SQUID and the resonatoris reduced to the point that the resonant frequency of the line does not change by morethan the width of the line as the flux into the SQUID is modulated. This makes the read-out electronics simpler, since the excitation frequency can be kept fixed. In the case ofmagnetic calorimeters, the better SQUID noise is preferred. This can be achieved byincreasing the strength of the coupling between the resonator and the SQUID, whichshould make it possible to approach quantum limitations in SQUID sensitivity. However,it may also require the implementation of phase-locked-loop electronics, in which theexcitation frequency is digitally adjusted to keep the transmitted phase fixed, so that theread-out can follow resonant frequency excursions large as compared to the bandwidth ofthe resonator. This optimization should make it possible to achieve energy an resolutionlimited by the thermodynamics of the magnetic calorimeter itself, and the multiplexing ofmany hundreds if not thousands of magnetic calorimeters in each HEMT amplifier.

Fig. 24. This circuit diagram depicts the current steering in a TES read-out circuit using single pole doublethrow switches that is used in code division multiplexing.

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A more recent idea in microwave read-out that has the potential to further extend thenumber of channels per read-out amplifier with the TES sensor approach, is theintegration of code division multiplexing, which is a technological approach used inCDMA cell phones. This approach, which is also being developed at NIST, is depicted inFig. 24. In this approach, each individual SQUID can be used to read out many differentpixels, potentially further reducing the number of HEMT amplifiers and read-outelectronics channels. It works by introducing code-division multiplexing for the pixelsattached to a single SQUID, and using single pole double throw (SPDT) switches todivert the current signal going to the SQUID so that it can either appear as a plus fluxsignal or a minus flux signal. These switches are similar to the low-power magnetic fluxactuated switches that are currently being demonstrated to simplify and improveconventional time division multiplexing [Beyer et al., 2008]. For 32 pixels there wouldbe a sequence of switching for the 32 input circuits to give arrangements of different plusand minus signals for the different channels. However all the pixels attached have asignal going into the summing coil going to just one SQUID at one time. The switchingof different pixels between +1 and –1 is chosen to be in different orthogonal patterns,such that the resulting signals form matrices known as Walsh matrices. By appropriatediagonalization of the matrix of signals for the different plus and minus biases, one canuniquely determine the signal from each individual channel for all times. Thus with thisapproach there is no energy resolution loss associated with multiplexing the 32 channels.If the output SQUID is a dissipationless rf SQUID in a microwave resonator, thisapproach dramatically increases the information that can be transmitted through eachHEMT amplifier.

The potential benefit of using this approach for Gen-X is astounding. To make veryrough estimates, one can conceive of an arrangement in which each HEMT can read out~ 1250 SQUIDs, each with ~ 2-5 MHz of bandwidth. Then each SQUID can read out ~32 pixels. Thus ~ 8 HEMTs and read-out electronics would be needed to read out ~3x105

TESs, which could be enough for an entire Gen-X array. The success of such an elegantapproach to the read-out could mean that as few as 20 coax cables would be needed forthe entire read-out. One of the biggest challenges to developing microwave multiplexingfor Gen-X is to accommodate all the components needed within a small enough pixelpitch. As is discussed in section 4.8, with the number of read-out channels needed forGen-X, the most likely geometry for bringing the sensor signals out of the focal plane isone in which the read-out components are directly beneath the pixels being read out,preferably spaced according to the pitch of the pixels in the array. The components of theread-out would potentially include the switches needed for code division multiplexing,the SQUID read-outs needed for TESs and MMCs, the input coils to these SQUIDs, andmicrowave resonators. Making all of these components small enough will be verychallenging. Lumped element components that are currently under development may helpto mitigate this challenge to some degree, and the use of position sensitive detectors andcode division multiplexing would mean that the space for SQUIDs and resonators wouldbe determined not by the pixel pitch, but by the some of all pixels that are read out byeach SQUID and resonator. For instance, if each TES or MMC has 10 absorbers, and 32

40

of these sensors are code division multiplexed, then the pitch needed for the SQUIDs andresonators could potentially be 600 µm.

