Build-up factor

These complex processes are treated as an attenuation process with a build-up factor, B(E,r), in equation 8.3-9 to give equation 8.3-11. 

Large computers calculated theoretical models of the secondary processes to produce tables of build-up factors. These tables are for neutrons and gammas of various energies in many geometries and material combinations. 

For example, a 1 MeV point isotropic source of gamma-radiation has a buildup factor ol 2.1 when penetrating a mean-free thickness of water. If the build-up factor is ignored, equation 8.3-11 is exp(-l) = 0.36. Hence, 36% of the radiation passes through the shield. But when the buildup factor is included, 2.1 0.36 = 76% of the radiation penetrates the shield. 

In practice, however, an equation expressing the relationship between the product of shield thickness and linear attenuation coefficient, and the build-up factor is used for calculating the build-up factor at a certain shield thickness. One of the typical approximations is as follows ... 

B = build-up factor due to broad beam geometry N = number of patients per week A = decay constant (fr1 j of the radionuclide... 

FIG. 6.15. Thickness of lead and concrete reducing 7-ray flux by different powers of 10, as function of 7-ray energy. The curves are for thick absorbers and include build-up factors. 

FIG. 6.20. Dose build-up factors in lead for a point isotropic source. Upper curves have scale to the left, lower curves to the right. (From Radiological Health Handbook. )... 

A freshly prepared small source of Ne had a measured decay rate of 1 GBq 1 s after its preparation. The source is shielded by 10 cm of Pb. e emits y-rays 8% with 0.878 MeV and 100% with 0.472 MeV. Estimate the total integrated dose received at 2 m distance during its life-time after preparation. Neglect build-up factors. 

The above equation is an approximation, and build-up factors of the material are not taken into accoxmt. Shielding calculation software can be used to greater effect. 

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. 

There are several computer models for package activity estimation based on exposure rates. Models can be simple if build-up factors are not taken into account, or very sophisticated if the Compton effect is included. 

The build-up factor, B, is the ratio of the total photons at a point to the number arriving there without being scattered. There are a number of empirical equations in use for estimation of the build-up factor, references to which are given later. 

Under open geometry conditions, a build-up factor must be included, as in Equation (2.17). 

---