Space science applications of cryogenic detectors

N.Rando, D.Lumb, M.Bavdaz, D.Martin, T.Peacock

Science Payload & Advanced Concepts Office, European Space Agency, ESTEC,

Noordwijk, The Netherlands Abstract The improved performance of cryogenic detectors has drastically enhanced their utilization range, allowing a number of space-based applications, with particular emphasis on astronomical observations. In this paper we provide an overview of the main applications of cryogenic detectors onboard spacecraft, together with a description of the key technologies and detection techniques used or being considered for space science missions. A summary of the cryogenic instrumentation technologies is also presented. Specific emphasis is given to space based astronomy in the soft X-ray regime, where superconducting tunnel junctions and cryogenic calorimeters offer well identified advantages. Possible instruments for future astrophysics space missions are also discussed, using XEUS (X-ray Evolving Universe Spectroscopy mission, presently proposed by ESA as a post XMM-Newton project) as a reference. To be published in the proceedings of the 2nd workshop on advanced Transition Radiation Detectors for accelerator and space applications (TRD 2003), held in Bari (Italy) on September 4-7, 2003.

Introduction Cryogenic detectors and cooling technologies have made a remarkable amount of progress over the last 20 years. The increased reliability and simplicity of operations of cryogenic equipment have allowed to install and operate them onboard spacecrafts, while the improved performance of cryogenic devices, such as sensors and cold electronics, has opened new applications [1]. Remarkable progress has been achieved in the fabrication of thin film based micro-devices (Low Temperature Superconductors such as Nb, NbN, Ta) showing highly uniform properties and excellent resistance to thermal cycles. Such devices include Superconducting Tunnel Junctions (STJs), SQUIDS (Superconducting Quantum Interference Devices) and simpler components, such as resonating cavities and low losses RF filters. Albeit at a lower pace than initially predicted, also High Temperature Superconductors are moving from a pioneering phase to more mature technological applications. In particular, the advent of radio-frequency devices based on HTS has opened up new perspectives for space-borne applications, in view of more energy efficient and better performing telecommunication platforms. Over the last 15 years several space missions involved cryogenic equipment, mostly in relation with astrophysics missions, at a wavelength range otherwise difficult to work with from ground. Among such missions we should mention Einstein (NASA’s X-ray observatory launched in 1978) [2], IRAS (Infrared Astronomical Satellite, launched in 1983), COBE (Cosmic Background Explorer, launched in 1989) and ISO (Infrared Space Observatory, launched in 1995) [3]. The Japanese satellite Astro-E (X-ray observatory) was designed to carry a spectrometer (XRS) built by NASA and operating at a temperature of 65 mK [4]. At this moment, several space missions involving cryogenic applications are in advanced development, in Europe as well as in Japan and US. Good examples are the ESA space science missions Planck (dedicated to the mapping of the cosmic background radiation) and Herschel (formerly known as FIRST- Far Infrared and Submillimetre Telescope) [5]. Both satellites will carry scientific payloads working at temperatures of 0.1 K and 0.3 K respectively. SIRTF (Space Infrared Telescope Facility, launch in August 2003), one of the four large NASA’s observatories, is based on a 4He cryostat and it will perform photometry, spectroscopy and imaging in the 3 – 180 µm spectral range [6]. On longer time-scales (post 2010-2015), other space missions with cryogenic equipment are under study, usually targeting low operating temperatures (down to 50-100 mK) and long lifetimes (5-10 years), such as ESA’s XEUS [7] (a post XMM mission) and NASA’s Constellation-X (the High Throughput X-Ray Spectroscopy mission), both envisaging micro-calorimeters working at T<100 mK. In the higher temperature range, between 100 and 10 K, many missions are already operational or in development. They include military reconnaissance satellites (such as Helios), earth observation satellites (SPOT) and meteorological spacecrafts (MSG, Meteosat Second Generation), with IR detectors operating at about 85 K.

