ESA Science & Technology - Publication Archive

Recent experimental results show a linear energy response in high quality Nb-Al-AlO_x-Nb superconducting tunnel junction detectors for photon energies between 1.5 and 6.4 keV. The experimental data are based on both direct x-ray illumination and on the escape and re-absorption of fluorescent photons created in the junction electrodes and in the silicon substrate. The observed linearity of the energy response raises questions on the validity of some theoretical models which describe the relaxation process occurring in a superconducting thin film after x-ray photoabsorption. Such models generally predict nonlinear effects due to large quasiparticle number densities and short recombination times.

The Compton Telescope (COMPTEL) telescope aboard the Compton Gamma-Ray Observatory (CGRO) is capable of imaging gamma-ray line sources in the MeV region with a sensitivity of the order 10^-5 photons/(cm²s). During two observations periods in July 1992 and February 1993 the Galactic plane in the region of the young supernova remnant Cas A was observed, showing evidence for line emission at 1.16 MeV from the decay of ⁴⁴Ti at a significance level of approximately 4 sigma. This is the first time a supernova remnant has been detected in the gamma-ray line from ⁴⁴Ti decay. Adopting a distance of 2.8 kpc to the Cas A remnant, the measured line flux (7.0 +/- 1.7) x 10^-5 photons/(cm²s), can be translated into a ⁴⁴Ti mass ejected during the Cas A supernova explosion, between (1.4 +/- 0.4) x 10^-4 solar mass and (3.2 +/- 0.8) x 10^-4 solar mass, depending on the precise value of the ⁴⁴Ti mean life time and on the precise date of the event. Implications of this result for supernova nucleosynthesis models are discussed.

Current research into x-ray detection using superconducting tunnel junctions indicates that the poor spectral resolution obtained so far, in comparison with theoretical expectations, is partly due to the excellent acoustic coupling of the junction and substrate. The substrate acts both as a source of noise and as a heat sink for the nonequilibrium junction, thus masking the intrinsic response of the superconducting electrodes to photoexcitation. A new design for a superconducting tunnel junction based on an x-ray detector is presented. The design effectively decouples the substrate and junction and should therefore eliminate many causes of spectral degradation, bringing resolution closer to that predicted theoretically, and thus allowing experimental investigation of the intrinsic superconducting film response to x-ray photoexcitation. An outline of the way in which the design can be optimized geometrically to achieve the decoupling is given. Further optimization of the intrinsic film response to x-ray photons is achieved through the introduction of specific absorbing and trapping regions to improve both the quantum efficiency and charge output of the new design. The use of "pairing potential barriers" within the electrode leads will also improve the intrinsic resolution of this device.

Preliminary results on the X-ray performance of Nb-based superconducting tunnel junctions (STJs) with a highly transmissive barrier are reported. The results show that the energy resolution of these detectors can be improved by collimating the X-ray photons onto the junction barrier area, thus reducing illumination of the surrounding substrate and leads. A charge output of about 50% of the theoretical maximum has been recorded for these STJs, with full width at half maximum resolution of about 200 eV at 6 keV. Several mechanisms which are believed to degrade the energy resolution are also discussed. X-ray events are also detected by other junctions on the same chip which are not illuminated. This may indicate the presence of a marked phonon transmission along the sapphire substrate which acts as a phonon waveguide analogous to the light transmission mechanism in fiber optics

An insert based on a top-loading, fully welded stainless-steel design, with a special steel-copper weld is described. It has been developed explicitly for pumped helium cryostats, down to a temperature of about 1.2 K. The principal characteristics of this insert are a large high vacuum sample space, the absence of any cold seal, and the reliability and flexibility of use. Typical applications are optical/UV/x-ray photon counting experiments based on cryogenic detectors and the development of advanced heterodyne receivers using Superconductor-Insulator-Superconductor junctions as mixers in the millimeter and submillimeter regimes.

Observational evidence is presented indicating that beyond L = 1.7-2 plasma corotating in the plasmasphere expands continuously with a small outward directed bulk velocity perpendicular to geomagnetic field lines. A numerical simulation of plasmaspheric flux tube drift motion is presented in support of such an outward plasma expansion. The maximum expansion velocity inside the plasmasphere is determined by the maximum value of the plasma interchange velocity, which is inversely proportional to the value of the integrated Pedersen conductivity.

