Our Lawrence Livermore National Laboratory/Sandia National Laboratories collaboration has deployed a cubic-meter-scale antineutrino detector to demonstrate non-intrusive and automatic monitoring of the power levels and plutonium content of a nuclear reactor. Reactor monitoring of this kind is required for all non-nuclear weapons states under the Nuclear Nonproliferation Treaty (NPT), and is implemented by the International Atomic Energy Agency (IAEA). Since the antineutrino count rate and energy spectrum depend on the relative yields of fissioning isotopes in the reactor core, changes in isotopic composition can be observed without ever directly accessing the core. Data from a cubic meter scale antineutrino detector, coupled with the well-understood principles that govern the core's evolution in time, can be used to determine whether the reactor is being operated in an illegitimate way. Our group has deployed a detector at the San Onofre reactor site in California to demonstrate this concept. This paper describes the concept and shows preliminary results from 8 months of operation.

High-energy photons and neutrons can be used to interrogate for heavily shielded fissile materials inside sealed cargo containers by detecting their prompt and/or delayed fission signatures. The FIND (Fissmat Inspection for Nuclear Detection) active interrogation system is based on a dual neutron+gamma source that uses low-energy (< 500 keV) proton- or deuteron-induced nuclear reactions to produce high intensities of mono-energetic gamma rays and/or neutrons. The source can be operated in either pulsed (e.g., to detect delayed photofission neutrons and gammas) or continuous (e.g., detecting prompt fission signatures) modes. For the gamma-rays, the source target can be segmented to incorporate different (p,γ) isotopes for producing gamma-rays at selective energies, thereby improving the probability of detection. The design parameters for the FIND system are discussed and preliminary accelerator-based measurements of gamma and neutron yields, background levels, and fission signals for several target materials under consideration are presented.

Sandia is evaluating methods for identifying and quantifying trace signatures in field collection samples to support national deterrence policies. The first step in this process is to identify which combination of major, minor, and trace elements in a recovered collection sample provides the most reliable forensic information, and then to be able to quickly, accurately, and, in some cases, nondestructively measure these components. Conventional approaches have typically required a long, complex series of sample preparations followed by radiochemical analysis, often yielding only qualitative results. We report on our investigations to assess accelerator-based ion beam analysis methods by cross-calibrating with other methods, performing in-air analyses of bagged samples in anticipation of inspecting poorly constituted radioactive materials, and quantifying the uncertainties for detected elements.

Active neutron interrogation is an effective technique used to locate fissionable material. This paper discusses a portable system that utilizes a AmBe neutron source. The AmBe source consists of an americium alpha source and a beryllium target that can be switched into alignment to turn the source on and out of alignment to turn the source off. This offers a battery operated backpack portable source. The detector system that has been fabricated for use with this source is a fifteen tube ³He neutron detector. The results of initial experiments with the detector and MCNP calculations are discussed.

We have constructed a fast-neutron double-scatter spectrometer that efficiently measures the neutron spectrum and direction of a spontaneous fission source. The device consists of two planes of organic scintillators, each having an area of 125 cm², efficiently coupled to photomultipliers. The four scintillators in the front plane are 2 cm thick, giving almost 25% probability of detecting an incident fission-spectrum neutron at 2 MeV by proton recoil and subsequent ionization. The back plane contains four 5-cm-thick scintillators which give a 40% probability of detecting a scattered fast neutron. A recordable double-scatter event occurs when a neutron is detected in both a front plane detector and a back plane detector within an interval of 500 nanoseconds. Each double-scatter event is analyzed to determine the energy deposited in the front plane, the time of flight between detectors, and the energy deposited in the back plane. The scattering angle of each incident neutron is calculated from the ratio of the energy deposited in the first detector to the kinetic energy of the scattered neutron.