Cryogenics in Space This section provides a brief review of the main scientific missions using cryogenics in chronological order, starting from Einstein and IRAS, the first ‘cryogenic missions’, which flew respectively in 1978 and 1983. Mission in operations (or post-operations), missions presently under development and missions under study are grouped in different sections of table 2 provides a summary of all space missions that involve cryogenics. Additional details can be found in [1]. - Missions in operations / post-operations. Einstein (HEAO-2) was the second of NASA’s three High Energy Astrophysical Observatories and the first X-ray telescope put into space in 1978. Among other instrument, Einstein carried a Solid State Spectrometer using a Si(Li) crystal detector (range 0.5-4.5 keV) cooled at about 100K via a solid ammonia/methane cryostat [2]. IRAS (Infrared Astronomy Satellite, launched in 1983) was the first cryogenic scientific satellite. Its mission was to map the entire sky from 8 to 120 µm and it was equipped with a 0.6 m telescope cooled with liquid He to about 4 K. The focal plane assembly operated 62 photo-conductive elements at 3K. COBE (Cosmic Background Explorer) was developed by NASA’s to measure the cosmic background radiation. The satellite was launched in November 1989. It carried three instruments, operating at wavelengths between 1.25 and 240 µm with focal planes at 1.6 K, cooled by a 650 litre superfluid helium cryostat. ESA’s ISO (Infrared Space Observatory) operated at wavelengths from 2.5 to 240 µm between November 1995 and May 1998 in a highly elliptical orbit. The satellite is based on a cryostat containing about 2200 litre of superfluid helium and on a 0.6 m diameter telescope. The instruments made use of different photo-conductors based on InSb, Si and Ge and operating between 1.8 and 10 K [3]. NICMOS (Near Infrared Camera and Multi-Object Spectrometer) is a Hubble Space Telescope (HST) instrument based on three cameras designed for simultaneous operations, operating between 0.8 and 2.5 µm and using HgCdTe photo-conductive detectors, cooled down to 50-60 K via 120 kg of solid nitrogen. A mission study involving cryogenic equipment (not selected for flight) is represented by the ESA’s STEP (Satellite Test of the Equivalence Principle), aiming to verify the equivalence of gravitational and inertial mass to a precision of one part in 1017. Such a precision would have been guaranteed by superconducting accelerometers cooled at 1.8 K by a bath of superfluid He. WIRE (Wide Field Infrared Explorer), one of the NASA small explorers, was launched in February 1999 and lost during its commissioning phase. It was supposed to survey the sky at mid-infrared wavelengths, between 12 and 25 µm. A two-stage, solid hydrogen cryostat maintained the optics colder than 19 K and the 128x128 Si:Ga detector array below 7.5 K. Astro-E (NASA and Japan’s effort) is a satellite for X-ray astronomy launched in 2000 and not operational due to a launcher problem. Astro-E was designed to provide X-ray images along with high resolution spectra from 0.4 to 700 keV. One of the instruments onboard, was based on an array of 2x6 micro-calorimeters, operating at 65 mK. Such a temperature is maintained by an ADR (Adiabatic Demagnetisation Refrigerator) hosted in a liquid helium cryostat and thermally shielded by solid-neon cooled outer jacket [6].

INTEGRAL (International Gamma-Ray Astrophysics Laboratory, a medium size ESA science mission dedicated to spectroscopy and imaging between 15 keV and 10 MeV) was launched in December 2002 and uses space qualified Stirling cryo-coolers [8]. The spectrometer installed on the spacecraft (SPI) is based on about 30 kg of germanium detectors maintained at a temperature of 85K. The satellite is presently operating nominally with an expected lifetime extending to 2005. - Missions under development. ESA is presently developing (phase C/D, 2003) two important cryogenic astronomical missions: Planck and Herschel [5]. Planck’s main objective is to map the temperature anisotropies of the Cosmic Microwave Background (CMB) over the whole sky, with a sensitivity ∆T/T = 2.10-6 and an angular resolution of 10 arc-minutes. Such goals require bolometers operating at 0.1K, HEMT at 20 K and a cooled telescope (60 K). Planck’s cryogenic system uses pre-cooling to 60 K by passive radiators, cooling to 20 K with a H2 Joule Thomson Cooler (adsorption compressors), cooling to 4 K with a He Joule-Thomson cooler (mechanical compressors) and final cooling to 0.1 K with an open loop Dilution Refrigerator. Mission lifetime is 15 months. Herschel (formerly known as FIRST, see figure 1) is dedicated to astronomical observations in the far-infrared and sub-mm wavelength range, from 85 to 600 µm. Herschel is based on a superfluid helium dewar at 1.65 K and on a 3He sorption coolers delivering a base temperature of 0.3 K. The scientific goals will be achieved with three instruments operating respectively at 2 K (HIFI, heterodyne receiver based on SIS mixers), 1.7 K (PACS, spectro-photometer partly based on photo-conductors) and 0.3 K (again PACS and SPIRE, another spectro-photometer using bolometers). Herschel is presently scheduled for launch in 2007 with a mission lifetime of 4.5 year. NASA’s SIRTF (Space Infrared Telescope Facility) is designed to perform imaging and spectroscopy in a large wavelength range, from 3 (NIR) to 180 (FIR) µm via a 0.85 m diameter, helium cooled telescope [6]. The detectors’ temperature is 1.4 K, while the cryogenic system makes use of 360 litre of superfluid He, for a lifetime of 2.5 years. SIRTF is scheduled for launch in August 2003. Finally, we should mention the re-flight of Astro-E (named Astro-E2), presently in development phase and scheduled for launch in February 2005. - Missions under study. NASA's JWST (James Webb Space Telescope) is based on a 6 to 8 m diameter passively cooled telescope enabling observations in the NIR and Medium-IR (MIR) from 1 to 30 µm. The science objectives of the JWST are the study of galaxies, stars and planets formation. The science instruments are a NIR camera, a NIR low-resolution spectrograph (both operating at 30K) and a MIR camera-spectrograph combination (at 8K, achieved by a solid hydrogen cryostat). NASA's WISE (Wide-Field IR Survey Explorer, formally NGSS) candidate medium-class Explorer program, is presently undergoing a Phase A study. WISE would provide a sky survey from 3.5 to 25 µm. The satellite used a solid hydrogen cryostat, a total mass of about 890 kg, a 0.5 m telescope and a four-channel imager based on HgCdTe and Si:As photo-conductive 1024x1024 elements arrays. XEUS (X-ray Evolving Universe Spectroscopy mission) is the potential follow-on mission to the ESA XMM cornerstone and is described in the next section [7].

