Imaging devices which are used or planned for x-ray imaging in the laser-fusion program are discussed. Resolution criteria are explained, and a suggestion is made for using the modulation transfer function as a uniform definition of resolution for these devices.
The highlights of X-ray Astronomy are reviewed spanning the years 1960 to the present. Examples of various types
of X-ray sources are discussed both for galactic and extragalactic objects. The observational data and possible models
are presented. The future of X-ray Astronomy is explored. The planned observations for the 1970 fs and 1980 Ts are
presented as well as the connections between astrophysics and X-ray astronomy.
Young men of 1915 replaced old-fashioned film with "modern" ionization chambers. But unenlightened analysts soon retrogressed to film so young men of the 1940's again demon-strated the superiority of electronic detectors. By the 1960's a whole generation had grown up with electronics and should have forgotten film, but surreptitiously it crept back into use for quantitative diagnostics of important pulsed x-ray sources. Even today there are those who claim the right to use it because of its greater range of acceptable photon energy and intensity and the simplicity of using it.
Some techniques used to extract quantitative data from the information contained in photographic images produced by grazing incidence soft X-ray optical systems are described. The discussion is focussed on the analysis of the data returned by the S-054 X-Ray Spectrographic Telescope Experiment on Skylab. The parameters of the instrument and the procedures used for its calibration are described. The technique used to convert photographic density to focal plane X-ray irradiance is outlined. The deconvolution of the telescope point response function from the image data is discussed. Methods of estimating the temperature, pressure, and number density of coronal plasmas are outlined.
Soft x-ray emission from the Oak Ridge Tokamak (ORMAK) has been studied with an array of diode detectors. The thermal character of the plasma emission allows observation of phenomena associated with fluctuations in electron temperature. Measurements of internal disruption growth rates and repetition times corroborate a theoretical tearing-mode instability model. Heat pulses from the disruption evolve diffusively, but with a heat conduction coefficient larger than that inferred from the plasma equilibrium.
A brief review of recent studies of fluctuations in x-rav emission from tokamak plasmas of controlled thermonuclear fusion interest is given. The origin of the x-rays, the nature of the oscillations, and measurement and analysis techniques are discussed, with emphasis on the work performed on the ST and PLT tokamaks. Areas for future research, particularly in the region of reconstruction, are stressed.
Numerical image processing techniques have been applied to the restoration of x-ray pinhole photographs of solid targets irradiated by intense relativistic electron beams. An experimentally determined point spread function for the pinhole camera was utilized to eliminate the effects of high energy x-ray degradation of pin-hole resolution. Spatially resolved measurements of x-ray yield were made by measuring the dose to calibrated film and calculating the exposure using Monte-Carlo electron-photon transport calculations.
A brief discussion of the potential of the Zone Plate Coded Imaging (ZPCI) technique for x-ray and par-ticle microscopy of laser fusion plasmas is provided. In addition, the first successful application of ZPCI to high resolution (≤ 10 μm), tomographic imaging of the x-ray and alpha particle emission from a laser fusion plasma is reported.
A coded aperture imaging system for the measurement of nuclear fuel motion in simulated core disruptive accidents is described. A one dimensional Fresnel zone plate type coded aperture is used to modulate the fission gamma rays emitted from the fuel pin under test. Two active imaging systems have been developed to detect the coded image and record it on film using a high speed camera. The first system is an X-ray image intensifier which converts fission gamma rays to optical photons in a thin (0.1 mm) CsI(Tl) scintillator screen on which a photocathode is deposited. The photoelectrons thus created are collected and focused by electron optics and produce an image on the output phosphor. The second system utilizes a NaI(Tl) scintil-lator crystal and a separate high gain optical image intensifier to produce the desired coded image. With a few modifications both systems appear capable of achieving the preliminary design goals of 1 mm radial resolution, 5 mm axial (along the fuel pin axis) resolution and 1 msec temporal resolution. Presently, the X-ray image intensifier system offers the best resolution, but the second system provides higher gain and a wider selection of operating conditions. The temporal response of both systems is limited to a few milliseconds by the output phosphor. Experimental performance data for each system will be presented, and the results of pre-liminary nuclear fuel pin imaging experiment performed at the Sandia Pulsed Reactor will be shown.
