X-rays attenuation coefficients

In locating contraband the CT approach has two major advantages over conventional X-ray imaging. First, and most importantly, it can measure and record, unambiguously, the material property of individual objects packed in a bag. The quantity is the X-ray attenuation coefficient, which is proportional to a material s... 

Dual-energy is a CT imaging approach aimed at generating and extracting additional information beyond material density from image slices. The additional information, a second material property, is known as the effective atomic number, Zeff. Although the X-ray attenuation coefficient is proportional to a material s density p, it is also a function (i) of the atomic number Z of each of the material s... 

A numerical matrix correction technique is used to linearise fluorescent X-ray intensities from plant material in order to permit quantitation of the measurable trace elements. Percentage accuracies achieved on a standard sample were 13% for sulfur and phosphorus and better than 10% for heavier elements. The calculation employs all of the elemental X-ray intensities from the sample, relative X-ray production probabilities of the elements determined from thin film standards, elemental X-ray attenuation coefficients, and the areal density of the sample cm2. The mathematical treatment accounts for the matrix absorption effects of pure cellulose and deviations in the matrix effect caused by the measured elements. Ten elements are typically calculated simultaneously phosphorus, sulfur, chlorine, potassium, calcium, manganese, iron, copper, zinc and bromine. Detection limits obtained using a rhodium X-ray tube and an energy-dispersive X-ray fluorescence spectrometer are in the low ppm range for the elements manganese to strontium. 

Foster AL, Brown GE, Tingle TN, Parks GA (1997) Quantitative arsenic speciation in mine tailings using X-ray absorption spectroscopy. Am Mineral 83 553-568 Gaillard JF, Webb SM, Quintana JPG (2001) Quick X-ray absorption spectroscopy for determining metal speciation in environmental samples. J Synch Rad 8 928-930 Gerward L (1981) Analytical approximations for X-ray attenuation coefficients. Nucl Instr Meth Phys Res 181 11-14... 

Fig. 1.2. Measured linear X-ray attenuation coefficients of fat, fibroglandular tissue, and tumor in the breast (from Johns and Yaffe (1987). With permission, lOP publications)...

Creagh DC and Hubbel JH (1987) Problems associated with the measurement of X-ray attenuation coefficients. I. Silicon. Report on the International Union of Crystallography X-ray Attenuation Project. Acta Crystallo-graphica A43 102-112. 

In more realistic thick samples, the calculation must also take account of the energy loss of the particle as it penetrates the sample and also the absorption of the X-rays as they emerge from the sample. To do this, a knowledge of the bulk composition of the sample is required (together with any variation with depth) and both the stopping power of the ion in the matrix and the X-ray attenuation coefficient must be known. The calculation then involves a numerical integration of the total X-ray yield from each sublayer of the sample. 

Second, the use of meshed particles versus a pressed wafer will typically lead to nonuniformity of X-ray absorption thickness. This can be directly observed by placing an X-ray sensitive camera behind the sample a sample of a powder pressed into a wafer is spectroscopically more uniform than a catalyst bed of meshed particles. Naturally the contrast becomes more extreme as the meshed particles become larger. Moreover, if the sample is spatially nonuniform then severe constraints are placed on the positional stability of the X-ray beam. Any motion of the position of the X-ray beam will then probe different thicknesses of the sample, with direct consequences on the measured S/N. From the perspective of XAFS spectroscopy, any nonuniformity of the sample thickness could directly affect the accuracy of the measurement of the amplitude of the X-ray absorption coefficient. It is the amplitude that contains information about the coordination number and site disorder. As has been discussed elsewhere (Koningsberger and Prins, 1988), these amplitude distorting effects are given the general heading of "thickness effects." In brief, a thickness effect occurs when part of the incident X-ray beam is not attenuated by the sample. In the case of meshed particles this would be in the form of pinholes in the sample. 

Absorption coefficients define the X-ray attenuation effect from a given solids. We use both mass absorption coefficients, //m, and linear absorption coefficients, p, with these being related by ju = j.umpx, where p is the density of a given solid and x is the X-ray path length. The mass absorption for any material can be calculated from the elemental mass absorption coefficients by the relation pm = Z/ / where the f and the are the weight fraction and linear absorption coefficients, respectively, for each element in the material. 

The linear attenuation coefficient can be written as /x = (fN A, where Avagadro s number = 6.02 X 10 3 mol, A is the atomic weight, and p is the density. Given that the interaction cross section cF is not a function of the density of the medium, a mass absorption coefficient p/p = (fNJA can be defined. This is related to the mass of a material required to attenuate an x-ray beam by a given amount. It is the form most often quoted for x-ray attenuation in tables of physical constants. 

I x) = intensity of rays after traveling a distance x through the material /X = linear attenuation coefficient of the material... 

Figure 5 Projections, P t,d), obtained from X-ray attenuation measurements are used in computed tomography to reconstruct the cross-sectionai distribution of the iinear attenuation coefficient within investigated object.

Berger MJ, Hubbell JH (1987) NIST X-ray and gamma-ray attenuation coefficients and cross sections database, NIST standard reference database 8. National Institute of Standards and Technology,... 

A common contrast agent is barium sulfate [7727-43-7] although iodinated compounds have been used. Owing to the much higher linear attenuation coefficient of the contrast agent, a higher (120 keV) energy x-ray typically is used. 

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