An ALISA Vector Module (AVM) is trained on the discrete gamma-ray emission spectra of 61 commonly occurring radioisotopes generated by an analytical model. The trained AVM is then used to decompose the spectra captured from actual sources in the field using low-resolution thallium-activated sodium-iodide (NaI) detectors and/or high-resolution high-purity germanium (HPGe) detectors using QR Factorization to find the optimal least-squares solution for an over-specified system of equations, even if inconsistent. For low-resolution NaI detectors, formal experiments conducted under carefully controlled laboratory conditions yield average classification (spectral decomposition) errors less than 6% in mixtures with up to 10 components in test samples consisting of 1,000 photonic events, which requires just a few seconds to obtain in typical situations. Preliminary experiments with the high-resolution HPGe detector yield dramatically smaller errors than with the NaI detector. Further improvements in the accuracy and precision of the training data, as well as fusion with other powerful classification methods, are expected to reduce the error without prohibitively increasing the computation time.

When neutrons interact with particular nuclei, the resulting energy of the interaction can be released in the form of gamma rays, which are characteristic of the nucleus involved in the reaction. The PELAN (Pulsed Elemental Analysis with Neutrons) system uses a pulsed neutron generator and an integral thermalizing shield that induce reactions that cover most of the entire gamma ray energy spectra. The neutron generator uses a D-T reaction, which releases fast 14MeV neutrons responsible for providing information on those nuclei that mostly respond to inelastic scattering. During the time period between pulses, the fast neutrons undergo multiple inelastic interactions that lower their energy making them easier to be captured by certain nuclei; this energy spectrum of gamma rays induced by these interactions are designated as the gamma ray thermal spectra. The PELAN system has been used for a number of applications where non-intrusive, non-destructive interrogation is needed. Although Pulsed Fast Thermal Neutron Analysis (PFTNA) has been around for approximately 30 years, the technology has never been successfully commercialized for practical applications. The following report illustrates examples of the performance of on a number of applications of interrogation of Unexploded Ordnance (UXO), mine confirmation, large vehicle bombs inspection and illicit drug smuggling detection.

Cerium-activated lithium glass with highly enriched ⁶Li dopant is very useful in detecting low-energy neutrons. The neutrons are identified by pulse height separation; however, in a mixed gamma and neutron field a simple pulse height separation severely degrades the gamma rejection ratio for the sensor. By using a glass element doped with natural ⁷Li that is exactly the same as the cerium-activated glass, the ambient gamma response can be obtained. ⁷Li does not show any thermal neutron response. Therefore, the pulse height spectrum due to net neutrons can be obtained by subtracting the pulse height response of the ⁷Li-doped glass from that obtained from a ⁶Li-doped glass element. The pulse height peaks at 1.3 MeV (electron equivalent) with a full width half maximum (FWHM) resolution of 15.5%. A prototype dual counter for gamma and neutron radiation has been built and operationally tested to be linear in the dose rate range of 0 to 1 mrem/hr. A simple conversion formula relating gamma or neutron counts to the corresponding dose rates contains non-significant second-order correction terms. For small hand-held detectors, GS-20 and GS-30 combination glass scintillators have performed well by increasing the effective gamma rejection ratio from neutron counting while increasing the gamma sensitivity (by addition of counts from two similar scintillators).

We have measured the neutron spectra of cosmic-rays and a spontaneous fission emitting source (Cf-252) using a neutron double scatter spectrometer. The energy range of measurements was 0.1-10 MeV where the spectrometer efficiency is determined to be up to 8.7%, depending on the separation between detection planes. Our cosmic-ray neutron spectrum measurement is in good agreement with the sea-level data reported by Goldhagen and his co-workers. In the energy range 0.1-1.0 MeV, the cosmic-ray and Cf-252 spectra are different and separable. This difference is expected from the applicable models that describe the phenomena, 'equilibrium slowing down' (cosmic-rays) and 'Maxwellian kinetic temperature' emission (spontaneous fission). We show that >80% of Cf-252 neutrons and <25% of cosmic-ray related neutrons are emitted in this energy range of measurement, and conclude that neutron spectroscopy provides effective ways to distinguish a fission source from the cosmic-ray background.

