Gamma buildup factors

The constants a( ), b E), A E Uj( ), U2( ) have been determined by fitting the results of calculations to these analytic expressions. Appendix E provides some values for the Berger formula constants. The best equations for the gamma buildup factor representation are based on the so-called geometric progression (G-P) form. The G-P function has the form... 

Trubey, D. K., New Gamma Buildup Factor Data for Point Kernel Calculations ANS-6.4.3 Standard Reference Data, ORNL/RSIC-49,1988. 

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. 

The third correction factor, which is the ratio of the adsorbed dose buildup factors in the sample and the dosimeter, is usually ignored, but is shown in this paper to be very important. The absorbed dose buildup factor is defined in this paper analogous to the dose buildup factor, a notation used when the unit roentgen was still the unit of radiation dose. This paper shows the magnitude of this third correction factor, which is caused by differences in gamma-ray attenuation coefficients and softening of the gamma-ray spectrum. As an illustrative example, the dose in different dosimeters is calculated as a function of the distance from a point isotropic cobalt-60 source in water. 

If the dose is corrected for the difference in energy transfer coefficient at 1.25 Mev. and for the differences in density of the solutions but not for the differences in buildup factors the G-value, even if actually constant, would behave as if it increased with concentration. This may partly explain the great increase in the G-values with concentration of ceric sulfate observed in the past by some authors. For poly(vinyl chloride) the buildup factors differ also greatly from those of water as is seen in Table III. This difference in buildup factors may explain some of the difficulties encountered in its use in gamma-ray facilities. 

Note that the only difference between energy and dose buildup factors is the type of gamma absorption coefficient used. For energy deposition, one uses the absorption coefficient for the medium in which energy deposition is calculated for dose calculations, one uses the absorption coefficient in tissue. 

Extensive calculations of buildup factors have been performed, and the results have been tabulated for several gamma energies, media, and distances. In addition, attempts have been made to derive empirical analytic equations. Two of the most useful formulas are as follows ... 

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