Publication archive

Publication archive

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
Images of an active region on the far side of the Sun were derived by applying seismic holography to recent helioseismic observations from space. Active regions are the centers of energetic phenomena such as solar flares and coronal mass ejections whose resulting electromagnetic and particle radiation interfere with telecommunications and power transmissions on Earth and can pose significant hazards to astronauts and spacecraft. Synoptic seismic imaging of far-side solar activity will now allow us to anticipate the appearance of large active regions more than a week ahead of their arrival on the east solar limb.
Published: 10 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
Published: 02 September 1999
Comet 46P/Wirtanen is the target comet of the ROSETTA mission. Here, we give an overview of the information currently available on this comet from remote-sensing observations. Main emphasis is put on the description of the coma in terms of morphology, composition and evolution. We also summarize the current knowledge of the basic properties of the nucleus, in particular its size and rotational properties.
Published: 02 September 1999
The International Rosetta Mission, approved by the Science Programme Committee of the European Space Agency as the Planetary Cornerstone Mission in ESA's long-term programme Horizon 2000, will rendezvous in 2011 with Comet 46P/Wirtanen close to its aphelion and will study the nucleus and the evolution of the coma for almost two years until it reaches perihelion.
Published: 02 September 1999
Astronomical observations of size and of related outgassing rates seem not to be compatible for the nucleus of comet 46 P/Wirtanen, the target comet of the ROSETTA mission. This possible disagreement has caused speculations about peculiar properties of this comet nucleus. It is shown by model calculations which also takes into account vertical heat fluxes into the nucleus that there is a possibility to combine the results of astronomical observations within a model of a freely sublimating ice surface of this comet with an outgassing area of about half the dayside surface. The resulting half-size parameter (i.e. the radius of an equivalent sphere) can be shown to be of about R H (725±230) m, and the nucleus is shown to have an active area of about half of the dayside surface, i.e. of about 25% of the total surface.
Published: 10 August 1999
This study is the first investigation of the dust collection by a spacecraft orbiting a cometary nucleus, which is based on a physically consistent ab-initio model of the dust distribution in the vicinity of an aspherical comet nucleus. The homogeneous bean-shaped nucleus of Crifo & Rodionov (1997a) is used, with updated parameter values adapted to comet 46P/Wirtanen, target of the Rosetta mission, but the conclusions of the study have a general significance.
Published: 02 June 1999
ESA's exciting and fast-track Mars Express mission is the first of the new flexible (F) missions, which are based on a new implementation scenario to maintain overall mission cost below a very stringent cost cap. The key features of an F-mission are streamlined management, up-front definition of the payload, and the transfer of more responsibility to industry. The cost ceiling is 175 MEuro for future F-missions, but only 150 MEuro for Mars Express. The scientific objectives of the mission include the remote and in-situ study of the surface, subsurface, atmosphere and environment of the planet Mars. ESA's Science Programme Committee preliminarily approved Mars Express in November 1998, provided sufficient funding would be available, and gave its full approval on 19 May 1999
Published: 01 July 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
Despite considerable progress over the past years, the detection of medium-energy X-ray photons (E>1 keV) with STJs near the energy-resolution limit, set by the Fano and tunnel noise, remains an elusive goal. There is presently little doubt that the spatially inhomogeneous response of the STJ is responsible for the degradation of the energy resolution. We review several proposed mechanisms against experimental data for Nb- and Ta-based STJs, of various sizes and in single or array-format. We argue against a single mechanism behind the resolution degradation. The experimental results presented here support a model in which quasi-particles are lost at the edges of the STJ, but also indicate that losses into the leads seriously degrade the energy resolution. Finally, an example is given of how fabrication details may play a role as well.
Published: 02 May 1999
The Infrared Space Observatory (ISO) Data Archive, developed by the ISO Data Centre in Villafranca, Spain, offers the astronomical community fast and easy access to all ISO products and related information through a pioneering World Wide Web (WWW) interface. The first release - issued on 9 December 1998 at http://www.iso.vilspa.esa.es/ - has already been accessed by many astronomers and visitors wishing to query the ISO database and to retrieve important scientific data.
Published: 02 May 1999
Two European survey missions are featured in the current ESA science program: Planck, a Cosmic Background Radiation mission, and GAIA, an astrometric mission. Both missions require global iterative processing over the spacecraft data in the spatial and time domains. The large data volumes and complex data structures involved demand novel analysis methods.
Published: 02 May 1999
We investigate whether the modelling of the immediate vicinity of an active nucleus -currently unobservable- can, as the modelling of the outer, observable coma, be based on unrealistic simple assumptions such as those of nucleus and dust grains sphericity. We point out the inconsistency of models based on such assumptions, which, to manage compatibility with the observations, have to introduce additional assumptions that conflict with the previous ones, such as the existence of active areas of the nucleus. We argue that, while the outer coma models being phenomenological in nature, can perhaps tolerate such inconsistencies, the circumnuclear coma models must be predictive, having to obviate the lack of observational data, and therefore must exclude implausible and ad hoc assumptions, and advocate only well-understood physical processes and duly validated modelling methods. We describe the first steps of development of a predictive circumnuclear coma model, and present a set of results obtained with parameters fitted to comet 46P/Wirtanen, the target of the Rosetta mission, but of a quite general significance.
Published: 02 May 1999
We summarise the results of a number of X-ray experiments on an epitaxial GaAs device carried out in both our laboratory and at the PTB Radiometry Laboratory at the BESSY Synchrotron Radiation Source. The detector has a diameter of 1.5 mm and is fully depleted to a depth of 40 micron. It has been characterized as a function of energy, bias and temperature. At -35 °C we determine the charge collection efficiency to be 97% and find that energy resolutions ranging from 730 to 930 eV fwhm can be readily achieved using conventional pre-amplifiers over the energy range 6-60 keV. By considering the various contributions to the fwhm, we show that leakage current and charge trapping noise dominate the resolution function. From detector modelling we determine the effective electron and hole density/cross section products to be 7 and 3 cm-1, respectively.
Published: 02 May 1999
The infrared emission of various comets can be matched within the framework that all comets are made of aggregated interstellar dust. This is demonstrated by comparing results on Halley (a periodic comet), Borrelly (a Jupiter family short period comet), Hale-Bopp (a long period comet), and extra-solar comets in the beta-Pictoris disk. Attempts have been made to generalize the chemical composition of comet nuclei based on the observation of cometary dust and volatiles and the interstellar dust model. Finally, we deduce some of the expected dust and surface properties of comet Wirtanen from the interstellar dust model as applied to other comets.
Published: 02 May 1999
31-May-2020 08:28 UT

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