The distance covered when 22 MeV gamma rays travel through air

In a beam of 22 MeV gamma rays passing through dry air at sea level, half will be absorbed in about 350 meters. Photons at lower energies are absorbed in a shorter distance (for 2 MeV, it's about 130 meters).

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Calculation details: NIST gives the mass attenuation coefficient $mu/rho$ for 20 MeV photons as $1.705 times 10^{-2} mathrm{ {cm}^2/g}$ (it's a little bit lower at higher energies). The density of air at sea level, $rho$, is about 1.225 kg/m$^3$. Therefore, for 20 MeV photons, half will be absorbed in a distance

$$frac{ln 2}{(1.705 mathrm{ {cm}^2/g})(1.225 mathrm{ {kg}/m^3})} approx 330 mathrm{ m}$$

and the distance is a little higher at 22 MeV.