Constellation-X is a next generation X-ray observatory under study by NASA. Such a mission is based on 2-4 identical satellites orbiting at the L2 and achieving a large photon collection area between 0.25 and 40 keV. At the focal plane of the telescopes, micro-calorimeter arrays operating at 50 mK would ensure high resolution imaging spectroscopy. The cryogenic system would adopt a design similar to Astro-E (outer solid neon, intermediate 4He and inner ADR stage). Cryogens should ensure a mission lifetime between 3 and 5 years. Launch would take place around 2010. DARWIN (Infrared Space Interferometry Mission) is a cornerstone candidate in the ESA ‘Horizon 2000+’ science plan (launch after 2012). Its goal is to detect terrestrial planets in orbit around other stars and to allow high resolution imaging in the medium infrared, between 5 and 30 µm. Nulling interferometry would be performed over a 50-500 meter baseline, including 6 free flying 1.5 m telescopes. Both the telescopes and the focal plane detectors would be cooled to about 20-30 K [9]. TPF (Terrestrial Planet Finder) is a similar mission under study at NASA, based on 5 spacecrafts flying in formation at about 1 AU from the Sun and focusing on the identification of terrestrial planets outside our solar system. As such, TPF if facing technological issues very similar to DARWIN, including cold optics and IR detectors. The present system baseline relies on passive cooling to about 40 K and a Brayton cryo-cooler to cool the IR detectors down to 5 K. Figure 1. An artist’s view of ESA’s Herschel spacecraft (launch in 2007)

