diff --git a/CODE/stepping.cc b/CODE/stepping.cc
index 6b98846ad29d3584df11c6ad92c0a96b5fe84c20..8353db3416be5d5066e8d8ec9a5b7e7cf002528d 100644
--- a/CODE/stepping.cc
+++ b/CODE/stepping.cc
@@ -17,8 +17,8 @@ void MySteppingAction :: UserSteppingAction(const G4Step* step)
     {
         
         G4ThreeVector position = step->GetPreStepPoint()->GetPosition();
-
-        G4double distanceThreshold = -400.*m;  // Punto de deteccion, debe coincidir con la interfaz de las capas de la atmosfera (-1600 (sea level) ,-1200,-800,-400,0,400,800,1200,1600,2000)
+//////// He cambiado el signo 
+        G4double distanceThreshold = 400.*m;  // Punto de deteccion, debe coincidir con la interfaz de las capas de la atmosfera (-1600 (sea level) ,-1200,-800,-400,0,400,800,1200,1600,2000)
 
         // Verifica si la distancia recorrida supera el umbral
         if (position.z() == distanceThreshold)