diff --git a/compact/tof_endcap.xml b/compact/tof_endcap.xml
index bff944698d7ec0e0f281e3a12c2ea608da40410f..65159e41c876429e9334d5f802d426f1977eb142 100644
--- a/compact/tof_endcap.xml
+++ b/compact/tof_endcap.xml
@@ -7,6 +7,7 @@
       --------------------------
     </comment>
     
+    <constant name="TOFSensor_thickness"         value="0.03*mm"/>
     <constant name="TOFEndcap_thickness"        value="0.25/100*9.37*cm"/>
     <constant name="TOFEndcapService_thickness" value="TOFEndcap_thickness-TOFSensor_thickness"/>