Cryogenic detectors for space applications: an overview Cryogenic detectors have found numerous applications onboard space missions. Such devices offer two key advantages over alternative sensors: 1) higher sensitivity (expressed by the Noise Equivalent Power, NEP); 2) better energy resolution (expressed in terms of resolving power, E/∆E = λ/∆λ). Low temperature detectors have then driven the utilization of cryogenics in space, dictating operating temperature, temperature stability and architecture of the payload system. Table 1 provides an overview of photon detectors, including spectral range, typical power dissipations, array size and operating temperature. Applications related to the lower energy end of the electromagnetic spectrum (i.e. sub-mm wave and IR) benefit most from the use of cryogenic detectors. Semiconductor bolometers have operating temperatures ranging from just above 50 K (NIR) to 0.1 K (sub-mm) and with NEP reaching values below 10-17 W/Hz1/2. Remarkable developments have taken place in infrared detector technology, mostly driven by the vast investments made by the US Department of Defense throughout the 1980s. Progress embraces a large spectrum range, from the NIR (λ=1 µm) to the FIR (λ=200 µm), focusing on low-background, high sensitivity and large format arrays. IRAS (IR Astronomy Satellite, 1983) used a total of 62 detector elements, while since 1995 large format arrays for IR astronomy are available with a total number of pixels in excess of 106 [10]. Astronomical observations in the far infrared (FIR) detect blackbody radiation in the 30 to 300 µm (corresponding to temperatures between 100 and 10 K). An example is the interstellar dust in our galaxy (at 20-30 K), detected by IRAS in 1983, confirming the existence of interstellar dust. FIR radiation is totally blocked by the atmosphere. Photo-conductors represent the main devices used throughout the IR range. At low temperatures the conductivity of these semi-conducting materials is influenced by the absorbed IR photons, which can ionize impurities and free charge carriers. The ionisation energy of the impurities sets the cut-off wavelength of these detectors, ranging from 200 µm (stressed lattice Ge:Ga) down to 40 µm (Blocked Impurity Band Si:Sb or Ge:Ga). Such photoconductors are typically operated at T < 3 K. In the case of ISOPHOT, a broad band photometer flown onboard ISO (Infrared Space Observatory, launched by ESA in 1995), Ge:Ga detectors were combined with low noise CMOS integrating preamplifiers and multiplexers operating at 2 K to achieve a NEP of order 10-18 W/Hz1/2[11]. Bolometers have also been used to detect sub-mm photons. Neutron-Transmutation-Doped (NTD) Ge detectors are well established and operate at temperatures between 300 and 100 mK, with NEP of order 10-17 W/Hz1/2. Such devices will be used onboard the ESA mission Planck (development by JPL, USA). In the sub-mm range heterodyne receivers provide very high sensitivities up to 500 GHz. Receivers based on Superconductor-Isolator-Superconductor (SIS) devices offer better performance that the conventional Schottky diode based systems. Nb based junctions have been used in the 300 to 500 GHz range, showing noise temperatures five times lower than the corresponding values of Schottky devices. Operating temperatures are of order 2 K. At frequencies ν > 500 GHz the Hot-Electron Bolometers (HEBs) compete with SIS and Schottky diodes for the next generation of heterodyne receivers (e.g. onboard the ESA missions Planck and Herschel). The incoming radiation excites the electron population (either in a semiconductor or in a superconductor absorber) without heating up the corresponding lattice, but determining changes in the resistance of the device according to a non-linear

behaviour, which is exploited to detect the signal. Operating temperatures range from 70K (2DEG, InSb HEBs) to 0.3 K (NIS HEBs). In the NIR (between 1 and 5 µm) other photo-conductors are used, mainly PtSi, HgCdTe and InSb. Over the last decade the introduction of two-dimensional InSb arrays has drastically changed the field of IR astronomy, with 1k x 1k pixel array, based on hybrid technology (e.g. InSb detector array bumped to a silicon Source Follower read-out in NMOS, PMOS or CMOS technology). These detectors have operating temperatures ranging between 77 and 35 K and have already been used onboard satellites such as the Hubble Space Telescope (NICMOS camera). While micro-bolometers produced via nano-technologies are now becoming available, offering good imaging performance, they are more indicated for remote sensing applications rather then for astronomical research [12]. Superconducting Tunnel Junctions (STJs) [13] and Transition Edge Detectors (TESs) represent a new generation of photon detectors [14], both photon counting in the visible and NIR, with intrinsic spectroscopic capability. STJs (see figure 2) operate between 0.5 and 0.1 K depending on the superconductors used (typically Nb, Al, Ta), with responsivities of order 104 e-/eV, resolving power of order 10 at λ = 500 nm and maximum count rate of order 104 event/sec. TESs operate at about 0.1 K, have also very large responsivities, comparable energy resolution and a max count rate of order 103 event/sec (see figure 3). Such a combination of performance is particularly attractive to modern astronomy, opening the way to new research activities [15]. Both STJs and TESs can operate over a large photon energy range, with very interesting performance in the UV and X-Ray. Both technologies offer distinct advantages over the traditional UV detectors, such as Multi-Anode Micro-channel Arrays and Intensified CCDs, with higher detection efficiency, the photon counting and intrinsic spectroscopic capability and the good imaging resolution (with 20 µm pixels). In the case of STJs an energy resolution of 15 eV at 6 keV has been demonstrated, while TESs have achieved even better performance (a few eV’s at 6 keV, see figure 4). On this basis, future space observatories are likely to make use of cryogenic detectors dedicated to non-dispersive spectroscopy (as baselined for one of the instruments onboard the future XEUS mission). Fundamental physics and planetary sciences can also benefit from the utilisation of cryogenic detectors, such as SQUID (Superconducting Quantum Interference Devices) based gravity gradiometers, to be used for low altitude Earth and planetary missions. Such devices allow the mapping of the intensity of the gravitational forces and can be used to verify the so-called Equivalence Principle, which postulates the coincidence of gravitational and inertial mass. To date SQUID devices based on low temperature superconductors are favoured, with operating temperature around 4K. SQUIDs based on high temperature superconductors (HTS) are also investigated, in view of their capability to operate at about 77 K [16].

Figure 2 A microscope picture of a 12x10 array of tantalum-aluminum based superconducting tunnel junction developed under ESA contract.