In summary, the development of microwave multiplexing techniques, whether it isdirected towards the read-out of TESs, MMCs, or MKIDs, is an absolutely criticaltechnology to develop over the next decade. This read-out is seen as critical to enable theread-out of the Gen-X microcalorimeter array, and forms a crucial part of our technologydevelopment program. At the moment the development of these techniques is beingspearheaded by the groups in infra-red astronomy in the US and in Europe. It is fortunatethat much of the research in this field will ultimately benefit detectors for a variety ofdifferent wavelengths, and as such it is recommended that a common effort between allgroups in the US is strongly supported.

4.7 Heat-Sinking of arrays

Heat sinking of the microcalorimeter array will be an extremely important issue thatneeds to be carefully engineered for the success of the Gen-X XMS instrument. The focalplane array will need to be heat-sunk at a uniform and stable temperature ofapproximately 50 mK. This requirement will put very stringent constraints of the strengthof thermal conductance needed between the array and the heat bath. As one example, for3x105 TESs that each requires 3 pW, there will be a total heat load of around 0.9 µW. IfMMCs are used, the heat-sinking of the focal plane array to the heat-bath is very muchless challenging. In order to keep the base temperature seen by TESs in an array within 1mK of the bath temperature the sum of all the thermal gradients between the array and theheat bath to be less than 1.0 mK.

The most difficult aspect of achieving this strength of thermal conductance is through theinterface that must exist from where the heat transport is through phonons to where it isthrough electrons. N�e�c�e�s�s�a�r�i�l�y� �t�h�e� �d�e�t�e�c�t�o�r�s� �a�n�d� �m�u�l�t�i�p�l�e�x�e�r� �a�r�e� �f�a�b�r�i�c�a�t�e�d� �o�n� �a�n��e�l�e�c�t�r�i�c�a�l�l�y� �n�o�n�-�c�o�n�d�u�c�t�i�n�g� �m�e�d�i�u�m� �(�e�.�g�.� �s�i�l�i�c�o�n�)� �a�n�d� �t�h�e� �t�h�e�r�m�a�l� �l�i�n�k� �w�i�l�l� �t�y�p�i�c�a�l�l�y� �b�e��h�i�g�h�l�y� �e�l�e�c�t�r�i�c�a�l�l�y� �c�o�n�d�u�c�t�i�n�g� �(�e�.�g�.� �gold or c�o�p�p�e�r�)�.� �I�n� �t�h�e�s�e� �s�i�t�u�a�t�i�o�n�s� �a�n�d� �a�t� �m�K��o�p�e�r�a�t�i�n�g� �t�e�m�p�e�r�a�t�u�r�e�s�,� �p�h�o�n�o�n� �s�c�a�t�t�e�r�i�n�g� �b�e�t�w�e�e�n� �d�i�s�s�i�m�i�l�a�r� �m�a�t�e�r�i�a�l�s� �a�n�d� �p�h�o�n�o�n� �t�o��e�l�e�c�t�r�o�n� �c�o�u�p�l�i�n�g� �m�a�y� �l�e�a�d� �t�o� �l�a�r�g�e� �t�e�m�p�e�r�a�t�u�r�e� �g�r�a�d�i�e�n�t�s� �i�n� �t�h�e� �b�o�u�n�d�a�r�y� �r�e�g�i�o�n� �b�e�t�w�e�e�n��t�h�e� �t�h�e�r�m�a�l� �l�i�n�k� �a�n�d� �d�e�t�e�c�t�o�r� �a�n�d� �c�a�n�n�o�t� �b�e� �i�g�n�o�r�e�d�.� �O�v�e�r�c�o�m�i�n�g� �t�h�i�s� �p�r�o�b�l�e�m� �g�e�n�e�r�a�l�l�y��r�e�q�u�i�r�e�s� �h�i�g�h� �c�o�n�t�a�c�t� �a�r�e�a�.� �T�h�i�s� �w�i�l�l� �t�e�n�d� �t�o� �i�n�t�r�o�d�u�c�e� �h�i�g�h� �m�e�c�h�a�n�i�c�a�l� �s�t�r�e�s�s�e�s� �w�h�e�n��m�a�t�e�r�i�a�l�s� �w�i�t�h� �h�i�g�h� �d�i�f�f�e�r�e�n�t�i�a�l� �t�h�e�r�m�a�l� �c�o�n�t�r�a�c�t�i�o�n� �r�a�t�e�s� �a�r�e� �f�i�x�e�d� �t�o�g�e�t�h�e�r� �(�e�.�g�.� �s�i�l�i�c�o�n��a�n�d� �c�o�p�p�e�r�)�.� �T�h�e�s�e� �s�t�r�e�s�s�e�s� �c�a�n� �b�e� �c�o�n�s�i�d�e�r�a�b�l�e� �a�n�d� �l�e�a�d� �t�o� �f�r�a�c�t�u�r�e� �o�f� �t�h�e� �a�r�r�a�y�. Thegeneral strategy currently in IXO is depicted in the figure below, in which the heat islaterally conducted out from the array to a copper heat bath. Here the heat is coupled intogold trenches within the array (overcoming Kapitza boundary resistance and electron-phonon decoupling impedance); then in needs to have sufficient lateral thermalconductivity within the trench metal, and then within the metallic heat-sinking layeroutside of the array; and then the metal on top of the silicon wafer needs to be heat sunksufficiently strongly though gold wire-bonds to the heat bath. The extension of thisapproach to accommodating the power of millions of pixels appears extremelychallenging.

