X-ray Filtration: Lead vs. Aluminum - Which is More Effective?

Lead.

The half-value layer (HVL) of lead is 0.5mm, which means that 2.5mm of lead will reduce the intensity of an x-ray beam by a factor of 2^5 = 32. The HVL of aluminum is 6.2mm, so 2.5mm of aluminum will only reduce the intensity of an x-ray beam by a factor of 2^2 = 4. Therefore, lead is a more effective filter for x-rays than aluminum.