Figure 3 Schematic diagram and microscope image of a Transition Edge Sensor (courtesy of SRON, the Netherlands). The thermometer (green) on top of the SixNy-membrane (yellow) is electrical connected by leads running over the support beams. The absorber is indicated by a purple square on the thermometer.

x ySi N membrane

Al wiring

Si

Cuabsorber

TES

slots

Figure 4 Pulse height spectrum for Mn-Kα and Kβ X-rays obtained with a TES microcalorimeter consisting of a 310 × 310 µm2 Ti/Au thermometer with TC = 95 mK and a 4.5 µm thick Cu absorber of 0.1 x 0.1 mm2. The insert shows a fit with 3.9 eV FWHM energy resolution through the Mn-Kα1,2 doublet at 5.89 keV (courtesy of SRON, the Netherlands).

0 2000 4000 6000 8000

Energy (eV)

0

20

40

60

Cou

nts

5870 5880 5890 5900 5910 5920Energy (eV)

0

20

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60

Cou

nts

FWHM = 3.9 eV

Figure 5 An artist’s view of XEUS in its deployed configuration. The mirror and the detector spacecraft are flying in formation and are separated by 50 m, while maintaining alignment within 1 mm3.

The XEUS mission The X-ray Evolving Universe Spectroscopy (XEUS) mission has been proposed as the next European X-ray astronomy mission, to be launched in about 2015 [7]. It is designed to match future IR and sub-mm facilities, such as JWST and ALM, in the exploration of the early Universe. While long wavelength measurements will concentrate on the discovery of the "cool" structures (such as forming galaxies and early stellar generations), XEUS will investigate the "hot" Universe comprising the first generation of Massive Black Holes, the first collapsing large scale structures that will form massive clusters of galaxies, and will investigate the chemical enrichment of the intergalactic medium through stellar explosions. These investigations will be facilitated by the deployment of a X-ray focusing telescope of unprecedented dimensions, providing 30 m2 of collecting area, in order to collect sufficient photons from the farthest, faintest targets. This large mirror system will be deployed on a separate spacecraft from the detector payload (see figure 5). With a focal length of 50 m, the 2 spacecraft (a low revolution speed spinning telescope module and a 3-axis stabilized detector module) will fly in a novel formation-flying mode, where the detector spacecraft will maintain a position at the focal spot, with an accuracy of < 1 mm3. In the current definition, the mirror system is too large to be deployed with a single spacecraft launch, so that an initial configuration with ~ 6 m2 area will be used. The telescope system will then be "grown" by the addition of extra outer petal segments in an assembly sequence that would rely on the robotic facilities provided available at the International Space Station [17]. Several instruments are foreseen for installation onboard the XEUS detector module: a Wide Field Imager (WFI, based on a DEPFET array), two Narror Field Imagers (both based on cryogenic detectors and described in the section below), an Extended Field Imager (EFI, conceived as an array of CCD imaging spectrometers), a High Time Resolution Instrument (HTRI, based on an array of silicon diodes), a Polarimeter (based on a compact gas micro-well detector) and a Hard X-ray Camera (HXC, based on an array of compound semiconductors). Very long imaging observations of selected fields will be made with the wide-field imaging camera, using a modestly cooled Si active pixel sensor. This will allow the detection of many objects (up to 105 / sq degree) with a coarse measure of spectra that will enable the preliminary identification of unusual objects. The detailed follow-up of such objects will be made using cryogenic spectrometers or "Narrow Field Imagers". These will enable plasma diagnostics, so that measurements can be made such as: 1) the centroid of emission lines can be measured to determine the cosmological red-shift of the target; 2) discrimination between power law AGN spectra and a thermal spectrum from star-forming regions allows the determination of relative contributions of accretion power and star formation to the early Universe energy budget; 3) resonance absorption lines measurements will test the red-shift dependence of absorbing systems of tenuous warm material between galaxies; 4) small groups of galaxies coalescing together in a large gravitational potential well may be surrounded by hot gas reservoirs whose thermal spectrum carries the signature of metal abundances resulting from chemical enrichment by the first generation of stellar explosions.

