Total Attenuation Coefficients

Etherington, 1958 gives the total attenuation coefficient in air p, = 0.0804 cmVgm, and the energy absorption coefficient = 0.0311 cmVgm for tissue. 

Fig. 5.9. Relative contributions of various photon interactions to the total attenuation coefficient for (a)...

Figure 7. Incident light dose at the surface versus depth of photodynamically induced tumor necrosis in vivo as a function of injected dose of Photofrin (hematoporphyrin oligomers). The value of a (total attenuation coefficient) was taken as 0.33 a typical value for tumors and other tissues with low levels of absorbing chromophores. Each curve represents a different injected dose of photosensitizer (Photofrin) in mg per kg of body weight as indicated on this graph.

A parallel beam of gammas impinges upon a multiple shield consisting of successive layers of concrete, Fe, and Pb, each layer having thickness 100 mm. Calculate the fraction of gammas traversing this shield. The total attenuation coefficients are /Li(concrete) = 0.002 m /kg, ju,(Fe) =... 

The reduction in intensity of a beam can occur by two mechanisms. One involves the deflection or scattering of the particles from the direct line of path between the source and the detector and is described by the scattering coefficient ii. The second mode of reduction is the complete transfer of the projectile energy to the absorbing material (the particles are "captured" ) and is designated by the energy) absorption co cient The (total) attenuation coefficient in (6.7) is the sum of both these modes. 

Both /ig and /x, can be measured independently. The (total) attenuation coeffici t is of primary interest in radiation shielding, while the (energy) absorption coefficioit is inqx>rtant in considering radiation effects on matter. 

Unlike heavy particles and electrons which lose their energy as a result of many collisions, y-rays are completely stopped in one or, at most, a few interactions. For thin absorbers the attenuation of y-rays follows relation (6.7), where is the number of photons m s The proportionality factor p is called the total) attenuation coefficient. When it has the dimension of m and the thickness x is expressed in meters, ii is referred to as the linear attenuation coefficient. The attenuation coefficient can be expressed in other ways ... 

As already mentioned, the total attenuation coefficient is the sum of the attenuation coefficients for each interaction process (eqn [1]). The percentage contribution of each process to the total attenuation changes with photon energy and depends on the atomic number of a single element absorber or the so-called effective atomic number in case of complex materials. 

Here fi is they-ray linear attenuation coefficient, usually measured in cm units. It is a sum of the interaction terms described in O Chap. 6 in this Volume, hence it is also called total attenuation coefficient. Its inverse is called the mean free path, while the thickness reducing the photon beam by half is the half-thickness di/y, both are measured in cm. Frequently the mass attenuation coefficient pip is used, because it does not depend on the physical state of the material. Its dimension is cm /g if the density p is given in g/cm units. Another important quantity is the mass-energy absorption coefficient p Jp, measured in the same units, which characterizes the energy deposition by photons. AH these quantities, their units and usage have been defined by the International Commission on Radiation Units and Measurements (ICRU) in ICRU Report 33 (ICRU 1980), which has recently been superseded by two new ones (ICRU 1993c, 1998). 

The effect on enhancement of both the imaging depth and contrast was found in in vivo studies of human skin optical clearing at topical application of flic 50%-propylene glycol solution [59]. The OCT images captured from the skin site of the volunteer at hyperdermal injection of 40%-glucose allowed one to estimate the total attenuation coefficient [see Eq. (8)] [59]. The attenuation behaviour with the time eourse well correlates with the spectral measurements shown in Fig. 12 and also reflects the index matching induced by the glucose injection. 

The total mass attenuation coefficients, p / p, and degree of attenuation. A, for the reference material (hematite) and the in-house RMs are presented in Table 4. They were calculated using equations 3 and 2, respectively, and the bulk chemical composition information from XRF and carbon-sulphur analysis. The total attenuation coefficients ranged from 1.01 to 1.85 and their associated degrees of attenuation from 0.65 to 0.78. 

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