Publication archive

Publication archive

Mercury is one of the least explored planets, and has many perplexing features. A conventional sample return mission requires significant launch mass, due to the large ¿v required for the outbound and return trips, and the large mass allocation necessary for a planetary lander and ascent vehicle. Solar sailing can be used to reduce the lander mass allocation by delivering the lander to low, thermally safe orbit close to the terminator. In addition, the ascending node of the solar sail parking orbit plane can be artificially forced to avoid out-of-plane manoeuvres during ascent from the planetary surface.
Published: 02 May 2004
The superconducting differential accelerometers both for the Equivalence Principle experiment and geodesy within the European STEP mission have common design principles. The test masses are suspended by stable superconducting magnetic levitation. The suspension design makes all the degrees of freedom of the masses stiff, except for the axial differential mode which is made compliant in order to obtain a high intrinsic sensitivity of the differential accelerometer, < 10-14m s-2Hz-1/2. Trimming of persistent currents circulating in the levitation system allows to achieve rejection ratios for the unwanted common and radial accelerations > 106. Two separate superconducting circuits couple the axial displacements of the test masses to two SQUIDs. Persistent currents are stored in the two circuits such that one SQUID is only coupled to the differential displacement while the other only senses the common one. By differencing the signal before its detection, one highly reduces the dynamic range needs of the SQUIDs, of the following amplifiers and of the final A/D converters.
Published: 22 December 2003
Few facts in science are more surprising and none has had a longer history than the apparent equivalence of the two kinds of mass in physics, gravitational and inertial. From Galileo and Newton to Eötvös and Einstein, it has been a compelling issue both theoretically and experimentally. Ground-based tests have now a precision of about 1 part in 1012. Even with this extraordinary agreement, there are profound theoretical reasons for carrying the measurements further. Our generation has the unique oppurtunity to make an advance of a factor of a million in testing the Equivalence Principle in space.
Published: 22 December 2003
Proceedings of the Fifth IAA International Conference on Low-Cost planetary Missions

Geosail is a small, low cost, innovative mission designed to exploit the versatility of solar sail propulsion for the exploration of magnetic reconnection and electron dynamics in the earth's magnetotail. The GeoSail mission requires only a very low performance solar sail to precess the major axis of an otherwise inertially fixed orbit, thus maintaining payload alignment within the geomagnetic tail. This constant rotation enables a near continuous observation window with the opportunity to probe the rapid dynamic evolution of energetic particle distributions in this critical region of geospace. An end-to-end system design study has been concluded and the key performance requirements identified. The level of solar sail performance required for GeoSail is typical of that currently being discussed within Europe for a near-term technology demonstration mission. GeoSail is therefore capable of providing both technology validation within the cost restrictions of a SMART mission while also returning unique science data from a first solar sail mission.

Published: 16 September 2003
We recently predicted the formation of a highly non-equilibrium quasiparticle (qp) distribution in low TC multiple tunnelling superconducting tunnel junctions (STJs) [1]. The situation arises through qp energy gain in cycles of successive forward and back tunnelling events in the absence of relaxation via sub-gap phonon emission. The qps can acquire sufficient energy to emit phonons, which break more Cooper pairs and release additional qps. In this paper we report theoretical and experimental studies of the effect of this process on photon detection by such an STJ. We derived a set of energy-dependent balance equations [2], which describe the kinetics of the qps and phonons, including the qp multiplication process described above. Solution of the balance equations gives the non-equilibrium distribution of the qps as a function of time and energy, and hence the responsivity of the STJ as a function of bias voltage. We compared the theoretical results with experiments on high quality, multiple-tunnelling Al STJs cooled to 35mK in an adiabatic demagnetisation refrigerator, and illuminated with monochromatic photons with wavelengths between 250 and 1000 nm. It was found that in the larger junctions with the longest qp loss time, both responsivity and signal decay time increased rapidly with bias voltage. Excellent agreement was obtained between the observed effects and theoretical modelling.