The XEUS cryogenic instruments: NFI 1 and NFI 2 At the current stage of developments, programmatic reasons dictate the provision of two different cryogenic spectrometer technologies, to ensure technological redundancy, force the development of different cooling technologies and also ensure that the very large energy dynamic range can be completely covered with the necessary energy resolution. The baseline design foresees a camera with an array of STJ detectors, and one with an array of micro-calorimeters with transition edge sensor (TES) readout. The STJ instrument (NFI 1, or Narrow Field Instrument 1) would be optimized for ~1eV FWHM energy resolution at soft X-ray energies, between 0,1-2keV. The TES camera (NFI 2) would offer <5eV resolution at higher X-ray energies, from 1-15keV. - The XEUS cryogenic chain Presently the XEUS cryogenic chain is based on a Stirling compressor providing a 20K pre-cooling stage and a Joule-Thompson expansion loop using 3He as a working fluid and achieving 2.5K. The 2.5K heat sink is then used to pre-cool a 3He sorption cooler (delivering 300-350 mK to NFI 1) and an Adiabatic Demagnetization Refrigerator (delivering 50 mK to NFI 2). The mechanical cooler design is preferred because of the long mission lifetime that can be achieved (not limited by cryogenic consumables). As an alternative, a pre-cooling approach based on cryogens (e.g. similarly to what is done for the Medium Infrared Instrument on the JWST) could also be considered. We should note here that the final choice of the cryogenic chain is related to the operational orbit and to the possibility to exploit, as efficiently as possible, passive cooling. - The Narrow Field Instrument 1 (STJ) The NFI 1 is based on an array of superconducting tunnel junction detectors (see figure 2) offering high collection efficiency and high-resolution spectroscopy at low X-ray energies. It covers a relatively small (1 arcmin) field of view to allow follow-up of specific sources already identified by the Wide Field Instrument present on XEUS or by other observatories. The array is operated at a temperature of 350 mK. A cross-section of the cooler system, including focal plane assembly and thermal shields, is shown in Fig 6. The drawing does not show the coil required to apply a bias magnetic field to the STJ array. The design of NFI1 is based loosely around a detailed design study, made for a proposal to NASA for an STJ camera on HST, with the exception that the large number of individual detectors are replaced by “distributed readout detectors” that significantly reduces the number of connections from the cold array to the outside electronics [18]. The electronics can be divided in 3 sub-systems: the coolers’ and mechanisms control electronics (CCE), the instrument computer and control system (ICU), and finally a front-end electronics dedicated to readout and processing of the STJ signal (FEE). This is sized to operate close to the STJ detector array, but is assumed to be at a room temperature stage, connected via cold harness that is tied to different cooling stages on its way between the STJ and the room temperature. The mass allocated for the 3 units is about 25 kg, for a total power of about 70W. Typical telemetry rate expected from NFI 1 is of order 3.5 kbit/s. - The Narrow Field Instrument 2 (TES) NFI 2 is the second narrow field imaging spectrometer, tuned to higher X-ray energies (1-15keV) than for NFI 1. NFI 2 is based on a bolometer detection array, read out via

superconducting transition edge thermometers (TES, see figure 3). Presently a 32 x 32 element array is considered. Such a detector will offer an energy resolution (FWHM) of order 3 eV at 7 keV, with an absorption efficiency exceeding 90%. The read-out scheme is based on SQUIDs. A multiplexing scheme (both time and frequency multiplexing are being explored [19, 20]) is being developed in order to simplify the array operations and read-out procedure. The focal plane array would be cooled down to about 50 mK by an Adiabatic Demagnetisation Refrigerator (ADR), presently being developed under ESA contract. The cryostat for the NFI2 instrument, including all thermal shields and the ADR cooler form an integral part. The instrument requires three electronics boxes containing the SQUID Flux-Lock-Loop Electronics (SEB, front-end part of the detector read-out electronics), the Bias Supply and Signal Read-out unit (BSSR) and the Instrument Control Unit (ICU). The SEB has to be situated as close as possible to the TES array. A mass of order 25 kg is expected for the SEB, BSSR and ICU units. Typical telemetry rate expected from NFI 2 is similar to NFI (of order 3.5 kbit/s). Figure 6 Cross-sectional view of the STJ cryo-cooler assembly (XEUS NFI 1). The height of the unit is approx 45cm, with largest diameter ~12cm. The 3He charged Joule-Thompson unit (2.5K) and Stirling (20K) compressors are not shown.

