MadSci Network: Physics

Re: Is it likely for a low powered x-ray to travel an unlimited distance?

Date: Thu Oct 14 00:35:44 2004
Posted By: Dr. Nagesh N Bhat, Post-doc/Fellow, Biophysics,
Area of science: Physics
ID: 1097077513.Ph

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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