41

�

Fig. 25. This cartoon illustrates the heat-sinking strategy being develop for TES arrays for IXO.

Considering first just the boundary (Kapitza) resistance, theory predicts that betweensilicon and gold at 50 mK, G/A ~ 7x10-6 W/K/cm2. So, to keep the temperature gradientcaused by 1.5 µW of power to less than 1 mK, the area of contact might need to be ~ 200cm2. This is one order of magnitude larger than the area of the array. Measurements in thematerials used in microcalorimeters have shown that this boundary resistance cansometimes be a few times higher than theory, compounding this problem. Thus, obtainingthe uniformity of temperature within the array will likely be very challenging for TESsensors. The use of three dimensional metallic structures with a sensor substrate likesilicon will be necessary. TES sensor designs that require less power may for currenttypical TES X-ray microcalorimeters may also be necessary.

One approach to overcoming this problem has been developed for the bolometersdeveloped for SCUBA-2 [Duncan, 2004]. They� �u�s�e� �a� �w�i�r�e� �m�a�c�h�i�n�e�d� �B�e�C�u “h�a�i�r� �b�r�u�s�h”�t�o� �s�u�p�p�o�r�t� �t�h�e� �a�r�r�a�y� �(��F�i�g�.� �26�)�.� �T�h�i�s� �c�o�n�s�i�s�t�s� �o�f� �2�0� �m�m� �l�o�n�g� �p�i�n�s� �w�i�t�h� �a� �f�l�a�t� �t�o�p� �m�a�c�h�i�n�e�d��o�u�t� �o�f� �a� �s�o�l�i�d� �b�l�o�c�k� �o�f� �m�e�t�a�l�.� �T�h�e� �p�i�n�s� �here are o�n� �a� �1� �m�m� �p�i�t�c�h�.� �T�h�e� �h�e�a�d�s� �o�f� �p�i�n�s� �a�r�e��e�p�o�x�i�e�d� �t�o� �t�h�e� �u�n�d�e�r�s�i�d�e� �o�f� �t�h�e� �a�r�r�a�y substrate�.�

�����F�i�g�.� �26�.� �B�e� �C�u� �h�a�i�r� �b�r�u�s�h : 4�0� �b�y� �4�0� of 1 �m�m� �s�q�.� �Ti�n�e�s� �each� �2�1� �m�m� �l�o�n�g�.�

42

��However, heat-sinking of the array to reduce thermal gradients is only part of the heat-sinking problem. To meet the performance requirements, it will be important to reducethermal cross-talk between adjacent sensors to less than 1 part in 1000. Thus, amidst thearray, it is very important that thermal conductance of the heat bath to the substratessupporting the sensors is very much stronger than the heat-links of sensors to thosesubstrates. If the sensors are designed so that heat is easily coupled into a highlyconducting metal, this problem becomes much easier to solve. This approach can berealized in MMCs as it is possible to electronically connect sensors to metallic heat bathswithout electrically connecting their read-out. The introduction of metals within theinsulating substrate of TESs would help to reduce thermal cross-talk (as well as reducingthe potential temperature gradients). The development needed to develop this the type offabrication processes for TESs in the Gen-X pixel geometry will be very challenging andwill form another significant component of the technology roadmap over the next decade.In summary, there will need to be a lot of careful design and development to determinethe best approach for achieving adequate heat sinking.

4.8 Wiring of pixels within arrays and within the FEA

Pixel wiring within the arrayThe wiring strategy being used by IXO is unlikely to be useful for wiring a full size Gen-X array. IXO is planning to use only planar microstrip wiring that pass between pixelsand underneath the cantilevered absorbers. This strategy is no longer possible since thespace between pixels is too small, and the number of wires needed to feed out too large.A more practical approach for contact wiring for Gen-X is to integrate microvias to thebackside of the substrate. This detector chip could then be “bump-bonded” to a carrierwafer on which microstrip leads bring the signal lines out to the SQUIDs/resonators, thatare read out using the microwave multiplexing techniques described in section 4.6. Acartoon of one possible configuration is depicted in Fig. 27 for a magnetic calorimeterarray. For fabrication of large-format infrared bolometer arrays, an indium bump bondingprocess has been developed at NASA/GSFC that yields high quality superconductingbump-bonds.

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Fig. 27. Cartoon of a single pixel in an MMC array. The pixel thermal link is connected through a gold viato a gold layer that is the thermal bath, underneath an isolation layer and silicon nitride membrane. Here wedepict a wiring scheme that uses superconducting microvias and bump bonds.

Fig. 28: Schematic top and cross-sectional view of a 3x3 segment of a TES microcalorimeter array (w/oabsorbers) with the proposed interconnecting scheme featuring a monolithic combination of microvias andmicrotrenches for vertical readout and heatsinking. Since the collected heat from a microtrench istransferred through a microvia, the trench structures can be disconnected.

Figure 28 presents a similar innovative concept for TES that combines a through-wafermicrovia for electrical readout and a silicon surface machined microtrench for array heatsinking into one monolithic structure. The main idea behind this concept is to maximizethe available spatial density for the electrical readout and at the same time to simplify and

44

shorten the thermal path from a microtrench to the heat sink. This is accomplished byletting the electrical and thermal component share the same physical conduit through thedevice chip and bump-bond connection. Therefore, the underlying readout chip not onlyserves as fan-out board, but also functions as the heat sink reservoir for the array.

This three-dimensional approach to the contact wiring was employed for the focal planearray of Scuba-2. A sketch showing how they were incorporated is shown in Fig. 28below. Here the pitch between pixels was 1.0 mm.

Fig. 28. This cartoon depicts the three-dimensional approach to the contact wiring that was employed forthe focal plane of Scuba-2.

4.9 Filters

The Gen-X filters will very likely be very similar to those needed for IXO (Gen-X filterswill be 3-4", IXO needs around 3"). Currently Luxel are in the process of developingthese kinds of filters. They are confident that they can produce filters that are about afactor of 2 thinner than was used for the calorimeter on Suzaku. It is reasonable toassume that there are 5 filters each with this design. The approach that is being developedincorporates a thin "mesh" that supports the filter. Above around 1 keV, the transmissionthrough this mesh will be very high. The curves for the transmission as a function ofenergy for the current Luxel designs are available and are shown below. The thicknessnumbers are for all filters together, assumed to be five. For a real set of filters, one wouldoptimize these based on mechanical issues (e.g., smaller diameter filters can be thinner).The "IXO" transmission does not yet take into account the transmission through the newpolyimide mesh, as this will depend upon the final design, but the new polyimide meshwill be very small and highly transmissive (even in area, the mesh filling factor isprobably much less than 5%).