An analysis is presented of factors affecting the specific loadability (W mm^-2 K^-1) of electron impact liquid metal anode x-ray sources (LIMAX). It is shown that in general, the limit to loadability is set by energy deposited in the electron window by inelastic electron scattering. Removal of this energy through convection cooling by the liquid metal stream represents the least efficient thermal transport process in LIMAX. As the electron window energy loss is approximately inversely proportional to the electron beam energy, the power loadability of a LIMAX source operated under otherwise constant conditions scales roughly with the square of the tube voltage. A comparison of the loadability of the liquid metal anode x-ray concept to conventional stationary anode x-ray tubes demonstrates the superiority of the former. The utility of LIMAX-based computed tomography in the field of air cargo container inspection is briefly discussed. In particular its characteristics relative to linac-based air cargo container inspection are highlighted: these include a higher contrast-to-noise ratio (CNR); compact radiation shielding and collimation; reduced detector cross-talk; improved image contrast; and the possibility of combining container CT with material-specific alarm resolution capability based on x-ray diffraction tomography.

A new Compton x-ray backscatter imaging technique called lateral migration radiography (LMR) has been successfully applied to the detection of voids and delaminations in the foam thermal insulation used on the shuttle external tank. LMR employs detection of selected scatter field velocity components, by using specially designed detectors and detector collimators, to achieve high image contrast. LMR is based on image contrast generated by migration of probe x-ray radiation in directions transverse to the illumination radiation beam. Because LMR is sensitive to electron density variations in these directions, thin, but large density variations, such as cracks and delaminations, generate signal-to-background ratios sufficient to produce images of features which are not even detectable in the usually interrogated thin dimension. The examined foam thermal insulation test panels consist of aluminum plates onto which the sprayed-on foam insulation (SOFI) is applied. Some of the test panels include structural features bolted to the base plate. The SOFI was layed down over the base plate and structure with a thickness varying from a few tens of mm up to a few hundred mm. The test panels included voids and simulated delaminations in the SOFI ranging in cross-sectional size from 6 x 6 mm to 50 x 50 mm. High quality images were acquired using pixels of 2 to 3 mm and irradiation times as low as 0.05 s per pixel.

The results of experimental research into characteristics of secondary penetrating radiation occurring when interacting primary X-ray beams from a solid-state cathode medium with targets made of various materials are reported. The experiments were carried out in a high-current glow discharge device with H₂, D₂ Kr, Xe gases and cathode samples made of Al, Sc, Ti, Ni, Nb, Zr, Mo, Pd, Ta, W, and Pt. The targets are shields made of various materials foil (Al, Ti, Ni, Zr, Yb, Ta, and W) with thickness of 10 - 30 μm and of 1- 3 mm. They were mounted at a distance of 21 and 70 cm from the cathode. In these experiments recording of the time radiation spectrums was carried out just before and after discharge current pulses (no discharge current). It was shown that the secondary radiation consisted of fast electrons. The secondary radiation of two types was observed. (1) The emission with a continuous temporal spectrum in the form of separate bursts with intensity up to 10⁶ fast electrons a burst. (2) The emission with a discrete temporal spectrum and emission rate up to 10⁹ fast electrons a burst. A third type of the penetrating radiation was observed as well. This type was recorded directly by the photomultiplier placed behind of the target without the scintillator. The abnormal high penetrating ability of this radiation type requires additional research to explain. The obtained results show that creating optically active medium with long-living metastable levels with the energy of 1.0-3.0 keV and more is possible in the solid state.

Transmission electron microscope (TEM) tomography provides three-dimensional structural information from tilt series with nanoscale resolution. We have collected TEM projection data sets to study the internal structure of photocatalytic nanoparticles. Multiple cross-sectional slices of the nanoparticles are reconstructed using an algebraic reconstruction technique (ART) and then assembled to form a 3D rendering of the object. We recently upgraded our TEM with a new sample holder having a tilt range of ±70° and have collected tomography data over a range of 125°. Simulations were performed to study the effects of field-of-view displacement (shift and rotation), limited tilt angle range, hollow (missing) projections, stage angle accuracy, and number of projections on the reconstructed image quality. This paper discusses our experimental and computational approaches, presents some examples of TEM tomography, and considers future directions.

