Calculating Photon Detection in Radiation Physics with 30 Degree Incident Angle

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_Andreas
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The problem: 1000 photons all having an energy of 100 keV hits a copper plate 2 mm thick. How many of these photons can be detected on the other side by a detector that absorbs all radiation? The incident angle is 30 degrees with respect to the plane of the copper plate.

If the angle had been 90 degrees I would have had no problem calculating this -- it would just have been N = N0*e^(mu*L), where N0 is the number of incoming photons, mu the linear attenuation coefficient, and L the thickness of the copper plate. But now that it says 30 degrees, I'm out of ideas.
 
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The path length is greater since the beam is not striking the face at 90 degress, just figure out the correct path length through the 2mm thick slab.
 
Thanks. I forgot that the photons are assumed either to be absorbed fully by the material, or to pass through without any interaction. So L should be be replaced by L'=4 mm, right? (That is, L'= 2mm/sin30).