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Fig. 29. This graph illustrates the total net transmission that is achieved with two different kinds of filterstack. The lower line is the transmission for the XRS2 filter stack on Suzaku. The upper is for the filterstack being developed for IXO.

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4.10 Cryostat design and wiring4.10 ADR mechanical design4.11 Electronics design4.12 Mass and Power4.13 Telemetry requirements4.14 Background and anti-coincidence detector

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5. Recommendations for development schedule, technology gates,and schedule and distribution of funding

5.1 Development schedule, technology gates and TRL definitions

The plan for maturing the technical readiness levels (TRL) to TRL-6 is roughly phasedby five, two-year technology demonstration gates (TG) that are defined in terms of 17specific performance requirements and design maturities that given below and labeled (a)through (q). Gate 0 represents the entry-level gate for beginning a developmentalprogram towards building the Gen-X calorimeter. It is considered to be already completefor a number of approaches towards meeting Gen-X goals, including TES and MMCapproaches, and is seen as a minimum requirement to possibly becoming part of the XMSdevelopment program. As well as describing gates, we have also defined the various TRLlevels with performance, read-out, and array size requirements that reflect the frameworkof what is needed for Gen-X, which is obviously very different to that of IXO. TheseTRL definitions are based upon the same list of criteria as the gates, although the phasingis somewhat different, and we have attempted to set TRL definitions to be consistent withthe general description that is used by NASA. Currently TES technology is the mostdeveloped towards meeting Gen-X goals, and is considered to be at TRL-2.

We have attempted to define the gates and TRL levels in terms that are as independent ofsensor technology approach as possible. For each sensor technology, there are differentapproaches to meeting these goals in terms of multiplexing and position sensitivedetectors. Since we do not a priori know which of these will be best suited for Gen-X atthis time, we have developed gates for two main approaches: one that seeks to populatethe Gen-X focal plane with individual sensors (one readout channel per pixel) and asecond approach that either uses some form of position-sensitive detector to sub-pixilateeach absorber or reads out multiple single pixels with the same readout channel. Themain difference between the two approaches is how many total readout channels need tocome out of the focal plane and the associated wiring and heat loads that come with thesedesign decisions. Thus, one technology path requires at minimum 3.24 x 106 readoutchannels (for 0.1” pixels), and the second technology path requires at minimum 3.24 x106 / N channels, where N is the number of focal plane pixels read out per channel.

(*) Indicates technology development that is also necessary for IXO.

Gate 0: Demonstrate feasibility of approach (minimum requirement to begin any Gen-X technology development funding): (a) & (b)

(a) Through modeling, demonstrate detector concepts that can potentially meetminimum Gen-X requirements.

(b) Produce a convincing multiplexing scheme that on paper can potentially meetGen-X requirements

- TRL 1 status achieved - basic principles observed and reported

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Gate 1: (c), (d), (e) and (f) successfully complete

(c) Multiplex 16 pixels with less than 3 eV energy resolution at 6 keV (*).(d) Demonstrate 2.0 eV energy resolution at 6 keV for any pixel size (*).

-TRL 2 status achieved - technology concept and/or application formulated

(e) If technological approach requires position sensitive detectors with N sub-pixelsper sensor, demonstrate such a detector that reads out N/2 subpixels, with sub-pixels of a size necessary to meet Gen-X requirements with an energy resolutionof 4 eV. If approach does not require sub-pixelation, demonstrate 4 eV energyresolution at 6 keV in 10 closed-packed single pixels that meets Gen-X pixel sizerequirements.

(f) Demonstration of a read-out concept that can be extended to meet Gen-Xrequirements, that can read out any sized x-ray detector with better than 6.0 eVenergy resolution at 6 keV.

