Using Carbon or Aluminum to block photons

We're looking to see which is better for letting photons through, Carbon or Aluminum.

20 MeV for Carbon range is 10.49 \frac{g}{cm^{3}} density of Carbon = ~2.3 \frac{g}{cm^{3}} thickness = \frac{range}{density} = \frac{10.49}{2.3} = 4.56 cm^{2}