Cygnus is a dual beam high-energy radiographic x-ray source. Ten years ago, three large zoom lenses were assembled to collect images from 200 mm x 200 mm square scintillators. The zoom capability allows zooming down to a 60 mm x 60 mm picture from the scintillator. Current radiographic imaging needs now require larger 270 mm x 270 mm square scintillators and the capability to use both 92 mm x 92 mm and 62 mm x 62 mm CCD cameras, and a new lens design to meet these needs. This zoom lens incorporates 11 elements and is designed to be telecentric. It images a scintillator emitting light peaking at 435 nm, so special glass types are required for the lens elements. Much larger elliptical pellicles are needed to deflect the scintillator light out of the x-ray path into the lens. The optical axis of the imaging system must be colinear with the x-ray axis. Two scintillators are positioned in each of two Cygnus x-ray axes, for a total of four scintillators and four lens systems. An optional configuration will be shown, enabling two lens systems imaging opposite sides of a single scintillator, for a total of four lenses and two scintillators. Although this configuration has advantages, it suffers from crosstalk. Care must be taken to analyze the anti-reflection coatings applied to all the elements in the imaging chain, including the CCD array and its vacuum window. The evolution of our Cygnus radiographic systems over the last two decades will be discussed.
Interrogating ejecta particles launched from target materials that are undergoing dynamic shock can be done with both xray imaging and visible shadowgraph imaging. Our dynamic testing must be done inside a containment vessel with limited access ports available. We designed an imaging system to relay both types of imaging systems through a single port using the same optical relay and then splitting the images onto three separate high-speed imaging cameras outside the containment vessel. X-ray imaging provides ejecta density measurements. Shadowgraph imaging that is done at two wavelengths (blue and red) constrains ejecta particle size distributions and provides areal density measurements of the ejecta cloud. The ejecta particles are positioned 225 mm before the x-ray scintillators; this arrangement permits a folded mirror system to allow the shadowgraph data to bypass the x-ray scintillators. This configuration results in spatial separations between the intermediate image planes of the x-ray and shadowgraph images along the optical axis. At the position of the x-ray intermediate image plane, mirrors are positioned such that the shadowgraph images are kicked out and their images are sent on to different cameras. Positioning of the large doublet relay lenses keeps shrapnel from impacting the vessel containment windows.
Often only a single physical process or component is investigated in the simulation of radiation detector systems. The results are then considered to be representative of what is expected in the correlating physical experiment. Although singular assessments may serve as a good estimate, the overall performance of a radiation detector system depends on several physical processes and the performance of all components within the system. Our Geant4-based multiphysics simulation toolkit couples radiation transport with optical photon processes, providing simulations of radiation detector systems components from the scintillator through the photocathode of the photodetector. Work to incorporate the backend detector components, including the complete photodetector and subsequent electronics (e.g., amplifiers, digitizers), is underway. Geant4 is used to model the radiation transport and optical photon processes that occur in the front-end detector system components when exposed to a chosen source. These components include the scintillator, detector housing, optical coupling to the photodetector, and photocathode of the photodetector. Characteristics of several detector systems that have been studied include time response; pulse height spectra; number of photoelectrons per MeV; detector efficiency versus incident quanta energy; and effects on detector response due to change in geometries, materials, and reflectivity. Comparison of these characteristics by means of this toolkit enables the selection of the optimal individual components; thus, it is possible to specify the radiation detector system best suited to meet the requirements of any physical experiment.
