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 = $\frac{range}{density} = \frac{10.49}{2.3} = 4.56 cm$

Therefore, the thickness of our Carbon is 4.56 cm

10 MeV hitting 4.56 cm of Carbon

n = $2.3 \frac{g}{cm^{3}} \times \frac{6.022 \cdot 10^{23} atoms}{12 g} = 1.2 \cdot 10^{23} \frac{atoms}{cm^{3}}$