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In Sabrina, geometries may be described by combinatorial (or constructive) solid geometry (CSG), also known as body geometry, or by boundary representation (BR), or surface geometry. In the combinatorial body geometry modeler, which is most often used when constructing models ab initio, models are defined in terms of finite, closed bodies. For users familiar with MCNP surface geometry, it may be useful to think of the body geometry in Sabrina as simple geometric macros, where a body is a predefined combination of geometric surfaces. For example, a right circular cylinder (RCC) is equivalent to two planes and a cylindrical surface.
Geometries may also be described in terms of surfaces, a la MCNP. This mode uses the syntax of MCNP, and MCNP input files may be read directly. Sabrina understands MCNP repeated structures, including lattices. The two geometry modes may not be mixed (except for the use of a cutter body in surface mode). Sabrina can write out a surface description of a model originally defined in body mode; this surface description is formatted for reading by MCNP. Usually surface mode is only used for problems that have already been set up for MCNP.
Boolean operations are used in either geometry mode to define a region of homogeneous material. These regions are referred to as zones in the body mode, and as cells in the surface mode. Note that we often use these terms interchangeably.