We used a pulsed 14 MeV neutron generator (NG) to acquire two concurrent gamma-ray spectra induced by inelastic neutron scattering (INS) and thermal neutron capture (TNC) in Si, O, and C, which are key elements in soil analyses. These separate spectra were acquired by gating the data acquisition system during the neutron pulse, to obtain an INS spectrum, and in between the neutron pulses, to obtain a TNC spectrum. Despite this separation, TNC gamma rays are still counted in the INS window due to the steady state achieved in the former reaction. With the NG operating at 10 kHz and a 25% duty cycle, the magnitude of the single-escape gamma rays from the Si 4.93 MeV gamma-ray peak in the TNC spectrum to the 4.43 MeV carbon region in the INS spectrum is 10.1% of the 4.93 MeV peak intensity. This percentage depends on the neutron repetition rate and duty cycle. It can be reduced to 4.9% by using a narrower gate-pulse that closely fits the neutron burst. We also show that under these conditions the net count rate in the individual peaks of soil elements, Si and O (6.13 MeV) of the TNC spectrum reaches a steady state between the neutron pulses, but the total count rate from the entire spectrum does not.

There is a need for high brightness neutron sources that are portable, relatively inexpensive, and capable of neutron radiography in short imaging times. Fast and thermal neutron radiography is as an excellent method to penetrate high-density, high-Z objects, thick objects and image its interior contents, especially hydrogen-based materials. In this paper we model the expected imaging performance characteristics and limitations of fast and thermal radiography systems employing a Rose Model based transfer analysis. For fast neutron detection plastic fiber array scintllators or liquid scintillator filled capillary arrays are employed for fast neutron detection, and ⁶Li doped ZnS(Cu) phosphors are employed for thermal neutron detection. These simulations can provide guidance in the design, construction, and testing of neutron imaging systems. In particular we determined for a range of slab thickness, the range of thicknesses of embedded cracks (air-filled or filled with material such as water) which can be detected and imaged.

Significant effort is currently being devoted to the development of noninvasive imaging systems that allow in vivo assessment of biological and biomolecular interactions in mice and other small animals. Ideally, one would like to discern these functional and metabolic relationships with in vivo radionuclide imaging at spatial resolutions approaching those that can be obtained using the anatomical imaging techniques (i.e., <100 μm, which would help to answer outstanding questions in many areas of biomedicine. In this paper, we report progress on our effort to develop high resolution focusing X-ray and gamma-ray optics for small-animal radionuclide imaging. The use of reflective optics, in contrast to methods that rely on absorptive collimation like single- or multiple-pinhole cameras, decouples spatial resolution from sensitivity (efficiency). Our feasibility studies have refined and applied ray tracing routines to design focusing optics for small animal studies. We also have adopted a replication technique to manufacture the X-ray mirrors, and which in experimental studies have demonstrated a spatial resolution of ~190 μm. We conclude that focusing optics can be designed and fabricated for gamma-ray energies, and with spatial resolutions, and field of view suitable for in vivo biological studies. While the efficiency of a single optic is limited, fabrication methods now are being developed that may make it possible to develop imaging systems with multiple optics that could collect image data over study times that would be practical for performing radionuclide studies of small animals.

Small animal dual modality microSPECT-micro CT has seen many technological advances during recent years. The design of small animal dual modality scanners is a multidisciplinary field, where several interrelated technological problems must be integrated in a complex instrument. This article describes the general concepts that must be taken into consideration during the design process of dual modality microSPECT- microCT scanners. A description of the contemporary scanner technology is presented using the recently designed dual modality micro SPECT -microCT at the Physics Research Laboratory at UCSF. The technology is described with a simple approach to introduce the reader to the complex process of the dual modality scanner design. This article includes a discussion of current technological challenges that have potential to improve or expand the microSPECT-microCT performance and its applications.