High-speed flows in the inner central plasma sheet (first reported by Baumjohann et al. (1990) are studied, together with the concurrent behavior of the plasma and magnetic field, by using AMPTE/IRM data from about 9 to 19 R(E) in the earth magnetotail. The conclusions drawn from the detailed analysis of a representative event are reinforced by a superposed epoch analysis applied on two years of data. The high-speed flows organize themselves in 10-min time scale flow enhancements called here bursty-bulk flow (BBF) events. Both temporal and spatial effects are responsible for their bursty nature. The flow velocity exhibits peaks of very large amplitude with a characteristic time scale of the order of a minute, which are usually associated with magnetic field dipolarizations and ion temeperature increases. The BBFs represent intervals of enhanced earthward convection and energy transport per unit area in the y-z GSM direction of the order of 5 x 10 exp 19 ergs/R(E-squared).

Pages: 485
Publisher: John Wiley and Sons
ISBN: 0-471-61011-9

The occurrence rates and typical characteristics of high-speed ion flows with velocities of 400-600 km/s are determined on the basis of the analysis of a large quantity of ion moments and magnetic field measurements acquired from the AMTE/IRM satellite data. It is determined that the occurrence rates in the plasma sheet boundary layer, the outer central plasma sheet and the neutral sheet neighborhood have 4:1:2 ratio, and they increase with AE in all three regions when going from low to moderate activity; that the largest occurrence rates are near the midnight meridian at the largest radial distances accessible to IRM; and that the high-speed flow occurrence rates and ion densities are anticorrelated. It is also determined that the high-speed flows are bursty, and their angular distribution is strongly peaked in the sunward direction. No tailward high speed ion flow was detected. Correlation between the obtained results and the existing models and theories is discussed.

Pages: 577
Publisher: Pergamon Press
ISBN: 0-08-034859-9

Present knowledge on gamma radiation that originates from cosmic-ray interactions with gas and photons in the ISM is reviewed. Basic principles of such radiation and the main observational results are described. The significance of the findings for molecular gas studies and cosmic ray studies is discussed. Present understanding of radiation from gamma-ray point sources is briefly addressed, emphasizing the possible diffuse nature of these sources.

A statistical study on the behaviors of ion and electron moments in the central plasma sheet was carried out using tail data obtained by the three-dimensional plasma instrument on board the AMPTE/IRM satellite in 1986. Results show that the ion temperature increases with increasing magnetic activity and the ion density decreases during disturbed intervals, except in the neutral sheet neighborhood at smaller radial distances. The ion and electron temperatures (Ti and Te, respectively) in the central plasma sheet were found to be highly correlated, with the Ti/Te ratio being constant over a wide range of temperatures and about twice as large as in the distant tail. The average ion flow speeds in the central plasma sheet (below 100 km/sec) are nearly identical to those found in the plasma sheet boundary layer, but their distribution functions for these regions are quite different.

Refractory metal Nb/Al/Al-oxide/Al/Nb junctions are shown to be sensitive to 6-keV X-rays over the temperature range from 2.8 to 1.4 K. For such junctions, which have an observed minimum ionizing energy of 12 MeV, a limiting energy resolution of 8 eV is predicted. Currently an energy resolution of 250 eV is observed at 1.4 K which is primarily dominated by system electronic noise. The Nb-based junctions are shown to be very stable with respect to thermal cycling while the nonequilibrium physics can be simply scaled from the theory of Sn junctions. It is concluded that on-chip arrays having a broad band pass and good energy resolution should be feasible to construct.

New results on the temporal and spectral characteristics of the high energy gamma ray emission from the Vela pulsar are presented. A new sensitive analysis method using maximum likelihood techniques has been applied to the final COS-B dataset. The whole pulsed flux is found to exhibit long term variability. The energy spectrum of this total emission cannot be described by a single power law and the data show strong evidence for a spectral break at approximately 300 MeV. Five discrete emission regions within the pulsar light-curve have been identified, with the spectral characteristics and long term behavior being different. These results support the idea that various source regions simultaneously exist in the pulsar magnetosphere and that the physical processes generating the gamma rays in these sites differ with the location.