Conclusions Both cryogenics and cryogenic detectors have achieved a remarkable progress over the last 15 years, with a technology readiness level moving from laboratory prototypes to flight proven or even commercial applications. Increased robustness, coupled to longer lifetimes and simplicity of operations opened the possibility to use cryogenics in space, albeit at the price of additional complexity and larger costs. The adoption of cryogenic devices onboard spacecrafts, allows unprecedented results, especially in the field of space science and low energy astrophysics. Over the last 20 years missions such as IRAS and ISO have demonstrated that these devices outperform any competing technology. New missions based on cryogenic detectors are being developed (such as ESA’s Herschel and Planck) or about to be launched (such as NASA’s SIRTF). In the previous sections we have provided a summary of the main space missions that made use of cryogenic detectors as well as a summary of the competing detector technologies across the electromagnetic spectrum, from sub-mm to x-ray. The X-ray Evolving Universe Spectroscopy (XEUS) mission, presently under study at ESA as a post XMM-Newton program, has been described in more detail, with particular regard to its cryogenic instruments and associated focal plane detectors. The activities undertaken by the leading space organizations show without any doubt that cryogenics and cryogenic detectors are going to play a strategic role onboard future space missions.

References [1] B.Collaudin, N.Rando, Cryogenics, 40, 797-819 (2001) [2] R.Giacconi et al., Astrophysical Journal, Part 1, vol. 230, June 1, 1979, p. 540-550 [3] M.F.Kessler et al., Astron. Astrophys. 315, L27-L31 (1996). [4] Y.Ogawara, International Astronomical Union, Symposium No.188, Kyoto, Japan – Kluwer Academic Publishers, 75-78, (1998). [5] B.Collaudin, T.Passvogel, Proceedings of SPIE, Vol.3358, (1998). [6] P.T.Timbie et al. Cryogenics, 271, 30 (1990). [7] M.Bavdaz et al., Proc. 2nd European Symposium on the Utilisation of the International Space Station, 621-627, ESA SP-433 (1999). [8] C.Winkler and W.Hermsen, Proc. 5th CGRO Symposium, Sep 1999, AIP. [9] A.Karlsson, Proceedings of the 36th Liege International Astrophysical Colloquium, July 2-5, 2001, page 37-49 (November 2001). [10] A.M.Fowler, Proceedings of ESA symposium on photon detectors for space instrumentation, 129-135, ESA SP-356 (1992). [11] D. Lemke, U.Klaas, J.Abolins et al., Isophot – Capabilities and performance. Astron.Astrophys. 315, L64-70 (1996). [12] R.I.Thompson, M.Rieke, G.Schneider, D.C.Hines, and M.R.Corbin, ApJ Letters, 492, L95 (1998). [13] N.Rando et al., NIM A 444, 441-444 (2000). [14] R.L.Kelley et al., NIM A 444, 170-174 (2000). [15] Jakobsen P., Ultraviolet-optical Space Astronomy beyond HST. ASP conference series, vol.164, 397-404 (1999). [16] J.Clarke, NATO ASI series, 87-147, Vol. F59, Springer-Verlag, (1989). [17] B.Aschenback, H.Brauninger, Proceedings of SPIE 982, 10-15 (1988). [18] R.den Hartog, A.Kozorezov, D.Martin, G.Brammertz, P.Verhoeve, A.Peacock, F,Scholze, D.J.Goldie, Proceedings of Low Temperature Detectors 9, AIP Conference Proceedings 605, 11-14, 2002. [19] M.Kiviranta, H.Seppa, J.v.d.Kuur and Piet de Korte, Proceedings of Low Temperature Detectors 9, AIP Conference Proceedings 605, 295-300, 2002. [20] K.D.Irwin, L.R.Vale, N.E.Bergren, S.Deiker, E.N.Grossman, G.C.Hilton, S.W.Nam, C.D.Reintsema, D.A.Rudman and M.E.Huber, Proceedings of Low Temperature Detectors 9, AIP Conference Proceedings 605, 301-304, 2002.

Table 1: Main characteristics of photon detectors

Detector type: Spectral range Temperature range (K)

Dissipation range (W)

Detector size (pixel and array)

Range Wavelength (m) Min Max Min Max Pixel (µm) Array (n x n)Ge crystal Gamma / hard X-ray λ < 1E-11 m 50 100 0 0 10000 <10

CCD X-ray / Vis 1E-10 < λ < 1E-6 m 150-200 300 0.1 20 10-30 106 STJs X-ray–UV-Vis-NIR 1E-10 < λ < 5E-6 m 0.01 1 10-9 10-6 20-50 <103

µ-Calorimeters X-ray 1E-10 < λ < 1E-7 m 0.05 0.3 10-12 10-11 100 <100 TESs X-ray-UV-Vis-NIR 1E-10 < λ < 1E-6 m 0.05 0.3 10-11 10-9 100 <100

Photo-conductors-NIR NIR 5E-7 < λ < 5E-6 m 30 100 0.01 0.02 30-50 106 Photo-conductors-MIR MIR 5E-6 < λ < 5E-5 m 2 20 0.01 0.02 50-100 <104 Photo-conductors-FIR FIR 5E-5 < λ < 1E-4 m 1 2 0.001 0.003 50-100 <103

