PENELOPE 2003 - OECD Nuclear Energy Agency

5.7. A short tutorial 175
Writing a geometry file is nothing more than routine work. After a little practice,
you can define quite complex systems by using only surfaces and bodies. You will soon
realize that the visualization programs (as well as the actual simulations!) slow down
when the number of elements in the geometry increases. The only way of speeding up
the programs is to group the bodies into modules. The best strategy for improving the
calculation speed is to build relatively simple modules and combine them into larger
parent modules to obtain a genealogical tree where the number of daughters of each
module is not too large (say 4 or 5).
You may save a lot of time by defining each body separately (and checking it carefully)
and then inserting it into the progressing module that, once finished, will be
added to the file. Notice that the input element labels are arbitrary (as long as they
are not repeated for elements of the same kind) and that we can insert new elements
anywhere in the file. Once the geometry definition is complete, we can generate an
equivalent file, with elements labelled according to their input order, by simply editing
the GEOMETRY.REP file.
The previous examples of geometry files (QUADRIC and ARROW) together with several
other files of more complex geometries are included in the distribution package. They
can be directly visualized by running gview2d and gview3d. The file GLASS (a glass
of champagne) shows that common objects can be described quite precisely with only
quadric surfaces; in this case, we do not use modules, which are useful only to accelerate
the calculations. WELL defines a scintillation well detector with much detail; we have set
an enclosure for the system, so that you can rotate the entire detector by editing the
definition file. Notice that, when the detector is tilted, it is very difficult to get an idea
of its geometry from the images generated by gview2d. SATURNE describes the head of
an electron accelerator, quite a complicated geometry with 96 surfaces and 44 bodies.
The structure MALE, which corresponds to a mathematical anthropomorphic phantom,
consists of 174 surfaces and 108 bodies, grouped into 11 modules.
We cannot finish without a word of caution about the use of pengeom, and other
general-purpose geometry packages. For simple geometries, they tend to waste a lot of
time. It is always advisable to consider the possibility of handling geometric aspects
directly; this may enable substantial reduction of the number of operations by taking
full advantage of the peculiarities of the material system.