# Changing KMESH/KINTS from azimuthal to polar direction in MCNP6

#### khary23

I am modeling a cylindrical source in MCNP6 and would like to use the FMESH tally in cylindrical coordinates. I am looking for the dose to water from the source as a function of radial distance as well as polar angle running from 0 to 180 degrees in the YZ plane not around Z. Is there a way to do this?

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#### DEvens

Gold Member
Sure. I'm looking at the MCNP6 version 1 user manual and it's Section 3.3.5.25, page 3-67. You use GEOM=CYL then the I direction is radial, J is the z direction, and K is the theta.

To get the dose you need the conversion factors. These allow you to convert from a flux at a particular energy to a dose. You will find a table that gives the energy range split up into a bunch of bins, and dose conversion factor for each bin. You can get those from the appropriate health physics standard. You enter them in the same way you would with a type F4 tally. That's detailed in Section 3.3.5.8, page 3-211.

#### khary23

Hello,
What I want to do is leave I(radial) and J(z) as they are in cylindrical coordinates , but change K so that is is not rotating about the z axis. What I want to do is have zero degrees pointing in the +z direction and 180 degrees in the -z direction and 90* at y =0. I drew a crude picture of what I want to do.

#### DEvens

Gold Member
Do you mean you want to have the axis of the cylinder along a line you specify? That's also in the FMESH description. Whatever the axis of the cylinder is, theta is defined to be the angle around the circumference of the cylinder.

#### khary23

No. I do not want theta to be defined as the angle around the circumference of the cylinder. I want theta to be defined as it is in spherical coordinates as the polar angle. Where theta =0 is along the z positive axis and theta = 180 is in the negative z direction.
If it is not possible I can use TMESH just thought it might more elegant to have the mesh and source geometry be the same.

#### DEvens

Gold Member
Oh. Except your first post said you were using a cylindrical source. And you drew a cylinder. If your source is cylindrical then why don't you want theta defined as the angle round the circumference?

If your source is really spherical, sadly FMESH does not have a spherical polar coordinate option.

#### khary23

Yes the source I am modeling is cylindrical. The formalism I am following dictates that (r,theta) specifies a point relative to the source longitudinal axis. Here is a picture of the geometry that the protocol calls for.

#### DEvens

Gold Member
I suppose you could turn the fmesh 90 degrees to the source cylinder. In your drawing you could have the z-axis of the fmesh coming out of the screen. That would give you a circular mesh in the plane of the source. It won't give you the other angle though.

"Changing KMESH/KINTS from azimuthal to polar direction in MCNP6"

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