FLUKA: 2.3.6} The geometry
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2.3.6} The geometry
---------------------
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:
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
XYP 4 2.5
END
BH1 5 +1 -2
VA2 5 +2 -3
BE3 5 +3 +4
BE4 5 +3 -4
END
GEOEND
The same geometry can be described in name-based free format as follows:
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
XYP cutplane 2.5
END
Blckhole 5 +blakhole -vacumbox
Vacarund 5 +vacumbox -betarget
UpstrBe 5 +betarget +cutplane
DwnstrBe 5 +betarget -cutplane
END
GEOEND
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