MCNP planar source (rectangular)

[damyoro]

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

 

[AndyW]

for your help.



Thank you for your question regarding the use of SDEF variables to define a planar rectangular source. The VEC (VEC=001) variable is used to specify the direction of the source particles, while the DIR variable is used to specify the direction of the source surface. In your example, it appears that you have only used the VEC variable, which means that the source particles will be emitted in the direction (1,0,0) from the origin (0,0,0). To take into account the fact that the source is emitting equally to the opposite surface plane, you can use the DIR variable to specify the direction of the source surface. For example, if your source surface is facing in the positive x-direction, you can use DIR 1 0 0 to indicate this. This will ensure that the source particles are emitted equally in both directions along the x-axis.

Regarding the cosine distribution mentioned in the primer, p(μ) = 2μ for μ [0,1], this is a probability distribution function that describes the angular distribution of the emitted particles. The value of p(μ) can be 2 when μ=1 because this is the maximum value of the distribution, meaning that the particles are most likely to be emitted in that direction. This does not violate the concept of probability, as the total probability for all directions is still equal to 1. The cosine distribution is commonly used to describe the emission of particles from a surface source because it takes into account the cosine of the angle between the emitted particles and the normal vector to the source surface, which affects the probability of emission.

In your example, the SI1 and SI2 variables specify the angular range in which the particles can be emitted from the source. The SP1 variable specifies the probability of emission for particles within that angular range. In this case, particles will be emitted from -45 to 45 degrees with a probability of 1, and from -70 to 70 degrees with a probability of 0. This means that particles will only be emitted within the range of -45 to 45 degrees.

I hope this helps to clarify the use of SDEF variables for defining a planar rectangular source. If you have any further questions, please do not hesitate to ask.