Setting up mixed materials

[Paidaxing123321]

If the protective material is a mixed material (with several different elements), how should I set up this material?

[AlanFord]

That is a very good question - there are several approaches that can be used. There are a few recent papers on creating interpolated buildup factors for mixed materials, but I haven't implemented any at this time as they haven't been endorsed by any of the international standards groups.

The approach I've seen most often is to model the shield (composed of several different elements) as a series of layers, each containing one of the materials. The layer thicknesses should be sized such that the mass of each material in the shield is conserved. This will ensure that the mean-free-paths of shielding between the source and the receptor are accurate.

The choice of a buildup factor is more problematic. Excluding any method to interpolate between buildup factors, my mentor of years ago suggested choosing the buildup factor of the outermost "thick" shield, as it will tend to control the spectrum of the photons reaching the receptor. In your case of a mixed shield, you may wish to choose the buildup factor of the material with the largest mass fraction. If you have two separate elements with similar atomic weights, their buildup factors are likely similar and, together, they may constitute the largest mass fraction of the shield.

Another approach when evaluating a shield is to choose a conservative buildup factor. When evaluating the maximum dose that could be received at the receptor, use the shield with the largest buildup factor. Or for a mixed shield, use the component element with the largest buildup factor. The reverse may be necessary if trying to determine an appropriate set point for a radiation monitor. Choose the shield of component element with the smallest buildup factor, resulting a conservatively low alarm set point.

I hope this helps!