Single photon emission computed tomography (SPECT) is an important technology for molecular imaging studies of small animals with an increasing demand for high performance imaging systems. We have designed a small animal imaging system based on position sensitive avalanche photodiodes (PSAPDs) detectors with the goal of submillimeter spatial resolution and high detection efficiency, which will allow us to minimize the radiation dose to the animal and to shorten the time needed for the imaging study. Our design will use fourteen 80×80 mm² PSAPD detectors, which can achieve an intrinsic spatial resolution of 0.5 mm. These detectors are arranged in two rings around the object and are equipped with pinhole collimators to produce magnified projection data. A mouse bed is positioned in the center of the detector rings and can be rocked about the central axis to increase angular sampling of the object. The performance of this imaging system and of a dual headed SPECT system has been simulated using a ray tracing program taking into account appropriate point spread functions. Projection data of a hot rod phantom with 84 angular samples have been simulated. Appropriate Poisson noise has been added to the data to simulate an acquisition time of 15 min and an activity of 18.5 MBq distributed in the phantom. Both sets of data were reconstructed with an ML-EM reconstruction algorithm. We also derived spatial resolution and detection efficiency from analytical equations and compared the performance of our system to a variety of other small animal SPECT imaging systems. Simulations show that our proposed system produces a spatial resolution of 0.9 mm which is in good agreement with the resolution derived from analytical equations. In contrast, simulations of the dual headed SPECT system produce a spatial resolution of 1.1 mm. In comparison to other small animal SPECT systems, our design will offer a detection efficiency which is at least 2-fold higher at better or comparable spatial resolution. These results suggest that detectors based on PSAPD technology can be used to improve the design of small animal SPECT imaging systems considerably. Our small animal system design is very compact and can achieve high resolution and detection efficiency.

Columnar CsI(Tl) screens are now routinely used in indirect digital x ray imaging detectors. The CsI(Tl) scintillator provides high density, high atomic number, and high scintillation efficiency. These properties, coupled with the fact that CsI(Tl) can be grown in columnar form, provide excellent spatial resolution, high x-ray absorption, and low noise resulting in detectors with high overall detective quantum efficiency (DQE(f)). While such screens are now commercially available, developments leading to further improvements in scintillator performance are ongoing at RMD. Here we report on the recent progress in developing very thin (10 μm) to very thick (~3 mm) columnar screens and discuss their application potential in digital radiology and nuclear medicine.

We have developed a CMOS-based x-ray imaging detector in the same form factor of a standard film cassette (18 cm × 24 cm) for Small Field-of-view Digital Mammography (SFDM) applications. This SFDM cassette is based on our three-side buttable, 25 mm × 50 mm, 48μm active-pixel CMOS sensor modules and utilizes a 150μm columnar CsI(Tl) scintillator. For imaging up to 100 mm × 100 mm field-of-view, a number of CMOS sensor modules need to be tiled and electronically synchronized together. By using fiber-optic communication, acquired images from the SFDM cassette can be transferred, processed and displayed on a review station within approximately 5 seconds of exposure, greatly enhancing patient flow. We present the physical performance of this CMOS-based SFDM cassette, using established objective criteria such as the Modulation Transfer Function (MTF), Detective Quantum Efficiency (DQE), and more subjective criteria, by evaluating images from a phantom study and the clinical studies of our collaborators. Driven by the strong demand from the computer industry, CMOS technology is one of the lowest cost, and the most readily accessible technologies available for digital mammography today. Recent popular use of CMOS imagers in high-end consumer cameras have also resulted in significant advances in the imaging performance of CMOS sensors against rivaling CCD sensors. The SFDM cassette can be employed in various mammography applications, including spot imaging, stereotactic biopsy imaging, core biopsy and surgical biopsy specimen radiography. This study demonstrates that all the image quality requirements for demanding mammography applications can be addressed with CMOS technology.