Two neutron microscope imaging experiments were performed at the Center for Neutron Research, at the National Institute for Standards and Technology (NIST) on the NG-7 30-Meter Small Angle Neutron Scattering Instrument. The NIST neutron source wavelength could be varied from 5 Å to 20 Å, and the neutron bandwidth could be varied. For both microscope experiments the image resolution was 5.0 mm, and was determined and limited by the NG-7 neutron detector’s 5.0 mm pixel size. The image acquisition times were set to 300 sec. In the first experiment the neutron source wavelength was set to 5 Å with an 11% bandwidth. A simple microscope with a 22.6x magnification, employing a compound refractive lens, composed of 201 aluminum (Al) biconcave lenses, was used to image a slit array in Cadmium (Cd) foil, located 139 cm downstream of the source. The Cd slit array consisted of 0.8 mm wide slits separated by 0.8 mm wide slats. The Al CRL had 1.98 mm radius of curvature, a 3.9 mm aperture, and a measured 1.2 cm field of view (FOV). An 85 lens version of this Al CRL had a measured 2.3 cm FOV and 9.4 x magnification, and was used to image at rat paw. The Cd slit array was placed upstream of the aluminum CRL at 74.5 cm object distance. In the second NIST experiment the neutron source wavelength was set to 8.5 Å with a 10% bandwidth. A simple microscope with a 22.5x magnification, employing a compound refractive lens, composed of 100 MgF₂ biconcave lenses, was used to image materials and specimens containing hydrogen, whose main contrast mechanism for neutrons is incoherent scattering. The MgF₂ CRL had a measured 2.4 cm FOV. The hydrogen-rich material imaged was a polypropylene (hydrogen-rich) grid, and the biological specimens were a scorpion, a rat paw, and a plant leaf, and they were situated 122 cm downstream of the source, and 78 cm upstream of the MgF₂ CRL.

Space-based gamma-ray spectrometers utilize active anticoincidence shielding to reduce the background caused by charged-particle interactions. Shielding improves the performance of gamma-ray spectrometers by reducing the effect of charged particle interactions which can not be distinguished from true gamma-ray interactions by the spectrometer. Active shields produce a blanking signal when a charged particle is detected, so that the signal from the spectrometer can be ignored during the spectrometer's charged-particle interaction. Anticoincidence shielding for space-born gamma-ray detectors requires a cylindrical-shell geometry and charged-particle sensitivity. To reduce the size, weight, and cost of the shielding we utilize a new direct-conversion charged-particle detector material, polycrystalline mercuric iodide. We present the results from planar film growth techniques for the particle-counting detection capabilities necessary for anti-coincidence shielding. We also show that films with similar detection properties were grown on curved substrates with the size and curvature needed to surround space-based spectrometer main detectors.

Amorphous selenium direct-conversion x-ray detectors have been used successfully for full field digital mammography (FFDM) and digital radiography (DR). Such detectors characteristically exhibit high spatial resolution and conversion efficiency that is a function of the applied electric field. About 50 electron volts of photon energy are required to generate each electron-hole pair in a typical amorphous selenium x-ray conversion layer biased at 10 volts per micron. At FFDM and DR imaging x-ray energies each absorbed photon can generate only about 250 to 1000 electron-hole pairs. Medical imaging applications must therefore employ low noise thin film transistor (TFT) arrays and charge integration amplifiers to achieve high signal-to-noise ratio (SNR) and detective quantum efficiency (DQE). To assure quantum-noise limited imaging results with the lowest practical x-ray exposure dose, it is desirable to include an additional low-noise gain stage in the x-ray conversion layer. We have proposed and studied a new structure for an amorphous selenium detector that employs an internal biased gain grid to cause avalanche-gain within the x-ray conversion layer. An amplification of at least 10X can be achieved without introducing excessive noise. Quantum-limited image detection should then be attainable for even very low exposures.

We report on recent advances in the development of powdered Lu₂O₃:Eu scintillator screens. The Lu₂O₃:Eu scintillator has excellent intrinsic material properties including high density (9.5 g/cm³), high average atomic number (63), and a peak emission of 610 nm. We have characterized the performance of screens fabricated from this material in comparison with commercial Gd₂O₂S:Tb screens. This paper presents resolution in terms of MTF(f), noise properties in terms of NPS(f), and overall NEQ(f), obtained by coupling the screens to a CCD detector. We have included sample radiographic images that demonstrate the ability to produce high quality images. These screens are currently under development, and further improvements in performance are expected with optimization of the scintillator and fabrication methods.

