## Attenuation Length

Hello, I am reading this article in wikipedia http://en.wikipedia.org/wiki/Attenuation_length about attenuation lengths and there is something I'm hoping someone here can explain to me.

Lets say I have one small solid sphere and one big hollow sphere. And lets say I put the small sphere within the hollow part of the big one.
Now, somehow the small sphere is radioactive and it emits gamma rays. The big sphere is made of a different material (not radioactive). Also, lets say I have solved the equation in the wiki article for the length L = -λln(rand(0,1)). Then, I solve for L using λ for the radioactive material. But what happens if this L happens to be big enough to exit the first material and it goes into the second one. Would I have to calculate L again using λ for the non-radioactive material?
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 Mentor You want to calculate a random length L (for a single particle) for the flight distance to the point of absorption? In this case, if both materials are equal, just add the lengths in the materials. Alternatively, if L is longer than the flight distance in the inner object, do the same calculation again for the outer object, with a new L' for the length in this material only.

 Quote by mfb You want to calculate a random length L (for a single particle) for the flight distance to the point of absorption?
Exactly.

 Quote by mfb In this case, if both materials are equal, just add the lengths in the materials. Alternatively, if L is longer than the flight distance in the inner object, do the same calculation again for the outer object, with a new L' for the length in this material only.
Thank you.