Semiconductor detector arrays made of CdTe/CdZnTe are expected to be the main components of future high-performance, clinical nuclear medicine imaging systems. Such systems will require small pixel-pitch and much larger numbers of pixels than are available in current semiconductor-detector cameras. We describe the motivation for developing a new readout integrated circuit, AEGIS, for use in hybrid semiconductor detector arrays, that may help spur the development of future cameras. A basic design for AEGIS is presented together with results of an HSPICE^TM simulation of the performance of its unit cell. AEGIS will have a shaper-amplifier unit cell and neighbor pixel readout. Other features include the use of a single input power line with other biases generated on-board, a control register that allows digital control of all thresholds and chip configurations and an output approach that is compatible with list-mode data acquisition. An 8x8 prototype version of AEGIS is currently under development; the full AEGIS will be a 64x64 array with 300 μm pitch.

Physical characteristics necessary to calculate the Detective Quantum Efficiency of a prototype imaging detector based on a 4 x 2 array of tiled CMOS sensors designed for small-field-digital-mammography (SFDM; 10 cm x 10 cm active area) are presented. Objective quantities such as modulation transfer function (MTF), noise power spectrum (NPS) and detective quantum efficiency (DQE) have been evaluated. The X-ray photon fluence per X-ray exposure was determined using Half-Value-Layer (HVL) techniques. At an X-ray beam characterized by 28 kVp, Mo-anode, a Mo filter of 0.025 mm and beam hardening by 4.5 cm Lucite, the detector is practically linear with x-ray exposure at least up to 40.7 mR. At an exposure of 40.7 mR and close to zero spatial frequency the DQE is in the vicinity of 60 to 70 %.

Micro-CT for bone structural analysis has progressed from an in-vitro laboratory technique to devices for in-vivo assessment of small animals and the peripheral human skeleton. Currently, topological parameters of bone architecture are the primary goals of analysis. Additional measurement of the density or degree of mineralization (DMB) of trabecular and cortical bone at the microscopic level is desirable to study effects of disease and treatment progress. This information is not commonly extracted because of the challenges of accurate measurement and calibration at the tissue level. To assess the accuracy of micro-CT DMB measurements in a realistic but controlled situation, we prepared bone-mimicking watery solutions at concentrations of 100 to 600 mg/cm³ K₂PO₄H and scanned them with micro-CT, both in glass vials and microcapillary tubes with inner diameters of 50, 100 and 150 μm to simulate trabecular thickness. Values of the linear attenuation coefficients μ in the reconstructed image are commonly affected by beam hardening effects for larger samples and by partial volume effects for small volumes. We implemented an iterative reconstruction technique to reduce beam hardening. Partial voluming was sought to be reduced by excluding voxels near the tube wall. With these two measures, improvement on the constancy of the reconstructed voxel values and linearity with solution concentration could be observed to over 90% accuracy. However, since the expected change in real bone is small more measurements are needed to confirm that micro-CT can indeed be adapted to assess bone mineralization at the tissue level.

This paper presents a few results of an initial study to compare softcopy displays with 8 bits contrast resolution with those of 11 bits contrast resolution. Particular interest was paid to failure to detect subtle objects and appearance of artifacts like false contours as a result of improper quantization. Objects like squares, discs and Gaussian nodules were simulated at different amplitudes on uniform backgrounds. Another approach was to place simple objects like disks and Gaussian objects into a clinical image. The study concluded that indeed subtle objects can be missed and artifacts such as false contours can occur, dependent on signal amplitude and noise. A comprehensive observer study is under development to confirm and refine these results.

LCDs are replacing CRTs as primary diagnostic softcopy displays in clinics. They exhibit higher spatial noise than CRTs, which can interfere with diagnosis and reduce the efficiency especially when subtle abnormalities are presented. We have reported recently a study on the LCD spatial noise¹. A high quality CCD camera was used to acquire images from the LCD. Noise properties were estimated from the camera images. Then an error diffusion based operation was applied to compensate for the display spatial noise. This paper presents the noise estimation and compensation results on five different LCDs using same processing protocol. These five different LCDs vary in terms of matrix size, pixel size, pixel structure and vendors. The purpose of this work is to demonstrate that the LCD spatial noise estimation and compensation scheme we proposed earlier is valid, robust and necessary for different medical grade LCDs used in clinics today.