One-dimensional coded apertures have been investigated for use in imaging long, narrow objects such as reactor fuel pins. Since coded apertures work better for small, point-like objects, while pinhole apertures work better for large extended objects, a hybrid aperture combining these properties seems attractive for imaging fuel pins. The one-dimensional aperture behaves like a coded aperture in the direction for which the object is small, while it acts like a pinhole in the direction for which the object is extended. A one-dimensional Fresnel zone plate has been investigated, and an iterative, nonlinear algorithm has been developed for removing the background produced by extra-core values in the point spread function. The algorithm is shown to have a decidedly nonlinear effect on the effective system transfer function. Uniformly redundant sequences have also been investigated as possible aperture codes. They can produce filled apertures and result in a decoded image with a uniform background level that can be easily removed by subtraction.
Photographs of coronal active regions,taken with Fresnel Zone plates were quantitatively analyzed. For this purpose the properties of absorption edge filters used for wave-length selection and of film sensitivity, as well as the imaging properties of Fresnel Zone plates were comprehensively investigated.The separation of the focused +1st order of diffraction picture from the zeroth and -1st order patterns is discussed. We put up the relation between temperature and density in the emitting regions and the energy density distributions in the corresponding pictures, including the effect of chromatic aberration. An iteration procedure was applied to solve the set of equations,which characterizes this relation, leading to two dimensional maps of temperature and emission measure as illustrated for one active region.
Grazing-incidence x-ray microscopy is used extensively at Lawrence Livermore Laboratory to diagnose laser fusion plasmas. The design, fabrication, and imaging characteristics of two microscope systems are discussed. These systems are based upon orthogonal, cylindrical mirror pairs and axisymmetric, hyperboloid/ ellipsoid x-ray optics.
Two sounding rocket experiments designed to study celestial sources of radiation between 100 and 1000 A are described. The Wolter-Schwarzschild Type II grazing incidence mirrors in the experiments have been built by the diamond turning technique and have achieved one arcminute resolution. Design considerations and fabrication procedures for each are discussed. The first, a survey instrument, has been flown once. The second, a spectroscopic instrument, is being readied for launch in summer 1977.
The x-ray image formed by total-external reflection from a concave spherical or cylindrical surface is faulted by numerous geometrical aberrations. Unfortunately the "blur" recorded on the film does not separate out the individual aberrations or reveal the controlling system parameters. Ray tracing with the aid of a high speed computer may aid in identifying a prominent type of aberration but will not in general yield the functional dependence of the system parameters on the blur. The present analysis develops a theo-retical expression for the blur as a power series in which the individual terms may be identified with well known aberrations defects. The coefficient of each term is expressed in terms of applicable system parameters and thus indicates almost at a glance what can be done to minimize a particular aberration. It is cautioned however that a favorable change in one aberration coefficient may cause an unfavorable change in another.
In recent years X-ray telescopes have been used successfully for high resolution observations of astronomical objects, in particular the solar corona. At the present time X-ray microscopes are being developed for diagnostics of laser and electron beam induced fusion plasmas. This paper describes both types of im-aging systems using as examples the AS&E X-ray telescope on Skylab and a 10X microscope being built for Los Alamos Scientific Laboratories. The designs, manufacturing tolerances, fabrication, testing, and per-formance are discussed. Steps for further development in both imaging areas are identified.
We describe how a glass strip may be bent to match any curve of large radius of curvature, to a high de-gree of accuracy, so that it may be used as a glancing incidence X-ray or extreme ultraviolet optical element. The desired match is obtained by applying a suitable combination of end couples and adjusting the cross-sectional moment of inertia along the length of the strip by varying its breadth. Two applications are described; a parabolic reflector for use as a laboratory X-ray collimator for testing mechanical collimators to be used in space instruments, and an elliptical reflector for point-to-point focussing. The latter device has been used for testing gratings in the X-ray and extreme ultraviolet (euv) regions and for concentrating radiation on to the entrance slit of a spectrometer, with a resultant gain in speed and resolution. We suggest that the technique may have other laboratory and space applications, which are briefly described.
The X-ray mirror for HEAO-B is now complete. The design, fabrication and visible light evaluation of the mirror are discussed. Final evaluation will depend upon X-ray calibration tests which will be performed in June-July, 1977.
Fresnel zone plate apertures have been used for coded imaging of x-rays generated by relativistic electron beams striking metallic targets. Calculations are presented which specify the design of the imaging system for 100 μm resolution. Results are given both for d.c. tests with a several μA, 0.75 MeV beam and for the 300 kA, 1 MeV Hydra machine which pulses for 80 nsec. An improvement in resolution compared to pinhole techniques is seen in the decoded images.
