A description is given of the principle of operation, design and technical realization of a Compton spectrometer. In
contrast to many other devices that have been discussed in the literature, the Compton spectrometer described here
combines an electron-impact x-ray source with a room-temperature semiconductor detector.
It is shown that the momentum resolution of the Compton spectrometer for the K characteristic lines emitted by the tube
anode is adequate to resolve the Doppler broadening originating in electron momentum distributions of low atomic
number elements, such as carbon, nitrogen and oxygen. Experimental Compton-broadened spectra from a range of
common materials are presented. Methods to extract Compton profiles from the experimental spectra, by accounting for
the continuous component of the x-ray tube emission and the multiplet nature of the characteristic lines, are illustrated.
The application of this Compton spectrometer to material characterization is briefly discussed.
SALOME (an acronym for Small Angle Lab Operation Measuring Equipment) is a versatile,
energy-dispersive x-ray diffraction imaging (XDi) test-bed facility commissioned and supported by the
Transportation Security Laboratory, Atlantic City, USA. In work presented here, the Inverse Fan-beam (IFB)
topology has been realized on SALOME and used to investigate the liquids identification capability of x-ray
Liquids were investigated from four classes of materials of relevance to security screening of aircraft
passenger luggage; namely: dilute aqueous liquids; concentrated aqueous liquids; hydrocarbon fuels; and
oxidizers. A set of features associated with the Molecular Interference Function (MIF) were used to classify the
liquids. Within the limited scope of this investigation, XRD proved to have excellent capability for
discriminating liquids from one another; in particular, for isolating the threat materials without raising false
alarms from either household or innocuous substances. Consequences for XRD-based screening of air passenger
luggage are summarized.
SALOME (an acronym for Small Angle Lab Operation Measuring Equipment) is a versatile, energy-dispersive
x-ray diffraction imaging (XDi) test-bed facility; commissioned, funded and supported by the Transportation
Security Laboratory, Atlantic City, USA. In work presented here, SALOME has been used to investigate the
photon collection efficiency of three beam topologies that have been proposed for Next-Generation XDi,
namely: Direct Fan-beam (DFB); Parallel Beam (PB); and Inverse Fan-beam (IFB).
The single channel replication unit for each of the three topologies was implemented on SALOME. The
apertures defining each topology were varied in width, influencing both the detector scatter signal and the
momentum resolution. A small powder graphite sample was used as reference object for these measurements, as
it provided simultaneous data on counting rate as well as peak resolution for the selected Bragg peak. The
photon collection efficiencies at constant momentum peak width for the DFB, PB and IFB topologies were
found to follow the trend (from lowest to highest, respectively) conjectured elsewhere in the scientific literature.
X-ray diffraction imaging (XDi) refers to the volumetric analysis of extended, inhomogenous objects by
spatially-resolved x-ray diffraction. Following a brief description of some of the areas in which x-ray diffraction
(XRD) is currently impacting on the detection of materials of interest in the security environment, the principles of
energy-dispersive x-ray diffraction tomographic systems of the 1st, 2nd and 3rd generation are described. The Multiple
Inverse Fan Beam (MIFB) topology for 3rd Generation XDi, in which the XRD properties of a 2-D spatial array of
volume elements are investigated in parallel without mechanical scanning, is described. 3rd Generation XDi is being
driven among other things by technological developments taking place in the field of Multi-Focus X-ray Sources
(MFXS) from which representative results are presented. MFXS source requirements for Next-Generation MIFB
XDi are summarized and the potential of 3rd Generation XDi for rapid, accurate and affordable screening in the
Checkpoint and Hold Baggage environments is summarized.
X-ray diffraction imaging (XDI) is a novel modality in which the local x-ray diffraction (XRD) properties of
inhomogenous objects are measured. Following a brief description of some of the areas in which x-ray diffraction is
currently impacting on the detection of materials of interest in the security environment, the principles of
energy-dispersive x-ray diffraction tomography employed in XDI are described. The Multi-Inverse Fan Beam (MIFB)
topology for 3rd Generation XDI, in which the XRD properties of a spatial array of 2-D volume elements are
investigated in parallel without mechanical scanning, is described. 3rd Generation XDI is being driven among other
things by rapid technological developments taking place in the field of spectroscopic, room-temperature, semiconductor
x-ray detectors. Detector requirements for Next-Generation MIFB XDI are summarized and the potential of 3rd
Generation XDI for rapid, accurate and affordable screening in the Checkpoint and Hold Baggage environments is
A novel detection technique employing x-ray diffraction (XRD) to screen for Special Nuclear Materials (SNMs),
in particular for uranium, is presented. It is based on the interesting fact that uranium (and incidentally,
plutonium) has a non-cubic lattice structure, in contrast to all other non-SNM, high-density elements of the
Following a brief review of the cubic crystal structures exhibited by high atomic number species such as lead,
the departure from cubicity exhibited by room-temperature uranium (orthorhombic) is discussed and its effect on
the uranium XRD pattern is examined. The XRD lines of uranium are compared with those of lead, a common
high-Z material found in container traffic. Significant differences are evident arising from their different crystal
In order to achieve adequate penetration, both of suspicious high-Z materials and their containers, high photon
energies must be used. Physical and technological considerations relevant to performing XRD at 1 MeV are discussed and a novel secondary aperture scheme permitting high-energy XRD is presented. It is concluded that the importance of the application and the prospect of its feasibility are sufficient to warrant experimental verification.
