What Makes Iron Effective at Blocking Radiation Compared to Lead?

[znc03]

Does anyone know how Iron compares to lead as a medium to block / absorb radiation? Do you know what wavelengths or spectrum iron is most effective against?

 

[mfb]

Photon radiation? In other words, X-rays and gamma radiation?
Lead is better per mass, and has a higher density in addition (so it is much better per length).
For high photon energies (>100keV), the radiation length is the relevant quantity. You can find numbers at the particle data group.

Iron: Radiation length 13.84 g ^-2 or 1.757 cm
Lead: Radiation length 6.37 g cm^-2 or 0.5612 cm

For low-energy photons, it depends on the energy.