The Focal Plane Crystal Spectrometer on HEAO-B is a moderate to high resolution, curved-crystal, Bragg spectrometer which operates behind a grazing incidence x-ray telescope. It is designed to allow detailed spectral studies of both point and extended celestial x-ray sources in the energy range 0.2-3.3 keV, with resolutions of 50 to 1000. The analyzing elements are six torroidal diffractors, and the detectors are position-sensitive, flow, proportional counters. HEAO-B is scheduled for launch in June, 1978.
A Hard X-Ray Imaging Instrument is described which is capable of high resolution imaging of solar and cosmic hard X-ray sources between 2 and 80 kev during Shuttle sortie flights. The properties of solar burst sources and the resulting instrument requirements are discussed. The instrument envelope of 1.2 x 1.2 x 3.0 meters includes a tungsten multigrid collimator which has 4" resolution, a 40' response envelope and a point source effective area of 26 cm2. A combination of periodic fan beans and non-periodic pencil beams enable a unique deconvolution to be achieved within a 128" x 128" field without mechanical scanning. The detector system is a set of direct-readout 40 atm-cm Xenon-filled proportional counters, designed to minimize background. The instrument is capable of refurbishment to optimize the collimator configuration for specific solar or cosmic scientific objectives, to upgrade the angular resolution or to extend the high energy response.
We have used two types of MOS detector arrays to sense directly patterns of soft x-rays, in the Lawrence Livermore Laboratory experimental laser-fusion program. A linear self-scanning photodiode array CSSPA) is used in a wave-length-dispersive spectrometer. A frame transfer charge-coupled device (CCD) facilitates the use of an x-ray microscope. Measurements and calculations of the x-ray sensitivity of these devices will be presented. Their linearity and dynamic range will be discussed, as well as data recovery systems for each detector. We will describe our experiences in using these devices to detect pulses of x-rays in laser-fusion experiments.
We have measured both the current response and the pulse response of a microchannel plate, MCP, to X-rays incident at an angle of 45° in the energy range of 8 to 100 keV. The X-ray detection efficiency, pulse height distributions, and sensitive depth of the MCP were determined from these data. We find that detection efficiency extrapolates to zero below 8 keV, and ranges between values of 1% and 2.6% above this energy, much higher than anticipated from previous measurements for oblique incidence below 6 keV. When compared to previous measurements at higher energies, we conclude that MCP's have a relatively constant detection efficiency in the energy range 10 to 600 keV, which is a rather unique property as compared to other X-ray detectors. We find that the shape of current response curves differs somewhat from the shape of efficiency curves, that pulse height distributions are proportional to pulse height to the -1.1 to -1.6 power, and that 90% of the current ori-ginates in the first 25 to 30% of the MCP thickness.
The quantum efficiency of microchannel plates has been measured at energies from 0.28 to 3.0 keV and at angles less than 6°. The quantum efficiency reaches a maximum of 27% at 3° and an energy of 0.86 keV. At all energies, the quantum efficiency increases as the angle of incidence decreases until a critical angle is reached, after which the efficiency decreases rapidly. The quantum efficiency, outside of the small angle dip, decreases with energy at a constant angle. The general behaviour of the quantum efficiency can be understood using a very simple physical model. The low angle dip is caused by reflection from the surface of the channel. The variation of the quantum efficiency as a function of energy can be explained by the variation of the X-ray absorption and of the photo- and Auger electron ranges. The data are reasonably reproduced by a simple model.
We describe the X-ray detector which will provide second-of-arc images for the HEAO-B X-ray observatory. The instrument uses microchannel plates as a photocathode surface and imaging photoelectron multiplier, and a crossed wire grid as a two-dimensional position-sensitive detector. Position determination is accomplished by electronic interpolation between the coarse grid wires. The detector provides the arrival time and position of each event which occurs with-in its field of view. We describe our measurements of the spatial resolution (15µm), temporal resolution (7.811s), image distortion (<10µm), uniformity of efficiency (< +10%), dark counting rate (0.2 cts cm-2 s-1), and quantum efficiency (10% at 1.54 key) of this detector. We also give a summary of the procedure which we have found useful during the initial turn-on of microchannel plates.