Published: 02 May 2003
We discuss the detector requirements for future X-ray astrophysics missions and present preliminary results from our compound semiconductor program designed to produce X-ray detectors with high spatial and spectral resolution across the energy range 1 keV to 200 keV. Several prototype detectors have been fabricated from monocrystalline TlBr and tested at hard X-ray wavelengths in our laboratories and at the ESRF synchrotron research facility. Energy resolutions of 1.6 keV (fwhm) at 5.9 keV and 2.6 keV (fwhm) at 26 keV have been achieved, although we find that performance is highly variable due to polarisation effects. The resolution function is dominated by high leakage current at all energies. From pulse height measurements of Am241 as a function of detector bias, we derive the electron mobility-lifetime product at -2 °C to be (2.9±0.2) x 10-4 cm² V-1. This is about an order of magnitude higher than previously reported values.
Published: 01 February 2001
S-Cam is a cryogenic camera for ground based astronomy designed around a 6x6 array of Ta-Al Superconducting Tunnel Junctions (STJs). The camera has been conceived as a technology demonstrator, aiming to prove the potential of this new generation of single photon counting detectors at a ground-based telescope as a possible precursor to space based applications. Following a first test campaign at the William Herschel Telescope in La Palma (Canary Islands, Spain), an improved version of the camera (S-Cam 2) has been developed and tested. In this paper we provide an overview of the latest camera performance, a description of the up-dated S-Cam 2 system and a summary of the main test results. An example of the novel astronomical data obtained during the second test campaign conducted in December 1999 are also shortly described.
Published: 16 August 2000
We report on the design and testing of a new readout scheme for Superconducting Tunnel Junction (STJ) arrays. By grouping the electrodes in rows and columns, this method drastically reduces the number of connections and electronic circuits reruired for reading out a large format array of pixelated detectors. It is a generic scheme in that it could be applied to different kinds of detector arrays. Using charge sensitive amplifiers with junction field-effect transistors (JFETs) we verify that the energy resolution degrades primarily due to capacitance increase at the amplifier's input node. However, since each detector is read-out by two independent circuits, these two outputs can be combined to increase the signal-tonoise level. The measurements reported here were carried out on an array of 6x6 junctions. All junctions were biased but only 2 rows and 2 columns read-out. We compare the results to measurements carried out on a similar 6x6 array fabricated from the same trilayer but with individual pixel read-out. The measurements show that stable biasing of STJs is possible with the new configuration and that the measured optical spectral line resolutions are consistent with our theoretical predictions.
Published: 16 August 2000
Superconducting tunnel junctions (STJs) have been demonstrated as photon counting detectors in the UV-NIR wavelength range (100-1000 nm). They combine a modest wavelength resolving power with fast response and high detection efficiency over a broad wavelength band. This makes this type of detector an interesting alternative to the present generation of detectors used in UV/optical astronomy, such as CCDs and micro-channel plates. Practical applications require imaging detectors with large sensitive area and good spatial resolution. While the feasibillity of small arrays of closely packed STJs which are individually biased and read-out has already been demonstrated, the development of large format arrays is limited by the large number of electronics chains and wire connections to the cold detector which would be required. An alternative approach is to use a large area absorber combined with a few STJs at the edges or corners. A photon's energy as well as its absorption position in the absorber can be derived from the signal amplitudes measured in the STJs. In this paper the performance in terms of wavelength resolving power and position resolution of four different linear geometries of Ta absorbers, read out with Ta-Al STJs, is investigated and compared with single STJs. The UV and optical spectra obtained with the absorbers show resolving powers within a factor of two of the theoretical limit. In particular, a measured resolving power at lambda=300 nm of ~16 with a position resolution of ~9 micron is achieved with a 100x50 micron² absorber in between two 50x50 micron² STJs.
Published: 16 August 2000
We present an experimental study of the performance of Distributed Read-Out Imaging Devices (DROIDs), 1- and 2- dimensional photon-counting imaging spectrometers, based on Ta/Al-based STJs placed on a Ta absorber. Results obtained with highly collimated illumination with 10 keV X-ray photons clearly demonstrate the imaging capabilities of 2-dimensional DROIDs. The derived spatial FWHM resolution is 7 micron for a 200 x 200 micron² absorber. With a 1-D DROID we have measured an intrinsic energy resolution of 15 eV FWHM for 6 keV photons. At high energies (E > 6 keV) the resolution is limited by spatial fluctuations in the qp recombination rate.
