prismatic mesh between two spheres

Hello everyone,
I'm new in NetGen/ngSolve, I would like to create prismatic (anisotropic) mesh between 2 spheres and import the mesh in Abaqus, for this i used "closesurfaces" but it dosn't work, this is the *.geo file:
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# RVE long fiber
algebraic3d

solid back   = plane (0,0,0;0,0,-1);
solid front  = plane (0,0,1;0,0, 1);
solid back_half   = plane (0,0,0.5;0,0,-1);
solid front_half  = plane (0,0,0.5;0,0, 1);

solid left   = plane (0,0,0;-1,0,0);
solid right  = plane (1,0,0; 1,0,0);
solid left_half   = plane (0.5,0,0;-1,0,0);
solid right_half  = plane (0.5,0,0; 1,0,0);

solid bot    = plane (0,0,0;0,-1,0);
solid top    = plane (0,1,0;0, 1,0);
solid bot_half    = plane (0,0.5,0;0,-1,0);
solid top_half    = plane (0,0.5,0;0, 1,0);

solid HalfFront   = top and bot and left and right and front and back and front_half and back_half;
solid HalfLeft    = top and bot and left and right and front and back and left_half and right_half;
solid HalfTop     = top and bot and left and right and front and back and top_half and bot_half;

solid sphCenter = sphere (0.5,0.5,0.5;0.25) ;
solid sphCenter_2 = sphere (0.5,0.5,0.5;0.255) ;

identify closesurfaces sphCenter_2 sphCenter;

solid Interf = sphCenter_2 and not sphCenter and not HalfFront and not HalfLeft and not HalfTop ;

tlo Interf;
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I get tetrahedral elements (C3D4) instead of wedge elements (C3D6) !!!
Is anyone know why ??
thank you