From 42f0ac75d4876c08c374216d2fcbb8e9aa956a95 Mon Sep 17 00:00:00 2001
From: Paul Romano <paul.k.romano@gmail.com>
Date: Tue, 24 Jul 2012 17:42:27 -0400
Subject: [PATCH] Changed calculation of scattering angle in elastic scattering
 to use dot product.

---
 src/physics.F90 | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/src/physics.F90 b/src/physics.F90
index efcb692988..dc2f9b815f 100644
--- a/src/physics.F90
+++ b/src/physics.F90
@@ -562,8 +562,9 @@ contains
     E = dot_product(v_n, v_n)
     vel = sqrt(E)
 
-    ! convert cosine of scattering angle from CM to LAB
-    mu = (ONE + awr*mu)/sqrt(awr**2 + TWO*awr*mu + ONE)
+    ! compute cosine of scattering angle in LAB frame by taking dot product of
+    ! neutron's pre- and post-collision angle
+    mu = dot_product(p % coord0 % uvw, v_n) / vel
 
     ! Set energy and direction of particle in LAB frame
     p % E = E