Published: 16 July 2000
We present an application of the generalised proximity effect theory. The theory has been used to determine the energy gap (Delta g) in proximised transition metal - aluminium bilayer structures such as Nb/Al, Ta/Al, V/Al and Mo/Al. These bilayers have different film thicknesses ranging from 5 to 260 nm. For the cases of Nb/Al, Ta/Al and V/Al bilayers, the interface parameters Gamma and GammaBN (here we define Gamma as the ratio of the products of normal state resistivity and coherence length in each film of the bilayer while GammaBN is the ratio of the boundary resistance between film 1 and 2 to the product of the resistivity and coherence length in the second film), which were used as input parameters to the model, were inferred experimentally from an existing bilayer of each kind and then suitably modified for different film thicknesses. This experimental assessment is therefore based on a comparison of measurements of the critical temperature and the energy gap at 300 mK with the predictions from the model for various values of Gamma, GammaBN. The energy gap of the bilayer was experimentally determined by using symmetrical Superconducting Tunnel Junctions (STJs) of the form S-Al-AlOx-Al-S, where each electrode corresponds to a proximised bilayer. However for the case of Mo/Al bilayers the interface parameters were determined theoretically since currently no STJ data for this configuration are available. The results for the Nb/Al, Ta/Al and V/Al bilayers have also then been compared to experimentally determined energy gaps found for a series of STJs with different film thicknesses. The correspondence between experiment and theory is very good.
Published: 16 July 2000
We discuss the observational requirements for future X-ray planetary and astrophysics missions and present preliminary laboratory results from our compound semiconductor program. The detectors used in the tests were simple monolithic devices, which are used in conjunction with a detailed material science and technology development program intended to produce near Fano limited, pixilated hard X-ray detectors. In practical terms, this means producing active arrays, comprised of over 10³ pixels each being of order 100 microns in size, with spectral resolving powers, E/DeltaE > 20 at 10 keV and high quantum efficiencies over the energy range 1 to 200 keV. Four materials are currently under study - GaAs, HgI2, TlBr and CdZnTe. In the cases of GaAs and CdZnTe, the detector energy resolution functions are approaching the Fano limit.
Published: 16 July 2000
S-Cam is a cryogenic camera for ground based astronomy based on a 6x6 array of Superconducting Tunnel Junctions (STJs). The camera has been designed as a technology demonstrator, aiming to prove the potential of this new generation of single photon counting detectors at a ground-based telescope. In this article we provide an overview of the detector performance, a description of the S-Cam system and a summary of the test results. The first astronomical data obtained at the William Herschel Telescope (WHT) in La Palma (Canary Islands, Spain) are also described.
Published: 08 April 2000
We report on hard X-ray measurements with two epitaxial GaAs detectors of active areas 2.22 mm² and thicknesses 40 and 400 microns at the ESRF and HASYLAB synchrotron research facilities. The detectors were fabricated using high purity material and in spite of an order of magnitude difference in depletion depths, they were found to have comparable performances with energy resolutions at -45 °C of ~1 keV fwhm at 7 keV rising to ~2 keV fwhm at 200 keV and noise floors in the range 1-1.5 keV. At energies < 30 keV, the energy resolution was dominated by leakage current and electromagnetic pick-up, while at the highest energies measured, the resolutions approach the expected Fano limit (e.g., ~1 keV near 200 keV). Both detectors are remarkably linear, with average rms non-linearities of 0.2% over the energy range 10-60 keV, which, taken in conjunction with Monte-Carlo results indicate that charge collection efficiencies must be in excess of 98%. This is consistent with material science metrology which show that the material used to produce them is of extremely high purity with impurity concentrations < 1013 cm-3.
Published: 11 March 2000
Proceedings of 1999 NASA/JPL International Conference on FUNDAMENTAL PHYSICS IN SPACE, April 29,30 and May 1, 1999, Washington DC, NASA Document D-18925. "GALILEO GALILEI" (GG) is a proposal for a small, low orbit satellite devoted to testing the Equivalence Principle (EP) of Galileo, Newton and Einstein. The GG Report on Phase A Study recently carried out with funding from ASI (Agenzia Spaziale Italiana) concludes that GG can test the Equivalence Principle to 1 part in 1017 at room temperature. The main novelty is to modulate the expected differential signal of an EP violation at the spin rate of the spacecraft (2 Hz). As compared to other experiments, the modulation frequency is increased by more than a factor 104, thus reducing 1/f (low frequency) electronic and mechanical noise. The challenge in this field is to fly an experiment able to improve by many orders of magnitude the current best sensitivity (of about 1 part 1012). This requires spurious relative motions of the test bodies to be greatly reduced, leaving them essentially motionless. Doing that with more than one pair of bodies appears to be an unnecessary complication. This is why GG is now proposed with a single pair of test masses. -- Remainder of abstract truncated --
Published: 15 January 2000
We present the first results obtained with a 2-dimensional X-ray imaging spectrometer consisting of a 200x200 micron² Ta absorber and read out by four Ta/Al superconducting tunnel junctions (STJs). A preliminary image reconstruction algorithm allows the visualisation of the diffraction pattern from a 5 micron pinhole illuminated with 10 keV X-rays. The image suggests a spatial resolution better than 10 micron. The algorithm does not take into account quasi-particle losses in the absorber. Hence, the pulse height reconstruction is not optimal and the energy resolution varies significantly across the absorber. The best energy resolution is obtained for a 20x20 micron² area in the centre of the absorber, and amounts to ~77 eV at a photon energy of 5895 eV, with a 70 eV electronic noise contribution.
