Isotropic distribution for a surface source MCNP

Hello everyone!
I have to use isotropic distribution for my mcnp program. But I didn't find how can I create that one.

Does it have to be a surface source? With a cell source, for example a point source, isotropic is the default.

Alex A said:

Does it have to be a surface source? With a cell source, for example a point source, isotropic is the default.

Yes, it has to be a surface..

What is the shape of the surface and can the source be a degenerate volume source?

That would be a surface but it has the advantages of not using a geometric surface and so the result is isotropic.

Alex A said:

What is the shape of the surface and can the source be a degenerate volume source?

That would be a surface but it has the advantages of not using a geometric surface and so the result is isotropic.

This is the surface of the cube, that is, one side of it.
I tried to make a cell source of it, but i don't understant how to do it correctly. For the cell source I need to write from which cell neutron flux starts and add some distributions of the cell, if I correctly understand. But I also have energy distribution "erg=d4". When I write distributions si1=-12 11, si2=-10 10, si3=-10 10 for x y z coordinates and sp1=-2 for energy I have some fatal errors.

I just now tried to use a degenerate cartesian volumetric source and immediately checked at the angles of the departure of the particles whether it is isotropic, and it seems that everything worked.