|MadSci Network: Physics|
As you rightly mentioned, when it comes to the real world, it is possible to stop X rays or gamma rays 'almost' completely. The shielding requirements depend on the initial intensity. Soft X rays, as in case of picture tubes are stopped mostly in the outer glass cover of the tube itself. This is because the absorption coefficient for X rays in these energy ranges is very high. Few mm of glass can offer several TVL (tenth value layer). Hence, if you calculate the total number of photons coming out of this thickness, you will find the number may be in fraction. For all practical purpose, if number of photons emerging out is less than 1 or more practically, if it is below the permissible limit, we can safely say that the X rays are stopped or shielded. However, theoretically, even the 'less than 1' fraction has some meaning. Suppose if the intensity is 0.001 photons/cm^2/s, it means that there is a probability of one photon coming out in 1000 seconds or 100 photons in 10^5 seconds!! X ray photons have infinite range because of their mode of interaction. The interaction processes are purely stochastic. Hence, theoretically, a single photon can pass through a thick material without getting affected. However, practically, we can stop (or in strict sense, reduce) the X rays to negligible intensity.
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