GEOBEGIN

Starts the geometry description

See also GEOEND, PLOTGEOM

     WHAT(1)        : not used

     WHAT(2)  > 0.0 : absolute accuracy parameter (Aa) in units of 1.E-6 cm.
                      It is used for tracking and boundary identification.
                      Aa should be larger than Ar*L , where L is the largest
                      coordinate value in the geometry description (excluding
                      the outer blackhole shell containing it) and Ar is the
                      relative accuracy achievable in double precision (of the
                      order of 1.E-14 - 1.E-15).
                      Default = 0.0001

     WHAT(3)  > 0.0 : logical unit for geometry input. The name of the
                      corresponding file must be input on the next card
                      if WHAT(3) is different from 5. Note that values of
                      WHAT(3) /= 5.0 and < 21.0 must be avoided
                      because of possible conflicts with FLUKA pre-defined
                      units.
                      Default = 5.0 (i.e. geometry input follows)

     WHAT(4)  > 0.0 : logical unit for geometry output. If different from
                      11, the name of the corresponding file must be input
                      on the next card if WHAT(3) = 0 or 5, otherwise on
                      the card following the next one. Values of
                      WHAT(3) /= 11.0 and < 21.0 must be avoided because
                      of possible conflicts with FLUKA pre-defined units.
                      Default = 11.0  (i.e. geometry output is printed on
                      the standard output)

     WHAT(5)        : parentheses optimisation level = i0 + i1 x 1000
                      i0 = 1: logical optimisation only activated
                           2: logical + "plane" optimisation activated
                           3: logical + "plane" + "bounding box"
                              optimisation activated
                             (Default: 3)
                      i1 > 0 forces optimisation even if no parentheses
                             are found in the region under scrutiny
                             (Default: 0)

*    WHAT(6)        : not used

     SDUM = COMBINAT  : Combinatorial geometry is used. See section
                        on Combinatorial Geometry for input description
                        Other geometries, available in older versions of
                        FLUKA, are no longer supported.
                        Default: COMBINAT
     SDUM = COMBNAME  : Combinatorial geometry is used in free format, and
                        names can be used instead of body and region numbers
     SDUM = FLUGG     : The FluGG interface to the Geant4 geometry is used.
                        In this case, the GEOEND card must immediately follow
                        the GEOBEGIN card. The FluGG interface is available
                        and described on the fluka web site
            Default   : COMBINAT

     Default (option GEOBEGIN not given): not allowed! GEOBEGIN and
           GEOEND must always be present.

Notes:

1) Geometry input and output:

             i) If WHAT(3) and WHAT(4) are both =< 0, the geometry input and
                output are part of the standard I/O streams: the GEOBEGIN
                card must be immediately followed by the Combinatorial
                Geometry input, and then by GEOEND. (See Example 1 below).
            ii) If WHAT(3) is > 0 (and not = 5) and WHAT(4) is =< 0, GEOBEGIN
                must be followed by one card with the name of the CG input
                file, and then by GEOEND. (See Example 2 below).
           iii) If WHAT(4) is > 0 (and not = 11) and WHAT(3) is =< 0, GEOBEGIN
                must be followed by one card with the name of the CG output
                file, then by the CG input, and then by GEOEND. (See Example 3 below).
            iv) Otherwise, if WHAT(3) is > 0 (and not = 5), and WHAT(4) is > 0
                (and not = 11), GEOBEGIN must be followed, in the order, by
                a line with the name of the CG input file, another line with
                the name of the CG output file and the GEOEND card.
                (See Example 4 below).

The first part of the CG output (9}) is essentially an echo of the input, and gives in addition some information on storage allocation of body and region data. Then follow an "Interpreted body echo" and an "Interpreted region echo", useful in the case where free geometry input format has been requested. It is recommended to check on this output that the geometry description has been correctly reproduced, especially if fixed input format has been chosen, since column alignment errors are critical due to the CG strict input format. In case of severe error ("next region not found", or similar) the CG output will also contain all error and debugging messages. Minor tracking problems, however, are reported on the error message file (logical unit 15, see Chap. (3) and Sec. 9.4}, unless reporting has been de-activated by setting WHAT(1) <= 0.0.

2) Special algorithms have been implemented in the Combinatorial Geometry package of FLUKA in order to minimise tracking errors due to rounding, to improve tracking speed and to handle charged particle transport near boundaries and in magnetic fields.

3) A voxel geometry is also available in FLUKA 8.2.11}. It is fully integrated into the FLUKA CG and it is even possible to describe a geometry partly as made of voxels and partly of conventional CG regions.

4) Information on how to set up Combinatorial Geometry input is given in Chap. (8)

5) End of geometry information must end with a GEOEND card. The same card can also be used to activate a geometry debugger.

6) Option PLOTGEOM allows to draw sections of the geometry on planes specified by the user.

7) Only one GEOBEGIN card is allowed.

8) The WHAT(2) parameter might need to be increased or decreased in case of very large or very small geometries respectively (i.e. L> 100 m or L< 10 cm).

Example 1:

* CG Input follows, output is printed as part om Main Output
*...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8
GEOBEGIN          0.        0.        0.        0.        0.        0.COMBINAT

Example 2:

* CG Input read from file BigHall.geo, output printed as part om Main Output
*...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8
GEOBEGIN          0.        0.       25.        0.        0.        0.COMBINAT
BigHall.geo

Example 3:

* CG Input follows, output is printed on file geo2.out
*...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8
GEOBEGIN          0.        0.        0.       26.        0.        0.COMBINAT
geo2.out

Example 4:

* CG Input read from file BigHall.geo, output printed on file geo2.out
*...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8
GEOBEGIN          0.        0.       25.       26.        0.        0.COMBINAT
BigHall.geo
geo2.out

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