Stuart A. Baker

Design and assembly of a telecentric zoom lens for the Cygnus x-ray source

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.

Proton Radiography Experiments on Shocked High Explosive Products

We studied the propagation of detonation waves and reflections of normal incident detonation waves in explosive products using the 800 MeV proton radiography facility at LANSCE. Using this system, we obtain seven to twenty‐one radiographic images of each experiment. We have examined the experimental wave velocity and density of the materials ahead and behind of the shocks as inferred from radiographs and compare them to standard explosive equations of state. Finally we compare the experiments with calculations of the experiments using the MESA hydrodynamics code.

Imaging detector systems for soft x-ray and proton radiography

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 CCDs 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.

Meeting the challenge of detecting ion plasma waves

Ion plasma waves—purely electrostatic ion waves with a wavelength of order of the electron Debye length and frequency of the order of the ion plasma frequency—have long been known in theory but have proven difficult to detect experimentally. The difficulties stemmed from the techniques used to produce the plasma and to drive and detect the waves. In the work reported here, these problems were overcome by using resonant laser scattering to detect ion plasma waves in a multiply ionized, laser‐produced plasma. This nonetheless required careful experimental design to minimize frequency smearing of the scattered signal by plasma gradients. The plasma was extensively characterized, allowing comparison of the theoretical dispersion relation with the wave data. The agreement of these two provides conclusive proof of the detection of ion plasma waves.

Alignment and testing of a telecentric zoom lens used for the Cygnus x-ray source

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.

Performance of image intensifiers in radiographic systems

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.

Large-format radiographic imaging

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.

X-ray emission from colliding laser plasmas

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.

Scintillator efficiency study with MeV x-rays

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.

Flash radiography studies with microcolumnar CsI

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.