"Cross-section" is used to express the likelihood of interaction between particles. For a specified target particle and for a specified reaction or process produced by incident charged or uncharged particles of specified type and energy, it is the mean number of such reactions or processes divided by the incident-particle fluence.
Applicable units are those of quantitykind:Area
http://en.wikipedia.org/wiki/Cross_section_(physics)
https://cdd.iec.ch/cdd/iec61987/iec61987.nsf/ListsOfUnitsAllVersions/0112-2---62720%23UAD026
"Cross-section" is used to express the likelihood of interaction between particles. For a specified target particle and for a specified reaction or process produced by incident charged or uncharged particles of specified type and energy, it is the mean number of such reactions or processes divided by the incident-particle fluence.
http://www.iso.org/iso/catalogue_detail?csnumber=31895