Why does silver have lower absorption than zirconium at 35 keV?

[Myrddin]

so just about finishing a report on x ray abosprtion on different materials, generally the linear attneuation coefficient increases with atominc number Z. However silver(47) had a lower attenuation coefficient then zirconium(40) despite having the significantly higher value of Z.

With some reading found that the absorption edges ;where there is a rapid increase in absorption from photon matching the binding of a shell, depends on the energy of the x ray your using. So we thought this must explain our result with silver. Looking at the K shell :

Zirconium has Kedge absortpion at 18KeV
Silver has kedge absorption 25 KeV

Our supply voltage was 35Kv---> 35KeV , so why does zircmonium have more absortion?

 

[member 553083]

The answer lies in the fact that (K) edges of zirconium and silver have different shapes. The K edge of silver has a very sharp rise at 25 keV, but then a gradual decrease as energy increases. So even though the absorption was lower for silver than for zirconium at 25 keV, it still had more absorption at 35 keV due to its gradual decrease in absorption. Thus, the higher atomic number of silver did not necessarily lead to it having a higher linear attenuation coefficient than zirconium.