"Angular Cross-section" is the cross-section for ejecting or scattering a particle into an elementary cone, divided by the solid angle $d\Omega$ of that cone.
Applicable units are those of quantitykind:AngularCrossSection
http://en.wikipedia.org/wiki/Cross_section_(physics)
https://cdd.iec.ch/cdd/iec61987/iec61987.nsf/ListsOfUnitsAllVersions/0112-2---62720%23UAD007
$\sigma = \int \sigma_\Omega d\Omega$
http://www.iso.org/iso/catalogue_detail?csnumber=31895