diff -u -r1.5 -r1.6
--- TPC.java	2 Jun 2008 20:37:00 -0000	1.5
+++ TPC.java	20 Jul 2011 21:53:57 -0000	1.6
@@ -41,6 +41,17 @@

         double rmin = dimensions.getAttribute("inner_r").getDoubleValue();
         double r = rmin;
         

+        // Displacement of TPC along z axis.  This can be positive or negative.
+        double zpos = 0;
+        Element position = node.getChild("position");
+        if (position != null)
+        {
+            if (position.getAttribute("z") != null)
+            {
+                zpos = position.getAttribute("z").getDoubleValue();
+            }
+        }
+        

         Tube envelope = new Tube(detectorName+"_envelope");
         Volume envelopeVolume = new Volume(detectorName+"_envelope_volume");
         envelopeVolume.setMaterial(air);

@@ -112,6 +123,7 @@

         envelope.setRMax(r);
         envelope.setDeltaPhi(Math.PI*2);
         PhysVol physvol = new PhysVol(envelopeVolume);

+        physvol.setZ(zpos);

         physvol.addPhysVolID("system",id);
         physvol.addPhysVolID("barrel",0);
         motherVolume.addPhysVol(physvol);

diff -u -r1.1 -r1.2
--- TPCTest.xml	12 Sep 2007 18:24:32 -0000	1.1
+++ TPCTest.xml	20 Jul 2011 21:53:58 -0000	1.2
@@ -26,6 +26,7 @@

   <detectors>
     <detector id="1" name="TPCTest" type="TPC" readout="TPCHits">
          <dimensions inner_r = "100.0*cm" outer_z = "150.0*cm" />

+         <position z="10.0" /> <!-- Just a simple test of z displacement. -->

          <layer repeat="10">
            <slice material="Nitrogen" thickness="1.0*cm" sensitive="yes" />
          </layer>