- TRL 3 status achieved – technology proof of concept

Gate 2: (g), (h) and (i) successfully complete

(g) If technological approach requires position sensitive detectors with N sub-pixelsper sensor, demonstrate 2.0 eV energy resolution at 6 keV in a single sensor withN sub-pixels that meets Gen-X size, quantum efficiency, and fill-factorrequirements.

(h) Demonstrate multiplexing in which one channel simultaneously reads-out 100sensors with design parameters that are commensurate with Gen-X requirementsand with an energy resolution of less than 10 eV at 6 keV. This read-out must inprinciple be expandable to read out 1000 sensors per channel.

(i) Design a front end-assembly that can accommodate the required level of heatsinking and wiring density that we be necessary to build the XMS instrument.

Gate 3: (j), (k) successfully complete

(j) Demonstrate multiplexing in which one channel simultaneously reads-out 1000sensors with an energy resolution that is commensurate with Gen-X requirements.

- TRL 4 status achieved - Component validation in laboratory environment

(k) Build a front-end assembly that can accommodate the required level of heatsinking and wiring density that we be necessary to build the XMS instrument.

49

Gate 4: (l), (m), (n), (o) successfully complete

(l) Demonstrate that all components of the XMS can survive vibration and radiationtesting.

(m) Demonstrate the pixel type used for (j) with an energy resolution of 2.0 eV at 10cps with the same read-out technology as used for (k).

(n) Space qualify electronics suitable for multiplexed read-out of pixels.(o) Demonstrate filters can be constructed with the required size and transmission

-TRL 5 status achieved - component and breadboard validation in relevantenvironment

Gate 5: (p) and (q) successfully complete

(p) Design cooling platform that can meet Gen-X-XMS requirements.(q) Build a 1-megapixel array with all detectors “biased”, with read-out of 10 k 0.1”

pixels with 2.0 eV energy resolution at 6 keV, and meeting all other Gen-Xrequirements.

-TRL 6 status achieved - subsystem model demonstration in a relevant environment

50

TRL 1Basic principles observed and reportedThis is the lowest “level” of technology maturation. At this level, scientific researchbegins to be translated into applied research and development.TRL 2Technology concept and/or application formulatedOnce basic physical principles are observed, then at the next level of maturation, practicalapplications of those characteristics can be ‘invented’ or identified.TRL 3Analytical and experimental critical function and/or characteristic proof-of-conceptAt this step in the maturation process, active research and development (R&D) isinitiated. This must include both analytical studies to set the technology into anappropriate context and laboratory-based studies to physically validate that the analyticalpredictions are correct. These studies and experiments should constitute “proof-of-concept” validation of the applications/concepts formulated at TRL 2.TRL 4Component and/or breadboard validation in laboratory environmentFollowing successful “proof-of-concept” work, basic technological elements must beintegrated to establish that the “pieces” will work together to achieve concept-enablinglevels of performance for a component and/or breadboard. This validation must devisedto support the concept that was formulated earlier, and should also be consistent with therequirements of potential system applications. The validation is relatively “low-fidelity”compared to the eventual system: it could be composed of ad hoc discrete components ina laboratory.TRL 5Component and/or breadboard validation in relevant environmentAt this, the fidelity of the component and/or breadboard being tested has to increasesignificantly. The basic technological elements must be integrated with reasonablyrealistic supporting elements so that the total applications (component-level, sub-systemlevel, or system-level) can be tested in a ‘simulated’ or somewhat realistic environment.From one-to-several new technologies might be involved in the demonstration. Forexample, a new type of solar photovoltaic material promising higher efficiencies would atthis level be used in an actual fabricated solar array ‘blanket’ that would be integratedwith power supplies, supporting structure, etc., and tested in a thermal vacuum chamberwith solar simulation capability.

