In a nuclear power plant, absorbers are used to control the fusion process. To be more precise, absorbers slow down the process. Some materials qualify as absorbers, one I remember is silver. What I would like to know is: What makes a material a good radiation absorber? How can the "strength" of absorption be measured? What quality - possibly in terms of (e.g. atomic or molecular) structure - of a material makes one absorber material a stronger absorber than another? How can absorbers and their strength be found by computation?