X-Rays (and gamma radiation) are penetrating radiation that is not readily stopped by the skin. Biologically, x-rays will cause cancer in cells that reproduce the most often, such as fetal cells and bone marrow. Skin cells are not reproduced/replaced often and are thus not affected by X-rays as are most internal organs.
Thus, X-rays do not adversely affect one race over another.
As to your assertion that whites fly in greater proportion than blacks (which I do not necessarily support) would only have relevance if you could show that these machines are installed only in airports from areas that have a greater proportion of whites. For example, are there any such machines in the New Orleans airport vs. the Salt Lake City airport?
We have them in Detroit.
I can tell you from personal experience the ratio of white fliers to other races is about equal.