Hello to everybody, I need some explanation on how to use SDEF variables to define correctly a planar rectangular source. Let say this source is emitting in all direction but I am interested in a side of the source surface where a point detector is located. I used in my example VEC (VEC =001) but not DIR. How to take into account the fact that in analog simulation the source is emitting equally to the opposite surface plane. I would like also to understand the cosine distribution mentioned in the primer for a surface source where it is written p(μ) = 2μ for μ [0,1]. That is means that p(μ) can the value 2 when μ=1. Is it not a probability? Which is supposed to be less than 1. Here is my example: SDEF SUR= 51 POS 0 0 0 X=0 Y=d1 Z=d2 PAR=2 ERG 1.25 VEC 1 0 0 $ Plane rectangular source SI1 -45 45 SP1 0 1 SI2 -70 70 Thank you in advance