Subject: Variables for X-ray absorption

I've been trying to understand what variables affect how much a material 
absorbs X-rays.  I've read online that it's the density of the material, or 
it's the size of the atoms that make up the material (or more fundamentally, 
the electron configuration of the atoms that make up the material), or who 
knows what else.  I'm imagining that all of these variables affect things, but 
is there one variable that affects things more than another?  I read on one 
page that the electron density in the material is the most important factor, 
and that seems likely to be the most important thing.

For instance, if you had a block of pure lithium and a block of pure calcium, 
and you adjusted the way that they were prepared and tested so that they were 
of exactly equal density and exactly equal thickness and everything else was 
exactly the same, would the calcium absorb more of the electrons because each 
atom has a greater number of electrons around it?

I'm not sure if I'm asking this clearly or not, and I apologize if it's kind of 
confusing.

Thanks.

Re: Variables for X-ray absorption