This paper presents results of psychophysical evaluations of an LCD with respect to its spatial noise. Spatial noise is quantified using a high-resolution CCD camera and compensated for using an error diffusion based algorithm. Psychophysical evaluation is performed in order to explore the dependence of human contrast sensitivity on display spatial noise. This evaluation uses the two-alternative forced choice (2-AFC) method. Gaussian-shaped objects, which simulate lung nodules, serve as stimuli. Three types of test images are used: Images containing simulated noise amounting to twice the amount of spatial noise present, images containing the original spatial noise and images compensated for the spatial noise. The probability of correct response and the detectability index, d', calculated indicates that spatial noise compensation leads to a lower contrast threshold.

Coherent Scatter Computer Tomography (CSCT) is a novel x-ray imaging method revealing structural information on the molecular level. More precisely, the momentum transfer dependence of the coherent scatter cross section of the object material in each voxel of an object slice under investigation is determined. Compared to other x-ray diffraction techniques very large objects can be investigated which allows to apply the technique in medical imaging, material analysis or baggage inspection. The ratio of multiple scattered radiation over single scatter increases with object size. For large objects multiple scatter can become the dominant contribution. Since this part of the measured radiation cannot be reconstructed correctly, artifacts in the resulting images occur. The amount of multiple scattered radiation in CSCT and its dependence on the object size and material have been investigated by means of Monte Carlo simulations. A method to correct for multiple scattered radiation in energy-resolved CSCT is introduced. The benefit of this correction method to the quality of reconstructed data is demonstrated.

A method for constructing an x-ray telescope with exceedingly hgh spatial resolution is to use a pair of coaxial, Fresnel zone plates aligned with an imaging x-ray detector. This combination allows the high sensitivity imaging of x-ray and gamma-ray sources ranging in energy from 1 keV to several hundred keV over a field of view of several degrees with spatial resolution of a fraction of an arc minute. We have implemented a version of such a telescope using several relatively new technologies. These include specialized techniques for constructing Fresnel zone plates from thin sheets of tungsten, a 64-element, avalanche photodiode (APD) array coupled to a matching, segmented, CsI(T1) scintillator, a new ASIC which provides 16-channels of low noise amplification, and image processing software that provides the user not only with localized intensity information, but also with localized spectral information.

The most complex components of an energy dispersive x-ray diffraction imaging (XDI) system are in general the radiation source and the spectroscopic detector array. Hence it is important to determine the geometrical factors affecting the size and shape of these components in arbitrary XDI configurations. These factors strongly influence system design parameters such as complexity, size and cost. Following an introduction to the physical principles of x-ray diffraction imaging (XDI) a generic 2-D cross section of an arbitrary tomographic XDI system is proposed. It is shown that a 3-D XDI arrangement can always be synthesised from identical 2-D generic sections when these are replicated along lines running through the vertex of an axially symmetric conic surface. The geometry of the cone and thus of the corresponding XDI system is determined by three arbitrary, independent parameters. There is thus an infinite number of possible alternative XDI configurations. The design of several variant XDI systems of potential interest for checked baggage inspection is discussed with reference to component size and complexity. These alternative configurations are described in this paper and their relative merits assessed. The procedure described here is useful both for optimising the performance of an XDI system of given complexity and for adapting the geometry of an XDI system to components of given specifications.