TRL 6System/subsystem model or prototype demonstration in a relevant environment(ground or space)A major step in the level of fidelity of the technology demonstration follows thecompletion of TRL 5. At TRL 6, a representative model or prototype system or system— which would go well beyond ad hoc, ‘patch-cord’ or discrete component levelbreadboarding — would be tested in a relevant environment. At this level, if the only‘relevant environment’ is the environment of space, then the model/prototype must bedemonstrated in space. Of course, the demonstration should be successful to represent atrue TRL 6. Not all technologies will undergo a TRL 6 demonstration: at this point thematuration step is driven more by assuring management confidence than by R&Drequirements. The demonstration might represent an actual system application, or itmight only be similar to the planned application, but using the same technologies. At thislevel, several-to-many new technologies might be integrated into the demonstration.

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5.2 Schedule and distribution of funding

The estimated budget below assumes that funding for IXO microcalorimeter developmentand building we carry on throughout the first six years of this independent Gen-Xdevelopment program, and highly leverages the expected progress of this program. Overthis period, the relatively modest budget of approximately $1.9M is recommended todirect separate development efforts that directly will be needed to facilitate the Gen-XXMS instrument. Of this, approximately $1.2M could be directed towards detectordevelopment. During this period, we anticipate that at least two groups will receive fundsto develop two detector technologies, a primary technology that currently appears to havethe greatest potential and also some potential alternatives. We also anticipate somesupport of smaller, more focused research efforts that would work in collaboration withthose leading technology efforts. These would initially be in support of the detectortechnologies with most potential, and later would likely be to investigate alternativeparallel technical paths as new ideas evolve. At the same time it is expected thatapproximately $0.7M per year will be needed specifically to develop the necessarymicrowave multiplexing. From previous research programs that have lead to Astro-E,Suzaku, and now Astro-H and IXO, we know that approximately 80% of the programbudgets is needed for the loaded costs to support staffing of these programs (includingtravel). The remainder of the costs is typically needed for providing cryogens,development of cryogenic apparatus, purchase of commercial electronics, purchase offabrication upgrades, computer upgrades and materials and supplies needed for themicro-fabrication processes. We have assumed that a total of 8 people are needed tosupport the fabrication, testing, and analysis of the different technologies described.

In the following four years, once the essential components of the instrument have beendecided upon and demonstrated, somewhat greater funding will be needed to develop alarger prototype demonstration system in a relevant environment. It is anticipated thatthere will only be one platform to demonstrate the prototype instrument, and only onemultiplexing concept being refined and optimized.

The following budget reflects a first estimate of the budget and its schedule needed tocarry out the necessary development program over the next decade in FY09 dollars. Eachitem is in units of one million dollars.

1 2 3 4 5 6 7 8 9 10Detector

Development 1.2 1.2 1.2 1.2 1.2 1.2 0.8 0.8 0.8 0.8Microwave

Multiplexing 0.7 0.7 0.7 0.7 0.7 0.7 0.5 0.5 0.5 0.5FEA and

packaging 0.4 0.4 0.4 0.8 0.6 0.2 0.2 0.2Prototype

demonstrationsystem

1.8 0.8 0.1 0.1

SpaceCoolingsystem

Concept0.1 0.1 0.1

Flightqualificationof necessary

hardware

0.4 0.4

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hardwareFilter

Development 0.2 0.2 0.2

Total 1.9 1.9 2.3 2.3 2.3 2.7 3.9 2.6 2.3 2.1

Total : $ 24.3 M

FY11 FY12 FY13 FY14 FY15 FY16 FY17 FY18 FY19 FY20 FY21 Total

Microcalorimeter 1.9 1.9 2.3 2.3 2.3 2.7 3.9 2.6 2.3 2.1 24.3

Total 1.9 1.9 2.3 2.3 2.3 2.7 3.9 2.6 2.3 2.1 24.3

TRL Legend TRL-1 TRL-2 TRL-3 TRL-4 TRL-5 TRL-6

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6. Acknowledgements

This is a working document has originally been drafted by Simon Bandler and EnectaliFigueroa-Feliciano. Many scientists have read through various parts of this manuscript,and we would like to acknowledge their important suggestions. These include Kent Irwin,Ben Mazin, Megan Eckart, Caroline Kilbourne, Stephen Smith, Richard Kelley, ScottPorter, Fred Finkbeiner, Thomas Stevenson, George Seidel.

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