# MCNP planar source (rectangular)

1. ### damyoro

12
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