Physical characteristics necessary to calculate the Detective Quantum Efficiency of a clinically used flat panel imager for full-breast digital mammography 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 was determined using Half-Value-Layer (HVL) techniques. At an X-ray beam characterized by 28 kVp, Mo-anode and Mo filter as well as beam hardening by 5 cm Lucite, the detector is practically linear with x-ray exposure at least up to 33 mR. At an exposure of 33 mR and close to zero spatial frequency the DQE is in the vicinity of 60%.

Conformal proton radiation therapy requires accurate prediction of the Bragg peak position. This problem may be solved by using protons rather than conventional x-rays to determine the relative electron density distribution via proton computed tomography (proton CT). However, proton CT has its own limitations, which need to be carefully studied before this technique can be introduced into routine clinical practice. In this work, we have used analytical relationships as well as the Monte Carlo simulation tool GEANT4 to study the principal resolution limits of proton CT. The GEANT4 simulations were validated by comparing them to predictions of the Bethe Bloch theory and Tschalar's theory of energy loss straggling, and were found to be in good agreement. The relationship between phantom thickness, initial energy, and the relative electron density uncertainty was systematically investigated to estimate the number of protons and dose needed to obtain a given density resolution. The predictions of this study were verified by simulating the performance of a hypothetical proton CT scanner when imaging a cylindrical water phantom with embedded density inhomogeneities. We show that a reasonable density resolution can be achieved with a relatively small number of protons, thus providing a possible dose advantage over x-ray CT.

Cardiac function is an important physiological parameter in preclinical studies. Nuclear cardiac scans are a standard of care for patients with suspected coronary artery occlusions and can assess perfusion and other physiological functions via the injection of radiotracers. In addition, correlated acquisition of nuclear images with electrocardiogram (ECG) signals can provide myocardial dynamics, which can be used to assess the wall motion of the heart. We have implemented this nuclear cardiology technique into a microSPECT/CT system, which provides sub-millimeter resolution in SPECT and co-registered high resolution CT anatomical maps. Radionuclide detection is synchronized with the R-wave of the cardiac cycle and separated into 16 time bins using an ECG monitor and triggering device for gating. Images were acquired with a 12.5 x 12.5 cm² small field of view pixilated NaI(Tl) detector, using a pinhole collimator. In this pilot study, rats (N = 5) were injected with ^99mTc-Sestamibi, a tracer of myocardium, and anesthetized for imaging. Reconstructed 4-D images (3D plus timing) were computed using an Ordered Subset Expectation Maximization (OSEM) algorithm. The measured perfusion, wall motion, and ejection fractions for the rats matched well with results reported by other researchers using alternative methods. This capability will provide a new and powerful tool to preclinical researchers for assessing cardiac function.

Mercuric iodide (HgI₂) and lead iodide (PbI₂) materials as direct converter layers for digital x-ray imaging have been studied for several years. This paper present results of basic imaging parameters by comparing dark current, sensitivity and image lag properties of these materials. A difficult challenge of both lead iodide and mercuric iodide photon detectors is higher than desired leakage currents. These currents are influenced by factors such as applied electrical field, layer thickness, layer density, electrode structure and material purity. Minimizing the leakage current must also be achieved without adversely affecting charge transport, which plays a large role in gain and is also influenced by these parameters. New deposition technologies have been developed through which the leakage current has now decreased by more than an order of magnitude while showing no negative affects on gain. Other challenges relate to increasing film thickness without degrading electrical properties. The image lag of the polycrystalline PbI₂ is much larger than that of the polycrystalline HgI₂ material, however, no significant image lag is observed for single crystal PbI₂. Optical microscopy and SEM studies showed that the polycrystalline PbI₂ has a low density, randomly oriented morphology with small crystallites while the best HgI₂ has a much better oriented (single crystal-like) structure. We believe that the long image lag can be attributed to the large number of deep defect states generated on the surface of the small PbI₂ crystallites. The imagers were evaluated for both radiographic and fluoroscopic imaging modes. MTF was measured as a function of the spatial frequency. The MTF data were compared to values published in the literature for indirect detectors (CsI) and direct detectors (a-Se). Resolution tests on resolution target phantoms showed that for both materials resolution is mostly limited by the TFT array Nyquist frequency.