Monte Carlo transport codes have been used to model the detector blur and energy deposition in various detector geometries for applications in MeV radiography. Segmented scintillating detectors, where low Z scintillators combined with a high-Z metal matrix, can be designed in which the resolution increases with increasing metal fraction. The combination of various types of metal intensification screens and storage phosphor imaging plates has also been studied. A storage phosphor coated directly onto a metal intensification screen has superior performance over a commercial plate. Stacks of storage phosphor plates and tantalum intensification screens show an increase in energy deposited and detective quantum efficiency with increasing plate number, at the expense of resolution. Select detector geometries were tested by comparing simulation and experimental modulation transfer functions to validate the approach.
Los Alamos has used penetrating radiography extensively throughout its history dating back to the Manhattan Project
where imaging dense, imploding objects was the subject of intense interest. This interest continues today as major
facilities like DARHT1 have become the mainstay of the US Stockpile Stewardship Program2 and the cornerstone of nuclear weapons certification. Meanwhile, emerging threats to national security from cargo containers and improvised explosive devices (IEDs) have invigorated inspection efforts using muon tomography, and compact x-ray radiography.
Additionally, unusual environmental threats, like those from underwater oil spills and nuclear power plant accidents,
have caused renewed interest in fielding radiography in severe operating conditions. We review the history of
penetrating radiography at Los Alamos and survey technologies as presently applied to these important problems.
We present a unique, lightweight, compact, low-cost, x-ray imager: MiniMAX (Miniature, Mobile, Agile, X-ray). This
system, which exploits the best aspects of Computed Radiography (CR) and Digital Radiography (DR) technology,
weighs less than 6lbs, fits into a 6" diameter x 16" long carbon-fiber tube, and is constructed almost entirely from offthe-
shelf components. MiniMAX is suitable for use in weld inspection, archaeology, homeland security, and veterinary
medicine. While quantum limited for MeV radiography, the quantum-efficiency is too low for routine medical use.
Formats include: 4"x6", 8"x12", or 16"x24" and can be readily displayed on the camera back, using a pocket projector,
or on a tablet computer. In contrast to a conventional, flying-spot scanner, MiniMAX records a photostimulated image
from the entire phosphor at once using a bright, red LED flash filtered through an extremely efficient (OD>9) dichroic
Massachusetts Institute of Technology, Lincoln Laboratory (MIT LL) has been developing both continuous and burst
solid-state focal-plane-array technology for a variety of high-speed imaging applications. For continuous imaging, a
128 × 128-pixel charge coupled device (CCD) has been fabricated with multiple output ports for operating rates greater
than 10,000 frames per second with readout noise of less than 10 e- rms. An electronic shutter has been integrated into
the pixels of the back-illuminated (BI) CCD imagers that give snapshot exposure times of less than 10 ns.
For burst imaging, a 5 cm × 5 cm, 512 × 512-element, multi-frame CCD imager that collects four sequential image
frames at megahertz rates has been developed for the Los Alamos National Laboratory Dual Axis Radiographic
Hydrodynamic Test (DARHT) facility. To operate at fast frame rates with high sensitivity, the imager uses the same
electronic shutter technology as the continuously framing 128 × 128 CCD imager. The design concept and test results are
described for the burst-frame-rate imager.
Also discussed is an evolving solid-state imager technology that has interesting characteristics for creating large-format
x-ray detectors with ultra-short exposure times (100 to 300 ps). The detector will consist of CMOS readouts for high
speed sampling (tens of picoseconds transistor switching times) that are bump bonded to deep-depletion silicon
photodiodes. A 64 × 64-pixel CMOS test chip has been designed, fabricated and characterized to investigate the
feasibility of making large-format detectors with short, simultaneous exposure times.
A 512x512-element, multi-frame charge-coupled device (CCD) has been developed for collecting four sequential image frames at megahertz rates. To operate at fast frame rates with high sensitivity, the imager uses an electronic shutter technology developed for back-illuminated CCDs. Device-level simulations were done to estimate the CCD collection well spaces for sub-microsecond photoelectron collection times. Also required for the high frame rates were process
enhancements that included metal strapping of the polysilicon gate electrodes and a second metal layer. Tests on finished back-illuminated CCD imagers have demonstrated sequential multi-frame capture capability with integration intervals in the hundreds of nanoseconds range.
An overview of the radiographic capabilities with emphasis on electronic image detection and processing at the Los Alamos National Laboratory is presented. Fixed facilities and portable x-ray sources and imaging systems make up the Los Alamos capability. Examples of imaging with large area amorphous silicon imaging panels, a portable computed tomography system, high speed x-ray imaging applications and equipment, and small area, high resolution imagers are given. Radiographic simulation and reverse engineering from radiographic images to computer aided design files and solid models is also presented.
Advanced in solid state detectors and dense scintillators make radiographic cameras competitive with film in applications where the SNR is dominated by quantum statistics. In addition, these cameras offer the advantage of sub-microsecond time-gating required by present and future radiographic facilities (e.g. PHERMEX, and DARHT). We compare theoretical considerations in camera systems incorporating lens, and fiber coupling to MCPs, and CCDs. We also present Monte-Carlo blur calculations for various candidate scintillators and discuss the relative merits of the PHERMEX camera design.
Thick segmented scintillating converters coupled to optical imaging detectors offer the advantage of large area, high stopping power sensors for high energy x-ray digital imaging. The recent advent of high resolution and solid state optical sensors such as amorphous silicon arrays and CCD optical imaging detectors makes it feasible to build large, cost effective imaging arrays. This technology, however, shifts the sensor cost burden to the segmented scintillators needed for imaging. The required labor intensive fabrication of high resolution, large area hard x- ray converters results in high cost and questionable manufacturability on a large scale. We report on recent research of a new segmented x-ray imaging converter. This converter is fabricated using vacuum injection and crystal growth methods to induce defect free, high density scintillating fibers into a collimator matrix. This method has the potential to fabricate large area, thick segmented scintillators. Spatial resolution calculations of these scintillator injected collimators show that the optical light spreading is significantly reduced compared to single crystalline scintillators and sub-millimeter resolution x- ray images acquired with the segmented converter coupled to a cooled CCD camera provided the resolution to characterize the converter efficiency and noise. The proposed concept overcomes the above mentioned limitations by producing a cost-effective technique of fabricating large area x-ray scintillator converters with high stopping power and high spatial resolution. This technology will readily benefit diverse fields such as particle physics, astronomy, medicine, as well as industrial nuclear and non-destructive testing.