Use passive iron material for the mCBM pipe and add 3 vacuum foils in the beam

media.geo

@@ -38,7 +38,10 @@ lead               1  207.1  82.  11.34
                    0  1  20.  .001
                    0
 // ---------------------------------------------------------------------------
-
+pipeiron           1  55.847  26.  7.87
+                   0  1  20.  .001
+                   0
+// ---------------------------------------------------------------------------
 
 // -----  Air at 20 degrees C   ----------------------------------------------
 air                3  14.01  16.  39.95  7.  8.  18.  1.205e-3  .78  .21  .01

pipe/pipe_v20a_mcbm.geo.info

+SIS-18 mCBM beam pipe geometry created with create_bpipe_geometry_v20a.C
+Introducing the target chamber derived from CAD drawings.
+
+The beam pipe is composed of iron with a varying wall thickness.
+
+Material:  pipeiron