Depth dose distribution

Figure 4 Depth-dose distribution curves in CTA stack films with the irradiation of electron. (From Ref. 11.)...

The depth dose distribution is measured by irradiating a stack of radia-chromic film chips with a thickness slightly greater than the practical range at the energy of interest. The depth dose is determined by evaluating the individual chips. 

BARTLETT, D.T., FRANCIS, T.M. and DIMBYLOW, P.J. (1989). Methodol-ogy for the cahbration of photon personal dosemeters Calculation of phantom backscatter and depth dose distributions, Radiat. Prot. Dosim. 27, 231-244. 

FIGURE 5.2 Depth-dose distribution for electron energies 100-200 keV. (SITA Technology, U.K. With permission.)... 

The X-ray depth dose distributions in a thick water absorber with large area beams are shown in Fig. 3. These distributions indicate that the attenuation is essentially exponential, and that the penetrating quality increases with the incident electron energy. Depth dose distributions for irradiating materials from opposite directions show the minimum dose in the middle of the material. The dose uniformity ratio (DUR), also known as the Dmax/Dmin dose ratio, increases with the thickness of the material, as shown in Fig. 4 (lower set of curves). For any thickness, the DUR decreases as the incident electron energy increases. 

Besides these distinct microscopic features, particle radiation also differs considerably from photon radiation with respect to the macroscopic distribution, i.e., the depth dose distribution. The typical shape is caused by the velocity-dependent stopping power, as described by the Bethe-Bloch-formula [63, 64] ... 

The dose distribution in the materials is given as a depth-dose curve. An example of the curve is illustrated in Fig. 4 obtained with the irradiation of electron from 0.5 to 1.0 MeV using cellulose triacetate (CTA) film dosimeter [12]. The existence of the maximum dose is an important characteristic of the depth-dose curve. Irradiation from two opposite sides by using two accelerators was proposed in order to give better uniformity in water [13]. The uniform irradiation is also important for flue gas treatment. Better efficiency of NO removal was proved with both-side irradiation by using three accelerators for coal-fired flue gas than single-side irradiation at the same dose [14]. 

Figure 14 Proton beam irradiation of a deep-seated large tumor. Single Bragg peaks of different energy are combined, in adequate proportions, to obtain a homogeneous dose distribution at the level of the SOBP. The depth-dose curve of a photon beam, shown for comparison, is inferior compared to the proton curve. However, an optimized multifield photon treatment allows to reach better irradiation conditions. (From Ref 43.)...

Radiation cross-linking of polyethylene requires considerably less overall energy and less space, and is faster, more efficient, and environmentally more acceptable. Chemically cross-linked PE contains chemicals, which are by-products of the curing system. These often have adverse effects on the dielectric properties and, in some cases, are simply not acceptable. The disadvantage of electron beam cross-linking is a more or less nonuniform dose distribution. This can happen particularly in thicker objects due to intrinsic dose-depth profiles of electron beams. Another problem can be a nonuniformity of rotation of cylindrical objects as they traverse a scanned electron beam. However, the mechanical properties often depend on the mean cross-link density. ... 

Penetration of gamma-rays is more homogeneous. For E-beams, the dose depth distribution is related to the energy levels. If possible, double E-beams are recommended to flatten the dose distribution curve. Another way is to use scattering foils and reflection plates. The packaging material (vials and stoppers) must be suitable. 

Figure ll(a,b) shows the distribution of fluence (J cm ) for a broad-beam surface irradiation (where, again, the beam is at least several penetration depths in diameter), for two different cases high absorption relative to scattering (Figure 11(a)) and vice versa (Figure 11(b)). In the former case, the depth dose has a simple exponential form ... 

Effective dose fi-om y-emitters distributed in soil for depth d > 0 is less than assessed by the coefficient Ky due to absorption in the soil layer above the radionuclides. The attenuation coefficient and build-up factor in soil depends primarily on the y energy, depth of distribution, soil composition, and density. 

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