S. A. Collins

Voyager 2 in the Uranian system: imaging science results

Voyager 2 images of the southern hemisphere of Uranus indicate that submicrometersize haze particles and particles of a methane condensation cloud produce faint patterns in the atmosphere. The alignment of the cloud bands is similar to that of bands on Jupiter and Saturn, but the zonal winds are nearly opposite. At mid-latitudes (-70� to -27�), where winds were measured, the atmosphere rotates faster than the magnetic field; however, the rotation rate of the atmosphere decreases toward the equator, so that the two probably corotate at about -20�. Voyager images confirm the extremely low albedo of the ring particles. High phase angle images reveal on the order of 102 new ringlike features of very low optical depth and relatively high dust abundance interspersed within the main rings, as well as a broad, diffuse, low optical depth ring just inside the main rings system. Nine of the newly discovered small satellites (40 to 165 kilometers in diameter) orbit between the rings and Miranda; the tenth is within the ring system. Two of these small objects may gravitationally confine the e ring. Oberon and Umbriel have heavily cratered surfaces resembling the ancient cratered highlands of Earths moon, although Umbriel is almost completely covered with uniform dark material, which perhaps indicates some ongoing process. Titania and Ariel show crater populations different from those on Oberon and Umbriel; these were probably generated by collisions with debris confined to their orbits. Titania and Ariel also show many extensional fault systems; Ariel shows strong evidence for the presence of extrusive material. About halfof Mirandas surface is relatively bland, old, cratered terrain. The remainder comprises three large regions of younger terrain, each rectangular to ovoid in plan, that display complex sets of parallel and intersecting scarps and ridges as well as numerous outcrops of bright and dark materials, perhaps suggesting some exotic composition.

Discovery of Currently Active Extraterrestrial Volcanism

Two volcanic plumes were discovered on an image of Io taken as part of the Voyager optical navigation effort. This is the first evidence of active volcanism on any body in the solar system other than Earth.

Charge-Coupled Device Pinning Technologies

For most thinned silicon CCDs, the photosensitive volume is bounded on top and bottom by layers of silicon dioxide. The frontside oxide is grown to serve as an insulator beneath the conductive gates of the parallel array while the backside oxide forms naturally as the initially bare silicon oxidizes. This paper describes the characteristics of the interface between these oxides and the photo-sensitive silicon and indicates the extent to which CCD performance (e.g. dark current, spectral response, charge collection efficiency, charge transfer efficiency, pixel-nonuniformity read noise full well capacity blooming residual image and vulnerability to ionizing radiation damage) depend* upon these interfacial characteristics. Techniques are described to achieve optimum passivation of these interfaces and to thereby obtain superior performance in the areas just listed. Specifically an implanted structure (the Multi-Pinned-Phase, MPP) is described which provides excellent frontside passivation and several techniques (backside charging, flash gate, the biased flash gate and ion-implantation) are presented for back surface passivation.

The Future Scientific CCD

The charge-coupled device (CCD) dominates an ever-increasing variety of scientific imaging and spectroscopy applications. Recent experience indicates, however, that the full potential of CCD performance lies well beyond that which is realized in currently available devices. Test data suggest that major improvements are feasible in spectral response, charge collection, charge transfer, and readout noise. These properties, their measurement in existing CCDs, and their potential for future improvement are discussed in this paper.

Potential of CCDs for UV and x‐ray plasma diagnostics (invited)

A program is under way to develop charge-coupled device (CCD) sensors for space-based X-ray astronomy imaging spectrometers. To date, laboratory line-emission spectra have been acquired throughout the range of 277 to 8000 eV (carbon through copper K-alpha emission) and CCD sensitivity has been demonstrated throughout the range of 1.1 through 8000 eV. Image resolution is excellent, limited almost entirely by the 15 micron pixel size. These results are presented and specialized techniques are described which permit such low energy response, high spectral resolution, and efficient charge collection. Finally, analysis is presented of one particular CCD characteristic which currently limits UV and X-ray performance: charge diffusion.

Present and Future CCDs for UV and X-Ray Scientlfic Measurements

Significant advances in CCD performance are reported. Specifically, interacting quantum efficiency of ¿ 50% is demonstrated throughout the spectral range of 600-9,000 Å, with comparable sensitivity expected to continue to wavelengths as short as a few Angstroms. Non-dispersive X-ray spectra, throughout the 250-8000 eV range, have been obtained with an FWHM spectral resolution of 200-250 eV. At present, both spatial and spectral resolution is limited, at some energies, by the diffusion of photo-generated charge into more than one picture element. A detailed model of this diffusion is described and evaluated on the basis of measured performance.

Performance Characteristics Of CCD's For The Acis Experiment

The Advanced X-ray Astrophysics Facility CCD Imaging Spectrometer (ACIS) will use two arrays of CCDs to provide X-ray imaging and spectroscopy. Spectroscopy at medium resolution with the imaging array is accomplished by pulse height analysis of each X-ray interaction, while for high spectral resolution, an objective grating disperses the spectrum across a linear array of CCDs to provide a dispersed spectrum where the wavelength resolution is determined by the telescope imaging properties. Performance data for CCDs manufactured by Texas Instruments, MIT Lincoln Laboratory and Ford Aerospace Corp. will be presented. Plans for future CCD enhancements will be discussed.

Charge-Coupled Device Advances For X-Ray Scientific Applications In 1986

A theoretical model is presented that predicts the output response of a thinned CCD to soft x-ray spectra. The model simulates the four fundamental parameters that ultimately limit CCD performance: quantum efficiency, charge collection efficiency, charge transfer efficiency, and read noise. Simulated results are presented for a wide variety of CCD structures, and general conclusions are presented about achieving a practical balance of sensitivity, energy, and spatial resolution for an Advanced X-ray Astrophysics Facility (AXAF) instrument. We compare the results of the analysis to existing state-of-the-art CCDs and project improvements that will be made in the near future.

Nondispersive x‐ray spectroscopy and imaging of plasmas using a charge−coupled device

An 800×800 Texas Instruments virtual phase charge‐coupled device (CCD) was used to obtain pinhole images and x‐ray spectra of laser‐produced, solid target plasmas. With the CCD used in the single‐photon counting mode, the spectrum in the energy range 2–10 keV was obtained without a dispersive element. Typical spectra reveal two distinct temperatures: a ‘‘cold’’ component of approximately 200 eV and ‘‘hot’’ component of approximately 5 keV. Also, multiline spectra comprising characteristic line emission (Kα, Kβ) from a multilayer target bombarded by β rays were recorded using a three‐phase CCD. The results demonstrate the potential of CCDs as imaging spectrometers with application in space, laboratory, and fusion plasma research.