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Added description of surface crossings.

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Paul Romano 2012-07-27 13:07:16 -04:00
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docs/source/methods/geometry.rst
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@ -260,6 +260,8 @@ will then be either both positive or both negative. If they are both positive,
the smaller (closer) one will be the solution with a negative sign on the square
root of the discriminant.
.. _find-cell:
----------------------------
Finding a Cell Given a Point
----------------------------
@ -331,10 +333,32 @@ are satisfied, than the point is indeed inside the cell.
Handling Surface Crossings
--------------------------
A particle will cross a surface if the distance to the nearest surface is closer
than the distance sampled to the next collision. A number of things happen when
a particle hits a surface. First, we need to check if a non-transmissive
boundary condition has been applied to the surface. If a vacuum boundary
condition has been applied, the particle is killed and any surface current
tallies are scored to as needed. If a reflective boundary condition has been
applied to the surface, surface current tallies are scored to and then the
particle's direction is changed according to the procedure in :ref:`reflection`.
Next, we need to determine what cell is beyond the surface in the direction of
travel of the particle so that we can evaluate cross sections based on its
material properties. At initialization, a list of neighboring cells is created
for each surface in the problem as described in :ref:`neighbor-lists`. The
algorithm outlined in :ref:`find-cell` is used to find a cell containing the
particle except rather than searching all cells in the base universe, only the
list of neighboring cells is searched. If this search is unsuccessful, then a
search is done over every cell in the base universe.
.. _neighbor-lists:
-----------------------
Building Neighbor Lists
-----------------------
.. _reflection:
-------------------
Reflective Surfaces
-------------------