A novel identification technique suited to security screening of liquid and amorphous substances with x-ray diffraction (XRD) is presented. The starting point is to fit the high momentum region (independent atom regime) of the XRD profile with a free-atom scatter function corresponding most closely to the effective atomic number of the sample. The Percus-Yevick formulation of the molecular interference function for hard-sphere liquids then enables features to be extracted from liquid/amorphous XRD profiles. These features correspond to such molecular structure parameters as effective particle radius, packing fraction, effective atomic number, particle homogeneity and inter-particle potential. Amorphous substances may thus be classified into functional groups such as oxidisers and fuels. These considerations are illustrated with synchrotron XRD measurements of acetone and hydrogen peroxide, implicated in the London "transatlantic aircraft plot" of 2006 and representative of hazardous liquid fuel and oxidizer combinations.
X-ray diffraction (XRD) profiles are conventionally used to determine lattice spacings via Bragg's law in order to characterize crystalline materials. It does not appear to be widely known that they also permit compositional information, such as effective atomic number and a density descriptor, to be determined from materials having little or no crystal structure. A ratio method is introduced that relates x-ray scatter intensities in two adjacent momentum transfer bands of the diffraction profile corresponding to the "tip" region of the primary x-ray spectrum. It is shown that this ratio depends on the effective atomic number, Z, of the scattering sample. Conversely, Z may be derived from measurement of the ratio. Error sources in this ratio method are briefly analyzed. Once Z is known, the IAM (independent atom model) cross section can be extrapolated from the high momentum transfer region to lower values, where molecular interference effects manifest themselves in the XRD profile. This procedure enables the molecular interference function, s(x), to be determined. On the assumption that the structure of liquids and amorphous materials can be represented by the hard sphere (HS) model, the packing fraction, η, of atoms in the scattering sample can be ascertained from the second moment of s(x). The effective atomic number, Z and the density descriptor, η, usefully complement information provided by more traditional material analysis techniques based on XRD profiles.
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.
In the present study the applicability of x-ray fluorescence tomography for in-vivo medical imaging was investigated with respect to signal strength, background distribution and minimum detectable concentration. Tomographic imaging of the concentration distribution of suitable marker substances by the detection of the x-ray fluorescence emitted upon external excitation with x-rays has been demonstrated by other groups. However, most of these studies work with parameters that are not realistic for the medical in-vivo imaging of marker substances based on this principle; e.g. they use very small phantoms or gaseous markers. The investigated scenario uses the irradiation during a transmission computed tomography (CT) scan for the external activation of a suitable type and concentration of an x-ray fluorescence marker administered to the patient. During the irradiation, collimated and energy-resolving detectors acquire fluorescence radiation signals emitted along lines through the patient. By tomographic reconstruction of the fluorescence signal data-set, a concentration map of the marker is generated. This fluorescence image will be inherently co-registered with the high-resolution transmission CT image and can show functional or metabolic processes as an additional channel of information. The present study is based both on phantom experiments in a dedicated measurement set-up and on simulations, using various marker substances and detection concepts. Special focus was given to background reduction strategies. Moreover, the background signal in the spectral detection windows that limits the concentration resolution of the method was quantified. Signal-to-background ratios and minimum detectable marker concentrations for different scanner concepts will be presented.
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 novel type of electron-impact x-ray source based on the interaction of energetic electrons with a turbulently flowing liquid metal target is presented. The electrons enter the liquid through a thin (several microns thick) window, separating the liquid from the vacuum region in which the cathode is situated. Several electron window materials including diamond, tungsten and molybdenum were tested in combination with the liquid metal GaInSn. Satisfactory agreement has been obtained between the predictions of thermal transport models and the measured dependence of the loadability on fluid velocity. The liquid metal technology appears to represent a significant improvement in continuous loadability relative to stationary anode x-ray tubes.
