X-ray backscattering

X-ray backscatter imaging introduced in baggage inspection (AS E lOlZ)... 

R.D. Swift, Mobile x-ray backscatter imaging system for inspection of vehicles, in Proc. SPIE Vol. 2936, Physics Based Technologies for the Detection of Contraband, L.O. Malotky andJ.J. PenneUa (eds), 19-20 November 1996, Boston, MA (1997) 124-129. 

The lack of a capability to screen for explosives hidden on an individual is a major vulnerability in aviation and general security. Personal privacy issues and perceived health risks have deterred the use of bulk detectors, such as X ray, X-ray backscatter, and millimeter wave, for screening of individuals for concealed explosives. Consequently, the TSA is focused on trace detection as the best solution for passenger screening in airports. The TSA has determined that individuals carrying as little as 1 lb of concealed explosives get sufficiently contaminated to be detectable by portal devices that use trace detectors. The level of contamination on an individual s exterior clothing that can be routinely detected by the best portal devices is about 1 pg or about 1 part in 109 of the explosive mass. 

The vertical density profile provides a means of optimizing the press operation, as well as panel performance. While initially undertaken in the laboratory, the VDP can now be measured on the moving panel as it leaves the press, using an x-ray backscatter technique (Dueholm, 1996). This is an expensive tool, but the purchase is increasingly common as the value of real time VPD information is appreciated. 

A schematic drawing of the sensor system employed in the Sortex Ash Monitor is shown in Figure 4. The x-rays emitted by the Pu source irradiate the coal sample and penetrate up to 38 mm. This radiation is absorbed and scattered the fraction backscattered is counted with a gas proportional counter. The aluminum filter is used to compensate for iron fluorescent x-rays, which are excited by the incident x-rays. The filter preferentially absorbs most of the iron fluorescent x-rays ( 6 KeV), and its thickness is chosen based upon the iron content of the coal samples. The aluminum filter also compensates for sulfur variations in the coal. This occurs because a major fraction of sulfur is present as pyrite and the decrease in x-ray backscatter intensity due to sulfur is partially offset by an increase in iron fluorescence. Organic sulfur is generally constant, and it is corrected for in the calibration of the instrument ( 5). 

Scanning transmission electron microscopy (STEM) uses X-ray backscattering analysis to obtain information on the size, morphology, and chemical composition of the active components on support materials (Fig. 5-47). 

Kutami et al. (1997) pressed a single crystal (1 x 1 xO.3 mm ) with a tetragonal structure at lOOOkgf/cm for 120 h under an oxygen atmosphere. As a result, the crystal annealed witli the uniaxial compression was an orthorhombic phase without twins as confirmed by Laue X-ray backscattering, TEM observation and Tc measurements. Onset of 7 of the twin-free crystal was about 92 K and the Jc value at 1T was one fifth that of the twinned crystal, which indicates a high crystallinity. 

EXAFS Extended x-ray absorption fine structure [177, 178] Variation of x-ray absorption as a function of x-ray energy beyond an absorption edge the probability is affected by backscattering of the emitted electron from adjacent atoms Number and interatomic distance of surface atoms... 

By inserting a semiconductor x-ray detector into the analysis chamber, one can measure particle induced x-rays. The cross section for particle induced x-ray emission (PIXE) is much greater than that for Rutherford backscattering and PIXE is a fast and convenient method for measuring the identity of atomic species within... 

Figure Bl.24.1. Schematic diagram of the target chamber and detectors used in ion beam analysis. The backscattering detector is mounted close to the incident beam and the forward scattering detector is mounted so that, when the target is tilted, hydrogen recoils can be detected at angles of about 30° from the beam direction. The x-ray detector faces the sample and receives x-rays emitted from the sample.

Ema data can be quantitated to provide elemental concentrations, but several corrections are necessary to account for matrix effects adequately. One weU-known method for matrix correction is the 2af method (7,31). This approach is based on calculated corrections for major matrix-dependent effects which alter the intensity of x-rays observed at a particular energy after being emitted from the corresponding atoms. The 2af method corrects for differences between elements in electron stopping power and backscattering (the correction), self-absorption of x-rays by the matrix (the a correction), and the excitation of x-rays from one element by x-rays emitted from a different element, or in other words, secondary fluorescence (the f correction). 

We have already discussed a number of applications of the SEM to materials characterization topographical (SE) imaging, Energy-Dispersive X-Ray analysis (EDS) and the use of backscattering measurements to determine the composition of binary alloy systems. We now shall briefly discuss applications that are, in part, spe-... 

Because X-ray counting rates are relatively low, it typically requires 100 seconds or more to accumulate adequate counting statistics for a quantitative analysis. As a result, the usual strategy in applying electron probe microanalysis is to make quantitative measurements at a limited collection of points. Specific analysis locations are selected with the aid of a rapid imaging technique, such as an SEM image prepared with backscattered electrons, which are sensitive to compositional variations, or with the associated optical microscope. 

Figure 2 Molybdenum K-edge X-ray absorption spectrum, ln(i /i ) versus X-ray energy (eV), for molybdenum metal foil (25- jjn thick), obtained by transmission at 77 K with synchrotron radiation. The energy-dependent constructive and destructive interference of outgoing and backscattered photoelectrons at molybdenum produces the EXAFS peaks and valleys, respectively. The preedge and edge structures marked here are known together as X-ray absorption near edge structure, XANES and EXAFS are provided in a new compilation of literature entitled X-rsy Absorption Fine Structure (S.S. Hasain, ed.) Ellis Norwood, New York, 1991.

Figure 3 Schematic illustration of the EXAFS phenomenon (A) outgoing photoelectron (solid curve) from X-ray absorbing atom (B) destructive interference at the absorbing atom between outgoing (solid curve) and backscattered (dashed curve) photoelectron from neighboring atom (C) constructhra interference at the absorbing atom between outgoing (solid curve) and backscat-tared (dashed curve) photoelectron from neighboring atom. Adapted from T. M. Hayes and J. B. Boyce. Solid State Phys. 37.173,1982.

Rutherford Backscattering (RBS) provides quantitative, nondestructive elemental depth profiles with depth resolutions sufficient to satisfy many requirements however, it is generally restricted to the analysis of elements heavier than those in the substrate. The major reason for considering depth profiling using FIXE is to remove this restrictive condition and provide quantitative, nondestructive depth profiles for all elements yielding detectable characteristic X rays (i.e.,Z> 5 for Si(Li) detectors). 

Extended X-ray absorption fine structure (EXAFS) measurements based on the photoeffect caused by collision of an inner shell electron with an X-ray photon of sufficient energy may also be used. The spectrum, starting from the absorption edge, exhibits a sinusoidal fine structure caused by interferences between the outgoing and the backscattered waves of the photoelectron which is the product of the collision. Since the intensity of the backscattering decreases rapidly over the distances to the next neighbor atoms, information about the chemical surroundings of the excited atom can be deduced. 

A variety of other techniques have been used to investigate ion transport in conducting polymers. The concentrations of ions in the polymer or the solution phase have been monitored by a variety of in situ and ex situ techniques,8 such as radiotracer studies,188 X-ray photoelectron spectroscopy (XPS),189 potentiometry,154 and Rutherford backscatter-ing.190 The probe-beam deflection method, in which changes in the density of the solution close to the polymer surface are monitored, provides valuable data on transient ion transport.191 Rotating-disk voltammetry, using an electroactive probe ion, provides very direct and reliable data, but its utility is very limited.156,19 193 Scanning electrochemical microscopy has also been used.194... 

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