void hexaround() {
   TGeoManager * geom = new TGeoManager("hexaround", "");
   TGeoMaterial *mat = new TGeoMaterial("Al", 26.98,13,2.7);
   TGeoMedium *med = new TGeoMedium("MED",1,mat);
   TGeoVolume *top = gGeoManager->MakeBox("TOP",med,150,150,150);   
   gGeoManager->SetTopVolume(top);
   TGeoPgon *pgon = new TGeoPgon("pgon", 0.,360.0,6,2);
   pgon->DefineSection(0,-70,0,100);
   pgon->DefineSection(1,70,0,50);
   TGeoPcon *pcon = new TGeoPcon("pcon", 0,360, 11);
   // we need a 2'nd degree function of the pcon radius with Z satisfying:
   // r(z) = a*z*z + b*z +c
   // r(0) = Rpgon_out(0) = 75./TMath::Cos(TMath::Pi()/6.)
   // r(70) = Rpgon_in(70) = 50.;
   // dr/dz(z=0) = -25./70.
   
   Double_t c = 75./TMath::Cos(TMath::Pi()/6.);
   Double_t b = -25./70.;
   Double_t a = (50.-c-b*70.)/(70.*70.);
   for (Int_t iz=0; iz<=10.; ++iz) {
      Double_t z = 7*iz;
      // Make last section of pcon slightly further in Z to avoid subtraction
      // artefact in visualization
      if (iz==10) z += 0.001;
      pcon->DefineSection(iz, z, a*z*z+b*z+c, 100.);
   }    
   TGeoCompositeShape *hexa = new TGeoCompositeShape("hexa", "pgon-pcon");
   TGeoVolume *vol = new TGeoVolume("hexaround",hexa, med);
   vol->SetLineColor(kRed);
   top->AddNode(vol,1);
   //top->Draw("ogl");
   // Or you can replace the line above with the one below for raytracing
   top->Raytrace();
}