Sub-mm bolometers Sub-mm 1E-4 < λ < 1E-3 m 0.1 0.3 10-9 10-8 100-500 <102

Table 2: Cryogenic space missions (Space Science)

Mission: Application: Type/Class: Launch year:

Cryogenic system: In-flight T [K]

Lifetime Orbit: Status:

Einstein (NASA) Science / X Satellite (observat.) 1978 solid ammonia/methane 100 290 dd LEO Post-ops. IRAS (NASA, NIVR, SERC) Science / IR Satellite (surveyor) 1983 4He (λ) cryostat 3 290 dd Near Polar Post-ops. GIRL (FMST, D) Science / IR Spacelab payload --- 4He (λ) cryostat 2-4 --- LEO Not Approved COBE (NASA) Science / IR Satellite (surveyor) 1989 4He (λ) cryostat 1.4 - 1.6 305 dd Near-Earth Post-ops. ISO (ESA) Science / IR Satellite (observat.) 1995 4He (λ) cryostat 1.8 840 dd HEO Post-ops. SFU (ISAS/NASDA/MITI) Science / IR Instrument (IRST) 1995 4He (λ) cryostat +3He SC 0.3 30 dd LEO Post-ops. MSX (BMDO,US) MP/UV to FIR Satellite (observat.) 1996 sH2 cryostat < 8 600 dd LEO Post-ops HST (NASA) Science / NIR Nicmos, Instrument 1997 sN2 cryostat 60 700 dd LEO Post-ops. XQC (NASA-Wisc.Univ.) Science / X Sounding Rocket 1996-99 4He (λ) cryostat +ADR 0.065 1 d Suborbital flight Post-ops. WIRE (NASA) Science / IR Satellite (surveyor) 1999 Dual, sH2 cryostat < 7.5 120 dd LEO Post-ops/loss STEP (ESA) Science / FP Satellite --- 4He (λ) cryostat 1.8 180 dd LEO Not approved Astro-E (ISAS, NASA) Science / X Satellite (observat.) 2000 sNe+4He cryost.+ADR 0.065 730 dd LEO Loss INTEGRAL (ESA) Sci. / Gamma Instrument (observ.) 2002 Stirling cooler 85 2-5 yr HEO In operation SIRTF (NASA) Science / IR Satellite (observat.) 2003 4He (λ) cryostat 1.4 2.5 yr Earth trailing Commissioning WISE (NASA, old NGSS) Science / IR Satellite (surveyor). tbd Dual, sH2 cryostat < 7.5 400 dd Polar Study Submillimetron (ASC) Sci. / Sub mm ISS telescope > 2004 4He (λ) cryost.+3He SC 0.1-0.3 tbd LEO Study Astro-E2 (ISAS, NASA) Science / X Satellite (observat.) 2005 sNe+4He cryost.+ADR 0.065 730 dd LEO Development XEUS (ESA) Science / X Instrument (observ.) 2015 Stirling +ADR+3He SC 0.05 – 0.3 > 10 yr LEO Study Herschel (ESA) Science / IR Satellite (observat.) 2007 4He (λ) cryost.+3He SC 0.3 & 1.7 4.5 yr Sun-Earth L2 Development Planck (ESA) Science / FIR Satellite (surveyor) 2007 H2&4He JT + DR 0.1 & 20 460 dd Sun-Earth L2 Development JWST (NASA) Science / NIR Satellite (observat.) 2010 Passive + cooler + sH2 4 – 40 5-10 yr Sun-Earth L2 Study Constellation-X (NASA) Science / X Satellite (observat.) 2010 Astro-E like / coolers 0.05 3-5 yr Sun-Earth L2 Study ARISE (NASA) Sci. / Radio Satellite (VLBI) 2008 Cryo-cooler + H2 JT 20 tbd HEO Study DARWIN (ESA) Science / IR Satellite (VLBI) >2012 Cryo-cooler + H2 JT 4 tbd L2 / Earth trailing Study TPF (NASA) Science / IR Satellite (VLBI) >2010 Passive rad. + cooler. 30 5 yr L2 / Earth trailing Study Rosetta (ESA) Sci. / Comet Instrument (probe) 2004 Stirling cooler 80 10 yr Heliocentric Development LEDA/MORO (ESA) Sci. / Moon Satellite (surveyor) --- Cryostat + cooler 1.4 - 1.6 ? ---- Not approved

FP:= Fundamental Physics. MP:= Multipurpose mission (Defence + Science).