The digital time-resolved spot diagnostic (D-TRSD) is intended to be a fast-response, time-dependent beam diagnostic for measuring the spot size emitted from radiographic x-ray sources. It will measure spot sizes from submillimeters up to several millimeters for photon energies in the range of 150 keV to 20 MeV with doses as low as 0.03 rad up to 500 rad. The D-TRSD performs fast data acquisition as radiographic x-ray sources typically produce pulses in the tens of nanosecond ranges. The system will be optimized to capture multiple individual pulses in time within the same data record. The data collected will be used to support beam tuning and source optimization with on-the-fly evaluation of radiographic source performance. The diagnostic is based on penumbral imaging techniques. A scintillating fiber array of parallel aligned fibers is positioned in the beam path with a high-Z rolled edge inserted between the source and the array, thereby creating the penumbral x-ray signal that is captured by the D-TRSD system. The light from the scintillation conversion is recorded by individual avalanche photodiode detectors integrated into custom digitizer electronics. The penumbra is used to calculate the spot of the beam on the axis orthogonal to the fiber array based on the magnification from the rolled edge, the resolution of the spaced fibers, and the rotation of the array normal to the beam path. Many arrays can be used to capture additional spot widths about different rotations. The D-TRSD system is modular, software-driven, and easier to use than legacy TRSD systems.
There is growing interest in using low-energy flash x-ray sources in radiographic applications to provide high-contrast
images of low-density objects. Due to the low-energy nature of the detected photons, thin bright scintillators are desired.
In order to pursue an optimum radiographic system, experimental studies have been performed of the static imaging
properties of thin microcolumnar CsI using a Platts x-ray source. The Platts source is a nominally 300 keV endpoint rod
pinch diode x-ray source with a ~35 ns pulse time. The source was used to measure the imaging properties of
microcolumnar CsI with various thicknesses and backings. The experimental setup was modeled in GEANT4, and the
images were simulated to estimate system performance. Taking into account the source photon production, radiation
transport, and system optical performance, an accurate assessment of the detection system can be deduced.
The telecentric zoom lens system (ZLS) has proven to be invaluable in flash x-ray field operations and recent successful
experiments pertaining to stockpile stewardship. The ZLS contains 11 custom-manufactured lenses, a turning mirror
(pellicle), and an x-ray-to-visible-light converting scintillator. Images are recorded on a fully characterized CCD. All
hardware is supported by computerized, programmable, electro-mechanical mounts and alignment apparatus. Seven
different glass material types varying in chemical stoichiometry comprise the 11 ZLS lenses. All lenses within the ZLS
are out of the path of direct x-ray radiation during normal operation. However, any unshielded scattered x-ray radiation
can result in energy deposition into the lenses, which may generate some scintillating light that can couple into the CCD.
This extra light may contribute to a decrease in signal-to-noise ratio (SNR) and lower the overall fidelity of the
radiograph images. An estimate of the scintillation generation and sensitivities for each of the seven types of glass used
as lenses in the ZLS is presented. This report also includes estimates of the total observed background decoupling that
each of the lens material types contribute.
We are investigating scintillator performance in radiographic imaging systems at x-ray endpoint energies of 0.4 and 2.3 MeV in single-pulse x-ray machines. The effect of scene magnification and geometric setup will be examined along with differences between the detector response of radiation and optical scatter. Previous discussion has reviewed energy absorption and efficiency of various imaging scintillators with a 2.3 MeV x-ray source. The focal point of our study is to characterize scintillator blur to refine system models. Typical detector geometries utilize thin tiled LYSO:Ce (cerium-doped lutetium yttrium orthosilicate) assembled in a composite mosaic. Properties of individual tiles are being studied to understand system resolution effects present in the experimental setup. Comparison of two different experiments with different geometric configurations is examined. Results are then compared to different scene magnifications generated in a Monte-Carlo simulation.
The self-magnetic pinch (SMP) diode is an intense radiographic source fielded on the Radiographic Integrated Test Stand (RITS-6) accelerator at Sandia National Laboratories in Albuquerque, NM. The accelerator is an inductive voltage adder (IVA) that can operate from 2-10 MV with currents up to 160 kA (at 7 MV). The SMP diode consists of an annular cathode separated from a flat anode, holding the bremsstrahlung conversion target, by a vacuum gap. Until recently the primary imaging diagnostic utilized image plates (storage phosphors) which has generally low DQE at these photon energies along with other problems. The benefits of using image plates include a high-dynamic range, good spatial resolution, and ease of use. A scintillator-based X-ray imaging system or “gamma camera” has been fielded in front of RITS and the SMP diode which has been able to provide vastly superior images in terms of signal-to-noise with similar resolution and acceptable dynamic range.
