Nicholas S. P. King

Surface preparation methods to enhance dynamic surface property measurements of shocked metal surfaces

This effort investigates surface-preparation methods to enhance dynamic surface-property measurements of shocked metal surfaces. To assess the ability of making reliable and consistent dynamic surface-property measurements, the amount of material ejected from the free surface upon shock release to vacuum (ejecta) was monitored for shocked Al-1100 and Sn targets. Four surface-preparation methods were considered: Fly-cut machine finish, diamond-turned machine finish, polished finish, and ball rolled. The samples were shock loaded by in-contact detonation of HE PBX-9501 on the front side of the metal coupons. Ejecta production at the back side or free side of the metal coupons was monitored using piezoelectric pins, optical shadowgraphy, and x-ray attenuation radiography.

Hybrid image sensor with multiple on-chip frame storage for ultrahigh-speed imaging

A high-resolution hybrid visible imager, that is composed of a CMOS readout integrated circuit (ROIC) and a silicon photo-detector array, has been designed. The ROIC is fabricated with a standard 0.25 μm CMOS mixed-mode process with a back-illuminated silicon detector array that is produced at Rockwell Scientific Company (RSC) using RSCs HyViSITM process. The camera system is designed primarily to record images formed on a scintillator used in pulsed proton radiography experiments. In such experiments, the repetition rate of the proton beam can be as high as 2.8 MHz (358 ns). An imaging system with the desired 1440x1440 pixels resolution would result in an instantaneous readout rate in excess of 5.79 E12 samples/s. To address this issue we designed a pixel with three-frame in-pixel analog storage allowing for a deferred slower readout. The 26 μm pitch pixel imager is operated in a global shutter mode and features in-pixel correlated double sampling (CDS) for each of the three acquired frames. The CDS operation is necessary to overcome the kTC noise of the integrating node to achieve high dynamic range. A 65 fps continuous readout mode is also provided. The hybridized silicon array has close to 100% fill factor while anti-reflection (AR) coating maximizes its quantum efficiency at the scintillator emission wavelength (~415 nm). The ROIC is a 720x720, two-side buttable integrated circuit with on-chip 12-bit analog to digital converter (ADC) for digital readout. Timing and biasing are also generated on-chip, and special attention has been given to the power distribution of the pixel-array and snapshot signal buffers. This system-on-chip approach results in a compact and low power camera, an important feature to extend the number of imaged frames by synchronizing multiple cameras.

Development of multi-frame detector for ultra-fast radiography with 800 MeV protons

Two prototype small-area pixilated detectors, capable of recording signals produced by 800 MeV proton beam micropulses, with a repetition rate of 358 ns, were constructed. The first detector system was built around a two-dimensional photodiode array. The array was illuminated by light emitted by a monolithic 1.7 mm thick LSO scintillator and imaged by a simple mirror and lens optical system. Tests were carried out with an array of 8/spl times/8 pixels 1 mm/sup 2/ in area. The other detector was a hydrogen ion-chamber operated at 1 to 2.5 atm. The fast positive-ion drift velocity in hydrogen helps to minimize the space charge build up. However, for pulses spaced closely in time, less than 1 /spl mu/s, there appear to be a substantial accumulation of positive space charge. The electronic readout chain, for each of the 64 channels, consisted of an externally clocked fast gated integrator and an amplifier coupled to an on-board ADC and a FIFO. The detectors were tested with a beam of up to 6/spl times/10/sup 6/ protons per mm/sup 2/ in 30 to 120 ns-wide microbursts.

High-frame-rate intensified fast optically shuttered TV cameras with selected imaging applications

This paper focuses on high speed electronic/electro-optic camera development by the Applied Physics Experiments and Imaging Measurements Group (P-15) of Los Alamos National Laboratorys Physics Division over the last two decades. The evolution of TV and image intensifier sensors and fast readout fast shuttered cameras are discussed. Their use in nuclear, military, and medical imaging applications are presented. Several salient characteristics and anomalies associated with single-pulse and high repetition rate performance of the cameras/sensors are included from earlier studies to emphasize their effects on radiometric accuracy of electronic framing cameras. The Groups test and evaluation capabilities for characterization of imaging type electro-optic sensors and sensor components including Focal Plane Arrays, gated Image Intensifiers, microchannel plates, and phosphors are discussed. Two new unique facilities, the High Speed Solid State Imager Test Station and the Electron Gun Vacuum Test Chamber are described. A summary of the Groups current and developmental camera designs and R&D initiatives are included.