A summary is given of methods to manipulate the polychromatic radiation emitted from electron impact x-ray sources so as to generate a (quasi-)monochromatic beam. These methods include: differential attenuation of bremsstrahlung, differential reflection of x-rays from monochromating crystals, production of fluorescence x-rays from secondary targets and geometrical enhancement of characteristic radiation. Typical values for some of the parameters which characterize (quasi-)monochromatic sources i.e. monochromaticity, energy bandwidth and source radiance are presented. A brief description is given of some radiological techniques which either necessitate or benefit from monochromatic radiation. With the help of a figure of merit for monochromatic x-ray sources, the suitability of the candidates mentioned above for these techniques is assessed.
Fan-beam coherent scatter computed tomography (CSCT) is a novel X-ray based imaging method revealing structural information of tissue under investigation. The source of contrast is the angular-dependent coherent scatter cross-section, which is determined by the molecular structure. In this work a phantom consisting of water, tricalcium phosphate, collagen and fat was used to investigate the contrast resolution of these four tissue constituents. Scatter projections were measured in fan-beam 3rd generation CT-geometry using an experimental demonstrator set-up equipped with a 4.5 kW DC power X-ray tube and photon-counting detectors. Reconstruction was performed using two algorithms, one based on algebraic reconstruction technique (ART) and the other based on filtered back-projection (FBP). The reconstruction results of the two techniques are compared. Furthermore, scatter functions of the four components were extracted from the 3D data sets and compared to previous measurements. The applicability of this technique for medical diagnosis is discussed.
Fan-beam coherent scatter computer tomography (CSCT) has been employed to obtain 2-dimensional images of spatially resolved diffraction patterns in order to supplement CT images in material discrimination. A Monte Carlo simulation tool DiPhoS (Diagnostic Photon Simulation) was used to create 2-dimensional scatter projection data sets of high-contrast water and Lucite phantom objects with plastic inserts. The results were used as input to a reconstruction routine based on a novel simultaneous iterative reconstruction technique (SIRT). At the same time an experimental demonstrator was assembled to confirm the simulations by measurements and to show the feasibility of coherent scatter CT. It consisted of a 4.5kW constant power X-ray tube, a rotatable object plate and a vertical detector column that could be panned around the object. Spatial resolution was ensured by mechanical collimation. Phantoms similar to those simulated were measured and reconstructed and the contrast achieved by CSCT between the materials under examination substantially exceeded that achieved in CT. A further step was taken by examining an animal tissue sample in the same way, the results of which show remarkable contrast between muscle, cartilage and fat, suggesting that CSCT can also be used in a medical scenario.
Recently, fan-beam geometry based Coherent Scatter Computed Tomography (CSCT) was proposed. Coherently scattered X-rays are used in order to reconstruct the spatial distribution of the material dependent structure function, allowing for superior tissue discrimination. In the present paper, the proposed 'third generation' CT geometry and acquisition scenario is simulated, taking into account effects like quantum noise and spectral smearing. A modified iterative algebraic reconstruction algorithm is used for reconstructing the structure function distribution from a number of two-dimensional projections acquired at different viewing angles. The visibility of small objects is investigated for several dose values. In addition to simulation studies, first results of an experimental fan-beam CSCT set-up are shown and discussed. Experiment and simulations indicate the feasibility of CSCT. Future investigations will deepen our understanding of the possibilities and limitations of this new imaging modality.
We demonstrate for the first time the feasibility of Coherent Scatter Computed Tomography (CSCT) with a fan geometry primary beam. CSCT allows superior tissue characterization and diagnosis by reconstructing the structure function rather than the attenuation properties as in regular CT. To study the feasibility of CSCT, Monte-Carlo based simulations of scatter distributions of technical phantoms were carried out. The projection data were used as input fro a novel algebraic reconstruction technique which takes account of the details of the measurement geometry. First results are discussed showing the potential of CSCT. The influence of achievable angular collimation quality is investigated and an outlook on future work is given.
Thermal control is an important issue in small-scale satellite design, and thin film coatings suit the limited mass and volume constraints. Group IVB transition metal nitride films meet the criteria that the satellite surface must be mechanically and chemically stable, and electrically conducting. Thin film TixAlyNz coatings have been investigated and tailored for temperature control. The films were deposited by reactive sputtering on aluminum substrates in N2/Ar-atmosphere. The solar absorptance, (alpha) , and thermal emittance, (epsilon) , were calculated from spectral reflectance measurements. It was found that an optimization of film composition leads to a reduced equilibrium temperature. The composition temperature. The composition Ti0.16Al0.41N0.43 has a flatter reflectance curve than TiN, and was found to be close to optimal. By varying the film thickness, interference effects could further reduce the equilibrium temperature. A 650 nm Ti0.16Al0.41N0.43 film showed a reflectance interference minimum positioned at the maximum of the blackbody spectrum, resulting in an increased emittance. Neglecting internal heat contributions, the lowest calculated equilibrium temperature was 34.6 degrees C for this film.