We have investigated scintillator efficiency for MeV radiographic imaging. This paper discusses the modeled detection efficiency and measured brightness of a number of scintillator materials. An optical imaging camera records images of scintillator emission excited by a pulsed x-ray machine. The efficiency of various thicknesses of monolithic LYSO:Ce (cerium-doped lutetium yttrium orthosilicate) are being studied to understand brightness and resolution trade-offs compared with a range of micro-columnar CsI:Tl (thallium-doped cesium iodide) scintillator screens. The micro-columnar scintillator structure apparently provides an optical gain mechanism that results in brighter signals from thinner samples. The trade-offs for brightness versus resolution in monolithic scintillators is straightforward. For higher-energy x-rays, thicker materials generally produce brighter signal due to x-ray absorption and the optical emission properties of the material. However, as scintillator thickness is increased, detector blur begins to dominate imaging system resolution due to the volume image generated in the scintillator thickness and the depth of field of the imaging system. We employ a telecentric optical relay lens to image the scintillator onto a recording CCD camera. The telecentric lens helps provide sharp focus through thicker-volume emitting scintillators. Stray light from scintillator emission can also affect the image scene contrast. We have applied an optical light scatter model to the imaging system to minimize scatter sources and maximize scene contrasts.
Cygnus is a high-energy radiographic x-ray source. Three large zoom lenses have been assembled to collect images from
large scintillators. A large elliptical pellicle (394 × 280 mm) deflects the scintillator light out of the x-ray path into an
eleven-element zoom lens coupled to a CCD camera. The zoom lens and CCD must be as close as possible to the
scintillator to maximize light collection. A telecentric lens design minimizes image blur from a volume source. To
maximize the resolution of objects of different sizes, the scintillator and zoom lens are translated along the x-ray axis,
and the zoom lens magnification changes. Zoom magnification is also changed when different-sized recording cameras
are used (50 or 62 mm square format). The LYSO scintillator measures 200 × 200 mm and is 5 mm thick. The
scintillator produces blue light peaking at 435 nm, so special lens materials are required. By swapping out one doublet
and allowing all other lenses to be repositioned, the zoom lens can also use a CsI(Tl) scintillator that produces green
light centered at 540 nm (for future operations). All lenses have an anti-reflective coating for both wavelength bands.
Two sets of doublets, the stop, the scintillator, and the CCD camera move during zoom operations. One doublet has x-y
compensation. Alignment of the optical elements was accomplished using counter propagating laser beams and
monitoring the retro-reflections and steering collections of laser spots. Each zoom lens uses 60 lb of glass inside the 425
lb mechanical structure, and can be used in either vertical or horizontal orientation.
Cygnus is a high-energy radiographic x-ray source. The rod-pinch x-ray diode produces a point source measuring 1 mm
diameter. The target object is placed 1.5 m from the x-ray source, with a large LYSO scintillator at 2.4 m. Differentsized
objects are imploded within a containment vessel. A large pellicle deflects the scintillator light out of the x-ray
path into an 11-element zoom lens coupled to a CCD camera. The zoom lens and CCD must be as close as possible to
the scintillator to maximize light collection. A telecentric lens design minimizes image blur from a volume source. To
maximize the resolution of test objects of different sizes, the scintillator and zoom lens can be translated along the x-ray
axis. Zoom lens magnifications are changed when different-sized scintillators and recording cameras are used (50 or
62 mm square format). The LYSO scintillator measures 200 × 200 mm and is 5 mm thick. The scintillator produces blue
light peaking at 435 nm, so special lens materials are required. By swapping out one lens element and allowing all lenses
to move, the zoom lens can also use a CsI(Tl) scintillator that produces green light centered at 550 nm. All lenses are
coated with anti-reflective coating for both wavelength bands. Two sets of doublets, the stop, and the CCD camera move
during zoom operations. One doublet has XY compensation. The first three lenses use fused silica for radiation damage
control. The 60 lb of glass inside the 340 lb mechanical structure is oriented vertically.
