Attenuation correction (SLAB geometry)
Self-shielding attenuation factor As,s(theta) per equation 3.30 from Drewitt (2009) as derived by Carlile (1974)
Carlile, C.J. (1974) Technical Report RL-74-103, Rutherford Appleton Laboratory
Drewitt, J.W.E (2009) PhD Thesis, University of Bath, pp. 45.