[cnvogel]

For typical Gamma radiation of about 1MeV photon energy or higher, you need about 1cm thickness of lead to reduce the radiation to about a third (by a factor of 1/ℯ). One order of magnitude of radiation hardness (reduce radiation to 1/10th) needs two centimeters of lead. That's getting heavy pretty fast.

Radiation intensity inside your box depends on the thickness t: I(t) = I₀·exp(-tρμ)

t: thickness, say 1 cm ρ: density of the material, for Lead 11 g/cm³ μ: absorption coeffcient 0.1 cm²/g [see ref 1] I₀: intensity outside of the box

    ...import numpy as np...
    In [5]: np.exp(-0.1 * 11.34)
    Out[5]: 0.32174370422037013

I(1cm) = 32,2%, I(2cm) = 10,4%

[1] http://www.eichrom.com/PDF/gamma-ray-attenuation-white-paper...