now enable electrons and positrons to pass through empty domains

Author: GuySten

src/particle.cpp

@@ -220,7 +220,12 @@ void Particle::event_advance()
 
   // Sample a distance to collision
   if (type() == ParticleType::electron || type() == ParticleType::positron) {
-    collision_distance() = 0.0;
+    collision_distance() = INFINITY;
+    if (material() != MATERIAL_VOID) {
+      double density = model::materials[material()]->density();
+      if (density > 0.0)
+        collision_distance() = 0.0;
+    }
   } else if (macro_xs().total == 0.0) {
     collision_distance() = INFINITY;
   } else {