There is much interest in developing new scintillator detectors for radiation detection and radiographic imaging
applications. The knowledge of the electron mobility (μ) is important in the basic understanding of charge transport and
in the selection and optimization of many inorganic scintillator materials such as thallium-doped cesium iodide, CsI(Tl).
Performance measures are used to model various scintillator responses in an effort to predict the effect of doping
concentrations. Performance models will help in the new scintillator design process. Initial tests are done with cadmium
zinc telluride detectors to establish measurement techniques and baselines.
Silicon-based photodetectors offer several benefits relative to photomultiplier tube-based scintillator systems. Solid-state
photomultipliers (SSPM) can realize the gain of a photomultiplier tube (PMT) with the quantum efficiency of silicon.
The advantages of the solid-state approach must be balanced with adverse trade-offs, for example from increased dark
current, to optimize radiation detection sensitivity. We are designing a custom SSPM that will be optimized for green
emission of thallium-doped cesium iodide (CsI(Tl)). A typical field gamma radiation detector incorporates thallium
doped sodium iodide (NaI(Tl)) and a radiation converter with a PMT. A PMT's sensitivity peaks in the blue wavelengths
and is well matched to NaI(Tl). This paper presents results of photomultiplier sensitivity relative to conventional SSPMs
and discusses model design improvements. Prototype fabrications are in progress.
Nicholas King, Stuart Baker, Steven Jaramillo, Kris Kwiatkowski, Stephen Lutz, Gary Hogan, Vanner Holmes, Christopher Morris, Paul Nedrow, Peter Pazuchanics, John Rohrer, Dan Sorenson, Richard Thompson
Multi-pulse imaging systems have been developed for recording images from pulsed X-ray and proton radiographic sources. The number of successive images for x-ray radiography is limited to four being generated by 25 ns, pulsed sources in a close positioned geometry. The number of proton images are provided by the number of proton bursts (approximately 60 ns) delivered to the radiographic system. In both cases the radiation to light converter is a thin LSO crystal. The radiographic image formed is relayed by a direct, coherent bundle or lens coupling to a variety of electronic shuttered, cooled CCD cameras. The X-ray system is optimized for detecting bremmstrahlung, reflection geometry generated X-rays with end point energies below 300 keV. This has resulted in less than 200 μm thick LSO converters which are 25 x 25 mm2. The converter is attached to a UV transmitting fiberoptic which in turn is directly coupled to a coherent bundle. The image is relayed to a 25 mm microchannel plate image intensifier attached to a 4 image framing camera. The framing camera image is recorded by a 1600 x 1600 pixel, cooled CCD camera. The current proton radiography imaging system for dynamic experiments is based on a system of seven individual high-resolution CCD cameras, each with its own optical relay and fast shuttering. The image of the radiographed object is formed on a 1.7 mm thick tiles of LSO scintillator. The rapid shuttering for each of the CCD's is accomplished via proximity-focussed planar diodes (PPD), which require application of 300-to-500 ns long, 12 kV pulses to the PPD from a dedicated HV pulser. The diodes are fiber-optically coupled to the front face of the CCD chips. For each time-frame a separate CCD assembly is required. The detection quantum efficiency (DQE) of the system is about 0.4. This is due to the lens coupling inefficiency, the necessary demagnification (typically between 5:1 and 3:1) in the system optics, and the planar-diode photo-cathode quantum efficiency (QE) (of approximately 15%). More recently, we have incorporated a series of 4 or 9 image framing cameras to provide an increased number of images. These have been coupled to cooled CCD cameras as readouts. A detailed description of the x-ray and proton radiographic imaging systems are discussed as well as observed limitations in performance. A number of improvements are also being developed which will be described.