Nanosecond Image Shuttering Studies at los Alamos National Laboratory

Experimental results comparing gated imaging capabilities of proximity-focused microchannel-plate intensifiers and electrostatically-focused silicon-intensified-target vidicons are presented. A brief summary of previous response data obtained from several standard and modified versions of both image sensors and current efforts on (1) sector gating of segmented photocathodes (2) pre-pulsing of photocathodes with infrared light to increase conductivity and (3) gate pulse injection techniques are discussed. Segmented photocathodes increased gating speed by simultaneous turn-on of individual sectors whereas preliminary analyses indicate no improvements from infrared illumination.

Processing of multiport CCD video signals at very high frame rates

Rates exceeding 1000 frames/s can be achieved with multiport CCD state-of-art video sensors. In order to provide sufficient spatial resolution, sensor configurations of 512 X 512 pixels are typical. Image area is divided into segments with individual video ports. Each port includes a photocharge sensitive amplifier, typically comprising sample/hold and charge reset circuits. Some amplifiers are even provided with a double correlated sample circuit for improving the signal/noise ratio. Frame rates are proportional to the number of ports, since the individual sensor segments are run in parallel. Unfortunately, the amount of external circuitry required for signal processing increases accordingly, 16-port sensors are a quite common configuration. Cameras with even higher number of ports are prohibitively expensive. Therefore, in order to achieve very high frame readout rates with a moderate number of ports, the sensors charge transport clock frequencies must be increased to the limit. Horizontal charge transfer frequencies exceeding 30 MHz have been achieved. The quality of the video signal deteriorates with frequency due to bandwidth limitation of the photocharge detecting amplifier. Its sample/hold and double correlated sample circuits are useless at such rates. Methods and circuits for the processing of video signals under such conditions are described. The circuits include wide bandwidth video buffer amplifiers/level translator/line drivers, fast peak stretchers, 10-bit resolution (or more) A/D converters and fiber optic data links to a remote mass digital data storage and processors. Also, the circuits must satisfy a number of practical conditions (size, power dissipation, cost) in order to make such camera useful in applications where space is limited and multiple head high frame rate cameras are required.

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.

Intensified/shuttered cooled CCD camera for dynamic proton radiography

An intensified/shuttered cooled PC-based CCD camera system was designed and successfully fielded on proton radiography experiments at the Los Alamos National Laboratory ALNSCE facility using 800-MeV protons. The four camera detector system used front-illuminated full-frame CCD arrays fiber optically coupled to either 25-mm diameter planar diode or microchannel plate image intensifiers which provided optical shuttering for time resolved imaging of shock propagation in high explosives. The intensifiers also provided wavelength shifting and optical gain. Typical sequences consisting of four images corresponding to consecutive exposures of about 500 ns duration for 40-ns proton burst images separated by approximately 1 microsecond were taken during the radiography experiments. Camera design goals and measured performance characteristics including resolution, dynamic range, responsivity, system detection quantum efficiency, and signal-to-noise will be discussed.

Development of a multiframe optical imaging detector for proton radiography at LANL

In this paper we give a brief report on the development of simple direct- and indirect-detection imagers for proton radiography experiments. We outline a conceptual design for a novel, multi-frame 5 mega frames per second (Mfs) hybrid imager. The high-density interconnect is identified as a critical enabling technology. We present a description of a 3D electronics packaging cube, which was completed in a recent feasibility study.

A/D processing of video signals at very high pixel rates

The video wave-form of CCD photocharge, clocked out at very high pixel rates, is a sequence of narrow voltage pulses. Pulse height is proportional to the recovered charge. A method and circuits are described for digitizing such waveforms in a 10-bit range at rates exceeding 75 MHz. Signal processing includes a wide-band clamping amplifier, a self-clocked peak detector, a very fast track/hold amplifier and a 75 MSPS, 10-bit flash converter. Digital data can be stored either in a local high speed RAM or transferred by a fast fiber-optic link to the remote digital storage. Test results (such as linearity, dynamic range and signal/noise ratio) are also given. >