8.1} Introduction

 The Combinatorial Geometry (CG) used by FLUKA is a modification of the
 package developed at ORNL for the neutron and gamma-ray transport
 program MORSE [Emm75] which was based on the original combinatorial
 geometry by MAGI (Mathematical Applications Group, Inc.) [Gub67,Lic79].

 The default input format is fixed, and different from that adopted elsewhere
 in the FLUKA code. The input sequence must be completely contained between a
 GEOBEGIN and a GEOEND card (see the corresponding description in 7}).

 Two concepts are fundamental in CG: bodies and regions.
 Originally, MORSE bodies were defined as convex solid bodies (finite
 portions of space completely delimited by surfaces of first or second
 degree, i.e. planes or quadrics). In FLUKA, the definition has been
 extended to include infinite cylinders (circular and elliptical) and
 planes (half-spaces). Use of such "infinite bodies" is encouraged since
 it makes input preparation and modification much easier and less
 error-prone. They also provide a more accurate and faster tracking.

 Regions are defined as combinations of bodies obtained by boolean
 operations: Union, Subtraction and Intersection. Each region is not
 necessarily simply connected (it can be made of two or more non
 contiguous parts), but must be of homogeneous material composition.
 Because the ray tracing routines cannot track across the outermost
 boundary, all the regions must be contained within a surrounding
 "blackhole" (an infinitely absorbing material, in MORSE jargon
 "external void", designated by the FLUKA material number 1), so that all
 escaping particles are absorbed. It is suggested to make the external
 blackhole region rather big, so as not to interfere with possible
 future modifications of the problem layout. The external blackhole
 must be completely surrounded by the boundary of a closed body, and
 therefore cannot be defined by means of half-spaces or infinite
 cylinders only. Inside such outermost boundary, EACH POINT OF SPACE MUST
 BELONG TO ONE AND ONLY ONE REGION.
 Note that in MORSE the concept of "region" refers to a portion of space
 of homogeneous statistical importance or weight setting, which may
 extend over one or several "zones" (homogeneous in material
 composition). Since the two MORSE concepts of region and zone coincide in
 FLUKA (there is a one-to-one correspondence), the term "region" will be
 used here to define "a portion of space of uniform material composition,
 obtained by boolean operations on one or more subregions", while "zone"
 will indicate one of such subregions, obtained by boolean operations on
 one or more geometrical bodies.

 Repetition of sets of regions according to symmetry transformations is
 possible in FLUKA through the card LATTICE and through a user-written
 routine. This allows, for instance, to model in detail only a single cell
 of a calorimeter and to replicate it in the entire volume.

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