diff --git a/beam.mac b/beam.mac
index ba2e1c4f2fbbcb0a1148ceb7c187b3808910b04d..4ad67cae0f99c699b154209bbb86f05bbd7d1d7d 100644
--- a/beam.mac
+++ b/beam.mac
@@ -22,7 +22,7 @@
 /gps/pos/type Plane
 /gps/pos/shape Rectangle
 /gps/pos/halfx 1.0 mm
-/gps/pos/halfy 0.75 mm
+/gps/pos/halfy 0.5 mm
 
 # the incident surface is in the x-z plane just below the gas
 /gps/pos/rot1 1 0 0