A collimated gamma beam crosses a slab of shielding and thins out as I = I0e-μx. Change the energy, the material and the thickness, and watch how much gets through. The beam never reaches zero.
Transmitted I / I0
3.6 %
a factor of 28 weaker
Photon energy1.25 MeV
Shield thickness5.0 cm
Material
Source preset
Attenuation μ0.667 /cm
Half-value layer1.04 cm
Tenth-value layer3.45 cm
HVLs in this slab4.81
Mean free path1.50 cm
Lead · density 11.35 g/cm3 · μ/ρ = 0.05876 cm2/g Narrow-beam values interpolated from NIST mass attenuation tables.
Tip: push the energy up and the shielding gets worse, not better — μ falls as photons get more penetrating.