When and why do I need to input a MAT-PROP card?
The density which is provided by the user via the WHAT(3) of a MATERIAL card is used for transport, i.e. it is used to convert cross sections to mean free paths. Therefore, it must be the actual density in case of a gas at non-atmospheric pressure, or the average density in case of an inhomogeneous material. However, it is not sufficient because:
- if the material is a gas, the Sternheimer-Peierls formula must be evaluated first at Normal Temperature and Pressure (NTP) and then corrected for actual TP. The density at NTP used in the evaluation of the formula is derived as MATERIAL-WHAT(3)/MAT-PROP-WHAT(1)
- the actual material density can be inhomogeneous (for instance a material with voids (like a sponge or a powder). The transport (mean free path) is evaluated based on the average density MATERIAL-WHAT(3), while dE/dx must be evaluated based on local density MATERIAL-WHAT(3)/MAT-PROP-WHAT(2).