Intersection of 180 degree revolution of line and a plane giving a circle

I have created a surface by revolving a line up to 180 degrees and saved it as STEP file.
Reading this step file and intersecting the surface with a plane gives a full circle.
I have expected a semi circle but the result was a full circle.
Am I doing anything wrong.

Here is the code I am using

TopoDS_Face iFace = Part.Face[i].FaceData;
Handle(Geom_Surface) gSurface = BRep_Tool::Surface(iFace);
Handle(Geom_Surface) aPlane = GC_MakePlane(0.0,0.0,1.0,-0.1);
GeomAPI_IntSS Intersector(gSurface1, aPlane, 0.0001);

Standard_Integer nb = Intersector. NbLines();
if(nb)
{
Handle(Geom_Curve) C = Intersector.Line(nb);
TopoDS_Edge SliceEdge = BRepBuilderAPI_MakeEdge(C);
Handle(AIS_Shape) AISEdge = new AIS_Shape(SliceEdge);
myAISContext->Display(AISEdge);
}

I have tried "BRepAlgoAPI_Section " also got same result.
Please help me. Thank you.