Do nuclear reaction cross sections depend on the angle between incident beam and target-crystal? Shouldn't the absorption rate of a  beam of particles strongly depend on the angle between the beam and the target material's crystal-axis (if the target material is a monocrystal)? At certain angles, all the nuclei will be stacked behind each other, offering a very small cross section, whereas other orientations will expose all of the nuclei without any shadowing. However, I don't see any mention of this in cross-section tables, so I guess it doesn't matter. Why is this? 
 A: There is indeed something similar to what you describe, the so-called blocking effect (or shadow effect) - http://www.nupecc.org/iai2001/report/C3.pdf , http://encyclopedia2.thefreedictionary.com/Blocking+Effect
EDIT (08/24/2013): Let me expand on my answer. There are at least two important effects related to propagation of charged high energy particles (or, in particular, nuclei) through a crystal: blocking effect and channeling effect. I cannot describe these effects in detail here - you can google them. What's important, this propagation strongly depends on the angles between the velocity of the particles and the crystal axes. This dependence is mostly caused by the Coulomb interaction between the particles and the atoms (electrons and nuclei) of the crystal. However, due to the modified pattern of propagation, the conditions of nuclear reactions of the particles with nuclei in the crystals can also be modified - see, e.g., Nuclear Instruments and Methods in Physics Research B 99 (1995) 440-443 (http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=12&ved=0CC4QFjABOAo&url=http%3A%2F%2Fwww.researchgate.net%2Fpublication%2F235224869_Nuclear_reaction_channeling%2Ffile%2Fd912f510926a532e2c.pdf&ei=t5QYUumYJsiJ2AXf8YCQBw&usg=AFQjCNHbzRymgZPPk_Ii-9Bz380soA9bcA&sig2=zWghWinQfys3WwYQShnKJg&bvm=bv.51156542,d.b2I 
A: Let's say your target is a film $10^{-2}$ mm thick. Nuclei are about $10^{-14}$ m in diamater at most. This means that the alignment of the beam with the target would have to be $10^{-9}$ radians, which is not possible with realistic beam optics. Even if your beam optics were that perfect, the perfect parallelism would be destroyed by scattering once the beam entered the target.
A: Crossection tables are given for the bulk form of the materials, not for oriented crystals.
Indeed it has been found for the muons in particular, which only interact electromagnetically or weakly, that along the crystal axis there are few losses in the beam momentum. The phenomenon has been proposed for use in high energy muon colliders, bending muon beams by the use of special crystals. See this proposal and also here.
A: As long as the material has isotropic properties, the cross sections are independent of direction.  Cross sections are defined as a function of the material.
However, you are correct that the absorption rate will depend on the angle of the beam of the cross sections.  This must be accounted for when calculating the reaction rates, usually in a transport equation.