Bechtel Nevada, Los Alamos Operations, has developed a high-speed, nine-frame camera system that records a sequence from a changing or dynamic scene. The system incorporates an electrostatic image tube with custom gating and deflection electrodes. The framing tube is shuttered with high-speed gating electronics, yielding frame rates of up to 5 MHz. Dynamic scenes are lens-coupled to the camera, which contains a single photocathode gated on and off to control each exposure time. Deflection plates and drive electronics move the frames to different locations on the framing tube output. A single charge-coupled device (CCD) camera then records the phosphor image of all nine frames. This paper discusses setup techniques to optimize system performance. It examines two alternate philosophies for system configuration and respective performance results. We also present performance metrics for system evaluation, experimental results, and applications to four-frame cameras.
ImageTool is a software package developed at Bechtel Nevada, Los Alamos Operations. This team has developed a set of analysis tools, in the form of image processing software used to evaluate camera calibration data. Performance measures are used to identify capabilities and limitations of a camera system, while establishing a means for comparing systems. The camera evaluations are designed to provide system performance, camera comparison and system modeling information. This program is used to evaluate digital camera images. ImageTool proves basic image restoration and analysis features along with a special set of camera evaluation tools which are used to standardize camera system characterizations. This process is started with the acquisition of a well-defined set of calibration images. Image processing algorithms provide a consistent means of evaluating the camera calibration data. Performance measures in the areas of sensitivity, noise, and resolution are used as a basis for comparing camera systems and evaluating experimental system performance. Camera systems being with a charge-coupled device (CCD) camera and optical relay system and may incorporate image intensifiers, electro-static image tubes, or electron bombarded charge-coupled devices (EBCCDs). Electro-optical components provide fast shuttering and/or optical gain to camera systems. Camera types evaluated include grated intensified cameras and multi-frame cameras used in applications ranging from X-ray radiography to visible and infrared imaging. It is valuable to evaluate the performance of a camera system in order to determine if a particular system meets experimental requirements. In this paper we highlight the processing features of ImageTool.
KEYWORDS: Cameras, Imaging systems, Signal to noise ratio, Modulation transfer functions, Image processing, Image resolution, Calibration, Contrast transfer function, Modulation, Data processing
This paper describes image evaluation techniques used to standardize camera system characterizations. Key areas of performance include resolution, noise and sensitivity. This team has developed a set of analysis tools, in the form of image processing software used to evaluate camera calibration data, to aid an experimenter in measuring a set of camera performance metrics. These performance metrics identify capabilities and limitations of the camera system, while establishing a means for comparing camera systems. Analysis software is used to evaluate digital camera images recorded with charge-coupled device (CCD) cameras. Several types of intensified cameras systems are used in the high-speed imaging field. Electro-optical components are used to provide precise shuttering or optical gain for a camera system. These components including microchannel plate or proximity focused diode image intensifiers, electro-static image tubes, or electron-bombarded CCDs affect system performance. It is important to quantify camera system performance in order to qualify a system as meeting experimental requirements. The camera evaluation tool is designed to provide side-by- side camera comparison and system modeling information.
Electronic charge-coupled device (CCD) cameras equipped with image intensifiers are increasingly being used for radiographic applications. These systems may be used to replace film recording for static imaging, or at other times CCDs coupled with electro-optical shutters may be used for static or dynamic radiography. Image intensifiers provide precise shuttering and signal gain. We have developed a set of performance measures to calibrate systems, compare one system to another, and to predict experimental performance. The performance measures discussed in this paper are concerned with image quality parameters that relate to resolution and signal-to-noise ratio.
