Write an egsinp file

Once you have a mesh file, you're ready to write the input file. The three required input file sections are:

1. Geometry definition
2. Source definition
3. Run control

First, define the mesh geometry. An EGS_Mesh geometry block looks like:

:start geometry definition:
    :start geometry:
        library = egs_mesh
        name = my_mesh
        file = cube.msh # your mesh here
    :stop geometry:

    # define a 15x15x15cm vacuum envelope around the mesh
    :start geometry:
        library = egs_box
        name = my_box
        box size = 15 15 15
        # translate it from the origin to the cube's centre point (5, 5, 5)
        :start transformation:
            translation = 5 5 5
        :stop transformation:
        :start media input:
            media = vacuum
        :stop media input:
    :stop geometry:

    :start geometry:
        library = egs_genvelope
        name = my_envelope
        base geometry = my_box
        inscribed geometries = my_mesh
    :stop geometry:

    simulation geometry = my_envelope
:stop geometry definition:

For the mesh, the only input you need to adjust is the file. If you're following along, this should be named cube.msh. We also defined a vacuum envelope around the mesh, so that particles exiting the mesh are only discarded when they exit the vacuum too. This probably won't cause a noticable difference for this convex mesh, but defining an envelope is crucial for correctly simulating meshes in general. For your own simulations, make sure to adjust the envelope's size and position to bound the mesh.

Next, the source. Let's define an exterior point source. The meshed box starts at the origin and extends 10cm along x, y and z. So placing the source at (5, 5, -1) means the source will be 1cm away from the box's -z face. For this example, you'll simulate a 10MeV photon monoenergetic point source.

:start source definition:
    :start source:
        library     = egs_point_source
        name        = my_source
        position    = 5 5 -1
        :start spectrum:
            type    = monoenergetic
            energy  = 10
        :stop spectrum:
        charge      = 0
    :stop source:
    simulation source = my_source
:stop source definition:

And finally, the run control section. Let's run 1 million histories.

:start run control:
    ncase = 1e6
:stop run control:

The input file is done, and you're ready to run the simulation.

:start geometry definition:
    :start geometry:
        library = egs_mesh
        name = my_mesh
        file = cube.msh # your mesh here
    :stop geometry:

    # define a 15x15x15cm vacuum envelope around the mesh
    :start geometry:
        library = egs_box
        name = my_box
        box size = 15 15 15
        # translate it from the origin to the cube's centre point (5, 5, 5)
        :start transformation:
            translation = 5 5 5
        :stop transformation:
        :start media input:
            media = vacuum
        :stop media input:
    :stop geometry:

    :start geometry:
        library = egs_genvelope
        name = my_envelope
        base geometry = my_box
        inscribed geometries = my_mesh
    :stop geometry:

    simulation geometry = my_envelope
:stop geometry definition:

:start source definition:
    :start source:
        library     = egs_point_source
        name        = my_source
        position    = 5 5 -1
        :start spectrum:
            type    = monoenergetic
            energy  = 10
        :stop spectrum:
        charge      = 0
    :stop source:
    simulation source = my_source
:stop source definition:

:start run control:
    ncase = 1e6
:stop run control: