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2.2.6} The geometry

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 The input for the Combinatorial Geometry (bodies, regions and optional region
 volumes) must be immediately preceded by a GEOBEGIN card and immediately
 followed by a GEOEND card. These two cards follow the normal FLUKA format. It
 is recalled that the format for the geometry has its own special rules,
 described in Chap. 8}.

 Comment lines in the geometry input have an asterisk in first position as in
 the rest of FLUKA input (but on-line comments are not allowed). Body
 numerical data can be written in two different formats, a "short" one (field
 length 10) and a "long" one (field length 22). The latter one is to be
 preferred when higher precision is needed, for instance when using bodies
 such as truncated cones, cylinders or planes not aligned with axes. It must
 be realised that using too few decimals can cause geometry errors when bodies
 are combined into regions (portions of space not defined or doubly defined).

 The whole geometry must be surrounded by a region of "blackhole" limited by a
 closed body (generally an RPP parallelepiped). It is often a good idea to
 make this body much larger than the minimum required size: this makes easier
 to introduce possible future extensions. In some cases, as in our basic
 example, it is also useful to surround the actual geometry by a region of
 ideal vacuum, and to have the blackhole region surrounding the vacuum. This
 can be useful, for instance, in order to start the trajectory of the primary
 particles outside the physical geometry (a particle may not be started on a
 boundary).

 Both the body input section and the region input section must be ended with
 an END card. Optionally, region volumes can be input between the region END
 card and the GEOEND card (this option can be requested by setting a special
 flag in the Geometry title card, see Chap. 8}). The only effect of specifying
 region volumes is to normalise per cm3 the quantities calculated via the
 SCORE option (see below): for other estimators requiring volume normalisation
 the volume is input as part of the detector definition (USRTRACK, USRCOLL,
 USRYIELD), or is calculated directly by the program (USRBIN).  The GEOEND
 card indicates the end of the geometry description, but can also be used to
 invoke the geometry debugger.

 The geometry output is an expanded echo of the corresponding input,
 containing information also on memory allocation and on the structure of
 composite regions made of several sub-regions by means of the OR operator.

 A possible realisation of the geometry set up for our basic example can be
 seen in the Figure below:

     z ^      -----------------------------------
       |     /      ______________________     /|
       |    /      /                      /   / |
       |   -----------------------------------  |
       |   |     /   vacuum (region 2)  / |  |  |
       |   |    ------------------------  |  |  |
       |   |    |       ___________    |  |  |  |
       |   |    |      /   Be     /|   |  |  |  |
       |   |    |     ------------ |   |  |  |  |
       |   |    |     | (reg. 4) |/|   |  |  |  |
       |   |    |     -----------  |   |  |  |  |
       |   |    |     | (reg. 3) |/    |  |  |  |
       +-- |    |     ------------     |  |  |  |-------->
      /    |    |           ^          | /   |  |       y
     /     |    |           | Beam     |/    |  |
    /      |    ------------------------     | /
   /       |            Blackhole (region 1) |/
  /        -----------------------------------
  x


 Only four bodies are used here: an RPP body (Rectangular Parallelepiped, body
 no. 3) to define a volume which will be the Be target region, inside another
 larger RPP body (no. 2), which will be filled with ideal vacuum and in turn
 is contained inside another larger RPP body (no. 1), to define the blackhole
 region. The fourth body, an XYP half-space (defined by a plane perpendicular
 to the z axis), will be used to divide the target into 2 different regions:
 the upstream half will be defined as the portion of body 3 contained inside
 the half-space, and the downstream half as the portion outside it. Therefore,
 region "3" (the upstream half of the target) is the part of body no. 3 which
 is also inside body 4, while region "4" (downstream half of the target) is
 the part of body no. 3 which is not inside body 4. Region "2" (the vacuum
 around the target) is defined as the inside of body no. 2 from which body
 no. 3 is subtracted. Region "1" is simply the interior of body no. 1 from
 which body no. 2 is subtracted.

 Note that bodies and regions can be identified by numbers, as described
 above, or with names (alphanumeric strings). The latter option is
 recommended, since it makes the preparation of the geometry much easier,
 especially if free format is also chosen. Here below we will show both
 possibilities.

 The beam starting point has been chosen so that it is in the vacuum, outside
 the target region.  The geometry part of the input file can then be written
 as:

*...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8
GEOBEGIN COMBINAT 0 0 A simple Be target inside vacuum RPP 1-5000000.0+5000000.0-5000000.0+5000000.0-5000000.0+5000000.0 RPP 2-1000000.0+1000000.0-1000000.0+1000000.0 -100.0+1000000.0 RPP 3 -10.0 +10.0 -10.0 +10.0 0.0 +5.0
* plane to separate the upstream and downstream part of the target
XYP 4 2.5 END
* black hole
BH1 5 +1 -2
* vacuum around
VA2 5 +2 -3
* Be target 1st half
BE3 5 +3 +4
* Be target 2nd half
BE4 5 +3 -4 END GEOEND The same geometry can be described in name-based free format as follows:
*...+....1....+....2....+....3....+....4....+....5....+....6....+....7....+....8
GEOBEGIN COMBNAME 0 0 A simple Be target inside vacuum RPP blakhole -5000000.0 +5000000.0 -5000000.0 +5000000.0 -5000000.0 +5000000.0 RPP vacumbox -1000000.0 +1000000.0 -1000000.0 +1000000.0 -100.0 +1000000.0 RPP betarget -10.0 +10.0 -10.0 +10.0 0.0 +5.0
* plane to separate the upstream and downstream part of the target
XYP cutplane 2.5 END
* black hole
Blckhole 5 +blakhole -vacumbox
* vacuum around
Vacarund 5 +vacumbox -betarget
* Be target 1st half
UpstrBe 5 +betarget +cutplane
* Be target 2nd half
DwnstrBe 5 +betarget -cutplane END GEOEND

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