Radiographic imaging continues to be a key diagnostic in many areas at Los Alamos National Laboratory. Radiographic recording systems have taken on many forms, from high repetition-rate, gated systems to film recording and storage phosphors. Some systems are designed for synchronization to an accelerator while others may be single shot or may record a frame sequence in adynamic radiographic experiment. While film recording remains a reliable standby in the radiographic community, there is growing interest in investigating electronic recording for many applications. The advantages of real time access to remote data acquisition are highly attractive. Cooled charge-coupled (CCD) camera systems are capable of providing greater sensitivity with improved signal-to-noise ratio. This paper begins with a review of performance characteristics of the Bechtel Nevada large format imaging systems, a gated system capable of viewing scintillators up to 300 mm in diameter. We then examine configuration alternatives in lens coupled and fiber optically coupled electro-optical recording systems. Areas of investigation include tradeoffs between fiber optic and lens coupling, methods of image magnification, and spectral matching from scintillator to CCD camera. Key performance features discussed include field of view, resolution, sensitivity, dynamic range, and system noise characteristics.
Experiments have been conducted in Sarov, Russia, with a dynamic radiographic system designed to establish the volume of an imploding 14-mm-diameter tungsten cylinder. Images were formed using a 65-MeV gamma source, lutetium oxyorthosilicate doped with cerium (LSO:Ce) radiation-to- light converter, and a fiber optic imaging bundle. Three radiographs were recorded in the course of approximately 2 microseconds using an electronic streak camera with intensified CCD readout. Significant improvements in system performance were achieved over lens coupling of components by the introduction of coherent fiber optics.
Bechtel Nevada, in collaboration with Los Alamos National Laboratory, has designed a radiographic imaging system that takes advantage of large format electron optical elements to produce a highly sensitive system for large diameter radiographic fluxes. Using specially designed fast lenses, the system is able to observe scintillator screens as large as 300 mm in diameter.A gated microchannel plate intensifier allows the system to be synchronized to pulsed gamma, proton and neutron sources of radiation to help reduce background noise levels. The entire system is deployed in a transportable housing with sealed heat exchanger and electrical patch panel that is designed to be lighttight so that the electron optics can be operated at extremely high gain. External controls allow manipulation of system gain, gate width and focus. The resolution is about 1 to 2 line pairs per millimeter at the radiation-to-light converter, and the f-number of the optical system is f/1. The image is digitized from a fiber-optically coupled 1024 X 1024 cooled charge-coupled device array. The system will have interchangeable components so that system performance can be optimized to meet specific recording requirements. The major trade-off is between field of view and resolution.
Mark Wilke, Andrew Obst, Dan Winske, Michael Jones, Stuart Baker, Joseph Abdallah, Stephen Caldwell, Robert Watt, S. Robert Goldman, Bruno Bauer, Robert Gibson
Colliding Au, CD, and Ti-CR plasmas have been generated by illuminating two opposing foils each with an approximately 100J, 0.5 nsec, 2(omega) Nd-glass laser beam from the Trident laser facility at Los Alamos. The plasmas are being used to study plasma interactions which span the parameter regime from interpenetrating to collisional stagnation. X-ray emission during the laser target interaction and the subsequent collision is used to diagnose the initial plasma conditions and the colliding plasma properties. X-ray instrumentation consists of a 100 ps gated x-ray pinhole imager, a time-integrated bremsstrahlung x-ray spectrograph and a gated x-ray spectrograph used to record isoelectronic spectra from the Ti-Cr plasmas. The imager has obtained multiframe images of the collision and therefore, a measure of the stagnation length which is a function of the ion charge state and density and a strong function of the electon temperature. Other isntrumentation includes a Thomson scattering spectrometer with probe beam, neutron detectors used to monitor the CE coated foil collisions, and an ion spectrometer. We will describe the current status of the experiments and current results with emphasis on the x-ray emission diagnostics. We will also briefly describe the modeling using Lasnex and ISIS, a particle-in-cell code with massless fluid electronics and inter-particle (classical) collisions.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.