-------------- 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.
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