Published: 15 January 2000
We present two independent experiments, each of which suggests that the local energy gap in Ta (and Nb) has a lateral spatial variation on a scale of several micron. The first experiment is a series of current-voltage characterizations of Nb/Al/AlOx and Ta/Al/AlOx Josephson junctions, which reveals a dependence of the measured energy gap on the size of the junction. This implies a geometrical dependence of the energy gap. An extended version of the current theory of the proximity effect could explain this phenomenon when a lateral coherence length is introduced, which is of the order of the bulk coherence length of the materials. The second experiment is a series of coincidence measurements of photon absorption events in a Ta absorber between two Ta/Al junctions. There is a clear distinction in the pulse-height characteristics between events detected in the absorber and the junctions. Interestingly, there are also events indicating the presence of a transition region between the absorber and the junction. Event statistics imply that this region has a size of ~6 micron, independent of photon energy, which is quite a bit larger than even the bulk coherence length in Ta. It is argued that an additional effect due to 'smearing' by the relaxed quasi-particle cloud must also be present. These effects are interesting and intriguing; not only from a theoretical viewpoint, but also for energy-gap engineering of superconducting materials for practical applications, e.g. in a variety of photon and particle detectors.
Published: 15 January 2000
Modern cryogenic detectors, such as superconducting Tunnel Junctions and Transition Edge Sensors, provide single photon counting performance, medium to high energy resolution, high count rates and good photon collection efficiency over a wide wavelength range. In order to avoid background limited performance, it is necessary to shield the detectors from any thermal IR radiation originating from the surrounding warm surfaces. In this paper we analyse the contribution of the thermal radiation to the detector performance and describe the IR filters used in the S-Cam camera and in other experimental configurations. Future detectors may require very severe attenuation of the IR flux (lambda>1 micron). Solutions to this problem are proposed and their validity demonstrated with experimental results.
Published: 15 January 2000
In "Ultraviolet-Optical Space Astronomy Beyond HST" ASP Conference Series 164, ed. Jon A. Morse, J. Michael Shull, & Anne L. Kinney. Astronomical detectors based on Superconducting Tunnel Junction (STJ) technology are the next logical step beyond CCD and MCP detectors for optical and ultraviolet astronomy. Rather than merely registering each photon event with high efficiency, these devices also measure the wavelength of the detected photons throughout the far-UV through near-IR. Large format STJ arrays promise to provide the ultimate '3D' astronomical detector capable of covering simultaneously more than a decade in wavelength. The STJ detector holds particular promise in space-based ultraviolet applications where its inherent spectral resolution is the highest and its naturally high quantum efficiency overcomes the sensitivity limit set by available UV photocathode materials. The capability to separate spectral orders on the detector also opens up exciting new possibilities for novel and highly efficient grating spectrometer designs.
Published: 15 December 1999
A number of 6×6 element arrays of Ta-based superconducting tunnel junctions have been manufactured for photon counting applications with moderate energy resolution in ground-based optical astronomy. The individual array elements show low leakage, uniform responsivity across the array, good simultaneous Josephson current suppression and minor crosstalk between adjacent pixels. The same arrays have been characterized in the soft X-ray range (E=270-1500 eV). The base electrode response shows good energy resolving power (E/DeltaE = ~140). Unwanted spectral features originating from other parts of the detector can be largely eliminated by rise-time filtering. Modifications in the layering are necessary in order to improve the soft X-ray detection efficiency.
Published: 02 May 1999
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