Potential security screening applications of a novel fluorescent x-ray source are discussed. The instrument incorporates a secondary tantalum target within the tube head and its output energy spectrum shows only the Ta fluorescence lines superimposed on a smooth, low, background. Output radiance for optimum operation is 5.9 X 109 photons s-1sr-1mm-2. Densitometry measurements were made on the sample volume formed by the overlap of the highly collimated primary and scattered beams and the ratio of the elastically to inelasticity (Compton) scattered signal was found. This ratio varies approximately as the square of the atomic number and its use reduces errors due to attenuation and geometry. The two main limitations of the ratio method are statistical noise and systematic effects such as multiple scattering and self-attenuation of the primary and scattered beams by the sample. These can be minimized by employing a forward scattering geometry and using a K edge filter to separate the small angle elastically and Compton scattered signals. The feasibility of the use of cheap scintillation detectors in conjunction with filters as opposed to more expensive energy dispersive detectors is demonstrated for low density materials and the implications for contraband detection discussed.
Coherent x-ray scatter is a very powerful analytical tool for the unambiguous identification of explosives having a polycrystalline structure. Its use in a realistic security screening environment depends on optimum matching of the measurement arrangment to the detection problem in order to minimize erroneous recognition while fulfilling stringent scan time requirements. The influence of such parameters as x-ray tube energy, scatter angle, primary beam geometry, and detector arrangement on system performance are considered and ways in which they may be optimized are discussed.
The ratio method, in which the elastic x-ray scatter signal Ie from a localized region of bulk material is normalized against the Compton scatter Ic, has been widely evaluated as a densitometric technique for evaluating mean atomic number. An analysis is presented of two major error sources influencing the ratio method. It is shown that a forward scattering geometry minimizes errors of both types for substances composed of low and medium Z elements. We describe a novel technique in which a K edge filter is used to separate small angle elastic and Compton scatter induced in a sample on irradiation with the FLUOREX fluorescent x-ray source. The feasibility of this method is demonstrated by first experimental results. The potential application of this technique to the problem of explosives detection is briefly discussed.
Material characterization based on the interaction of x and gamma radiation with matter is outlined and the need for a high intensity, monochromatic source highlighted. The design and properties of a new, fluorescent x-ray tube are described. This gives a high intensity (5.9 X 109 photons s-1 sr-1 mm-2), quasi- monochromatic output, consisting of the K fluorescent lines of its secondary target, a tantalum cone situated inside the tube. The spectral purity, stability and radiance are reported and comparisons made with radioisotope sources and a conventional tube monochromatized with a curved Ge crystal. In terms of radiance, the `Fluorex' x-ray set compares favorably with the former but not with the latter. However, it offers other advantages over these sources and without being fully optimized already has sufficient monochromatic output to find extensive use for material characterization.
Using the novel technique of energy-dispersive X-ray diffraction tomography, measurements were made of the coherent X-ray scatter from various types of foodstuff (chocolate, bacon, cherry jam, chicken breast) with their typical contaminants (macrolon, blue foil, cherry stones/wood and bone, respectively). In addition, it is shown how the use of a window technique in the diffraction spectrum allows cancellation of the foodstuff contribution in scatter images, leaving only that of the contaminant. The extension to multicomponent systems, allowing arbitrary elimination of unwanted materials in coherent scatter images, is possible. Taken together, these results indicate the great potential of coherent X-ray scatter analysis for contamination detection in the foodstuff industry. By development of more efficient X-ray scatter geometries, using e.g. fan beam irradiation with simultaneous acquisition of spectra from different voxels, the requirements of industrial mass production with respect to inspection time and resolution are likely to be met.
Bulk objects can be investigated for their material constituents by applying high-energy (30 keV to 100 keV) coherent X-ray scattering. When aiming at the detection of explosives in airport baggage, the technique allows discrimination between explosives and other substances. Coherent X-ray scatter measurements are presented for a set of explosives and their constituents as well as for a variety of nonexplosive materials. They demonstrate the superior material discrimination power of this method. The measurements have provided a quantitative basis for the prototype design of an airport baggage scanner. Sensitivity (200 g) and inspection time requirements (a few seconds) demand a highly application-specific system design with parallel acquisition and analysis of scatter spectra from different volume elements.