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7.4} Materials

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 Materials in FLUKA are identified by a name (an 8-character string) and by a
 number, or material index. Both are used to create correspondences, for
 instance between region number and material number, or between material name
 and neutron cross section name.

 Some materials are already pre-defined. A Table in 5} lists the 25 available
 pre-defined materials with their default name, index number, density, atomic
 number and atomic weight. The user can either refer to any one of them as it is,
 or override it with a new number, name and other properties, or define a new
 material. In the latter two cases, the new material definition is done by
 option MATERIAL. If the material is not a single element or isotope, but a
 compound, mixture or alloy, a command COMPOUND, extended on as many cards as
 necessary, is needed to specify its atomic composition. The correspondence
 between the material and the composition is set using the same name in the
 MATERIAL and in the COMPOUND cards. Note that material names, if low-energy
 neutron transport is desired, cannot be assigned arbitrarily but must match one
 of the names available in the FLUKA cross section library (see Table in
 Chap. 10}.

 Once all the materials to be assigned to the various geometry regions have been
 defined (either explicitly with MATERIAL or implicitly in the pre-defined
 list), it is necessary to specify of which material each region is made, by
 setting a correspondence material index --> region number. This is done by
 command ASSIGNMAt.

 Command ASSIGNMAt is used also to indicate that a magnetic field exists inside
 one or more given regions: in this case a command MGNFIELD is needed to specify
 intensity and direction of a constant magnetic field, or a complex one defined
 by a user routine as explained below. Note that in practice at least one
 ASSIGNMAt command must always be present.

 A less common kind of correspondence is set by option LOW-MAT. By default, the
 correspondence between a material and a low-energy neutron cross section set
 established by name, but in some circumstances this cannot be done, for
 instance when two different materials share the same cross section set, or when
 two cross section sets have the same name. Option LOW-MAT can be used to set a
 different correspondence.

 Another FLUKA option concerning the definition of materials is MAT-PROP. It is
 used for a variety of purposes: to describe porous, inhomogeneous or gas
 materials, to override the default average ionisation potential, to set a
 threshold energy for DPA calculations and to request a call to a special user
 routine when particles are transported in a given material.
* Start_Devel_seq
 MAT-PROP is also used to do a rough temperature re-scaling of thermal neutron
 cross sections when no cross section set is available at a desired temperature
 in case of the old 72-groups neutron cross section library.
* End_Devel_seq

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