A new Compton x-ray backscatter imaging technique, backscatter radiography by selective detection (RSD), has been used for inspection of the spray-on-foam-insulation (SOFI) on the space shuttle external tank. RSD employs detection of selected backscatter field components, by using specially designed detectors with movable detector collimators, to achieve high image contrast. The optimization study utilized test panels with simulated and natural defects in the spray-on foam insulation. Some of the test panels include structural features, stiffener-stringers and connection flanges, which were bolted to an aluminum base plate representative of the external tank. The SOFI was then layed down over the base plate and structural components with thicknesses varying from a few tens of mm up to a few hundred mm. The simulated defects range in cross-sectional size from 6 × 6 mm to 50 × 50 mm. Natural defects including roll-over voids and knit-line delaminations have a wide range of sizes, geometries, and orientations with a minimum critical cross-sectional size of 6 mm. Imaging registration is currently obtained at 0.05 seconds per 2 mm pixel, or about 19 minutes per 0.093 m²(1 ft²). The current system is being evaluated to enhance the detection of natural defects of a minimal critical size. Monte Carlo (MC) simulations with MCNP5 are being used to determine the history and corresponding spectrum of the detected photons that are responsible for improving defect image contrast. The simulation results are used in combination with experimental data to select optimal detector configurations. Detector configurations are sensitive not only to the type of defect being detected, but also the defect's depth in SOFI, distance from aluminum substrate, and defect orientation. Additional parameters including detector type, detection mode, and x-ray illumination beam size were also evaluated. Both NaI and plastic (BC404) scintillation detectors in pulse and integral mode were used to determine their effect on image quality and defect detection sensitivity. The x-ray illumination beam geometry (round versus square) and beam spot size were varied to determine resolution and the effect on defect contrast. The current system using pulse mode NaI detectors, and a 2 mm round x-ray illumination beam can detect the presence of the smallest critical size defects at a scan rate of 0.05 seconds per 2 mm pixel.

The micro-CT system has been developed for small animal imaging. The system is mainly composed of CCD detector coupled with CsI (Tl) phosphor, X-ray source with micro focal spot, linearly moving couch, and rotation gantry. This system was developed as a gantry rotation type and designed to get CT images of small living animals. In this paper, the requirements of main parts of the system to acquire micro spatial resolution are described. The characteristics of the system, such as field of view, geometries of main components, gantry movement, and X-ray analysis are mainly considered. Resolution of the CT system was evaluated under variable conditions. Typically, the spatial resolution of the CT system was obtained about 37 micron at 10% of MTF curve.

The feasibility of creating a single, bulk detector from many smaller pieces of inorganic crystal scintillator was demonstrated in this research. The results show that light collection from multiple inorganic scintillators can be optimized via the properties of the non-scintillating medium, the packing factor of the crystals, and the cell geometry to yield a pulse-height spectrum of acceptable resolution. We were able to characterize changes in the ¹³⁷Cs gamma ray pulse height spectrum for increasing numbers of individual scintillators to the composite detector.

A directional scintillating fiber detector for 14-MeV neutrons was simulated using the GEANT4 Monte Carlo simulation tool. Detail design aspects of a prototype 14 MeV neutron fiber detector under development were used in the simulation to assess performance and design features of the detector. Saint-Gobain produced, BCF-12, plastic fiber material was used in the prototype development. The fiber consists of a core scintillating material of polystyrene with 0.48 mm × 0.48 mm dimension and an acrylic outer cladding of 0.02 mm thickness. A total of 64 square fibers, each with a cross-sectional area of 0.25 mm² and length of 100 mm were positioned parallel to each other with a spacing of 2.3 mm (fiber pitch) in the tracking of 14-MeV neutron induced recoil proton (n-p) events. Neutron induced recoil proton events, resulting energy deposition in two collinear fibers, were used in reconstructing a two dimensional (2D) direction of incident neutrons. Blurring of recoil protons signal in measurements was also considered to account uncertainty in direction reconstruction. Reconstructed direction has a limiting angular resolution of 3° due to fiber dimension. Blurring the recoil proton energy resulted in further broadening of the reconstructed direction and the angular resolution was 20°. These values were determined when incident neutron beam makes an angle of 45 degree relative to the front surface of the detector. Comparable values were obtained at other angles of incidence. Results from the present simulation have demonstrated promising directional sensitivity of the scintillating fiber detector under development.