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Both alpha and beta radiations can only travel short distances through air as they're not as penetrating as gamma radiations. How long gamma radiations with 22 MeV energy can travel in air? Is it meters, kilometers, miles, etc.

Is there a difference between a lower energy gamma ray (for example a 2 MeV gamma ray) and a higher energy gamma ray (22 MeV gamma ray) when it comes to the maximum distance covered in air?

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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).

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.

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  • $\begingroup$ Thank you, does the same applies to 22 MeV bremsstrahlung too? If 22 MeV gamma rays produced by pair production travel over a max distance of over a kilometer, what is the max distance that 22 MeV gamma rays generated by bremsstrahlung can travel? Is it just the same? $\endgroup$
    – SnoopyKid
    Commented Oct 30, 2020 at 4:04
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    $\begingroup$ @MohamedObeidallah Gamma rays produced by pair production are the same type of particle as gamma rays produced by bremsstrahlung. So yes, the distance is the same. $\endgroup$
    – Thorondor
    Commented Oct 30, 2020 at 17:05
  • $\begingroup$ Thanks, aren't gamma rays should be called as rays instead of particles? Sorry. So, if half of 22 MeV photons is absorbed at a distance of 350 m then the entire distance is twice than 350 m thus 700 m. Correct? $\endgroup$
    – SnoopyKid
    Commented Oct 30, 2020 at 17:32
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    $\begingroup$ @MohamedObeidallah Gamma rays are just high-energy electromagnetic radiation. Like all electromagnetic radiation, they are both waves (rays) and photons (particles). As an approximation, you can think of "gamma radiation" as a beam composed of many tiny high-energy particles, just like a water wave is composed of many tiny water molecules (but note that this analogy is not perfect and shouldn't be taken too literally). $\endgroup$
    – Thorondor
    Commented Oct 30, 2020 at 17:42
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    $\begingroup$ @MohamedObeidallah The second part of your comment is not right. If half of the photons are absorbed in a distance of 350 m, then half of the remaining photons are absorbed in the next 350 m. Therefore, 1/4 of the original photons are left after 700 m. After 1050 m, 1/8 of the photons remain, and so on. $\endgroup$
    – Thorondor
    Commented Oct 30, 2020 at 17:42

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