A statistical survey was performed on ion moments (including data on bulk velocity, ion beta, the ratio between the ion pressure and magnetic pressure, and ion temperature anisotropy) near the plasma sheet boundary layer and the immediately adjacent outer central plasma sheet, using four months of tail data obtained by the three-dimensional plasma instrument aboard the AMPTE/IRM satellite. It was found that there is a sharp gradient in ion density and ion beta at the interface between the plasma sheet boundary layer and the central plasma sheet. Average flow velocities in the plasma sheet boundary layer were found to be surprisingly low, typically below 100 km/sec.

Ultraviolet spectroscopy from the Pioneer Venus Orbiter shows a decline in the cloud top abundance of SO₂ from about 100 ppb to about 10 ppb in the period 1978-1986. A consistent decline in polar haze has occurred over the same period, with the correlation coefficient between these two observables of r = 0.8. Star calibrations determine the instrument sensitivity to within 10%, which rules out the possibility that this is an instrumental effect. Systematic errors could increase the SO₂ abundance to twice the inferred values in later orbits. Tracking of SO₂ features and power spectral analysis give rotation periods for the longer-lived features of 3.6-5.2 days, consistent with cloud-tracked winds observed at other wavelengths. The behavior of SO₂ and polar haze can be plausibly explained by episodic injection of SO₂ into the cloud top regions, for example, by active volcanism.

COS-B viewed the Cyg X-3 region seven times between November, 1975, and February, 1982; a search for steady gamma-ray emission pulsed at the characteristic 4.8-hour period did not reveal its source. Leiden-MIT balloon experiment observations of Cyg X-3 in May, 1979 show the 4.8-hour modulation with sinusoidal light curve and modulation depth of 0.30, for energies of up to about 140 keV. The strong variability of Cyg X-3 over more than one order of magnitude at energies below 20 keV does not emerge in the data collected at hard X-ray energies.

The gamma-ray emissions from the Crab pulsar and nebula are analyzed. The data were collected from 1975-1982 in the Galactic anticenter region by COS-B in the energy range 50-3000 MeV. The temporal behavior of the pulsed gamma-ray flux from PSR 0531 + 21 and the unpulsed gamma-ray emissions from the Crab region are examined. It is determined that the spectrum of the pulsar emission is represented by a single power law with an index of 2.00 + or - 0.10 over the energy range of the instrument, and no systematic variation of spectral index with pulsar phase is noted; the spectrum of the unpulsed gamma-ray emission is represented by a single power law with a spectral index of 2.7 + or - 0.3.

This is one in a series of papers prepared in association with the release of the final COS-B database. Various aspects of the performance of the COS-B gamma-ray telescope are studied using in-flight data. First, the time-variation of the sensitivity and background over the 6.7 years of the mission is investigated. COS-B observed many regions of the sky repeatedly over the mission lifetime, allowing the study of the temporal behavior of the instrument. Using a multivariate optimization algorithm, the relative sensitivity for the 65 observation periods is determined to an accuracy generally better than 10 percent. The temporal variations of the instrumental background, assumed to be related to a cosmic-ray scaler aboard the satellite, are simultaneously determined. Second, the dependence of the instrumental background on the inclination to the telescope axis is analyzed using observations of high galactic latitude areas. The resulting corrections to the instrumental parameters are a necessary prerequisite for the combination of the data into large-scale maps for scientific analysis. The implementation of the corrections to give such maps is outlined.

The cosmic ray (CR) distribution in the Galaxy inside the solar circle is determined by using the kinematics of both H I and CO to ascertain the spatial distribution of the interstellar gas. The conversion factor between integrated CO line intensity and the molecular hydrogen column density is found, and possible large-scale variations of this relationship throughout the Galaxy are investigated. The galactocentric CR distributions are shown to be described satisfactorily by exponential distributions for radii larger than about three, with a radial scale length of 4-11 kpc for electrons and a scale length greater than 18 kpc for nuclei.