ROOT geometry of the CBM magnet

Based on the CAD design of BINP Novosibirsk, November 2018.
MC model by Egor Ovcharenko (evovch@gmail.com).

Version v18b is the same as v18a, but without field clamps. It is meant for being used with the MUCH system (muon setup).

A GDML geometry was created from the original CATIA design files. The GDML geometry was converted into ROOT TGeo format by A. Senger, using the macro macro/geometry/gdml2root.C.

The most substantial difference to the previous geometry (v15b_much) is the increase in the vertical aperture to 1,440 mm.