Replace Rayleigh scattering implementation with that from SNOMAN. The

angular distribution is slightly different, and now fits with the
distribution given in the GEANT4 physics reference manual.

Unit test is now included to verify the correctness of the scattering.

1 files changed, 56 insertions, 0 deletions

diff --git a/tests/rayleigh.py b/tests/rayleigh.py
new file mode 100644
index 0000000..4394ada
--- /dev/null
+++ b/tests/rayleigh.py
@@ -0,0 +1,56 @@
+import unittest
+import numpy as np
+
+from chroma.geometry import Solid, Geometry
+from chroma.make import box
+from chroma.sim import Simulation
+from chroma.optics import water_wcsim
+from chroma.event import Photons
+import histogram
+from histogram.root import rootify
+import ROOT
+ROOT.gROOT.SetBatch(1)
+
+class TestRayleigh(unittest.TestCase):
+    def setUp(self):
+        self.cube = Geometry()
+        self.cube.add_solid(Solid(box(100,100,100), water_wcsim, water_wcsim))
+        self.cube.pmtids = [0]
+        self.sim = Simulation(self.cube, water_wcsim, bvh_bits=4, geant4_processes=0,
+                              use_cache=False)
+        nphotons = 100000
+        positions = np.tile([0,0,0], (nphotons,1)).astype(np.float32)
+        directions = np.tile([0,0,1], (nphotons,1)).astype(np.float32)
+        polarizations = np.zeros_like(positions)
+        phi = np.random.uniform(0, 2*np.pi, nphotons).astype(np.float32)
+        polarizations[:,0] = np.cos(phi)
+        polarizations[:,1] = np.sin(phi)
+        times = np.zeros(nphotons, dtype=np.float32)
+        wavelengths = np.empty(nphotons, np.float32)
+        wavelengths.fill(400.0)
+
+        self.photons = Photons(positions=positions, directions=directions, polarizations=polarizations,
+                               times=times, wavelengths=wavelengths)
+
+    def testAngularDistributionPolarized(self):
+        # Fully polarized photons
+        self.photons.polarizations[:] = [1.0, 0.0, 0.0]
+
+        photon_stop = self.sim.propagate_photons(self.photons, max_steps=1)
+        aborted = (photon_stop.histories & (1 << 31)) > 0
+        self.assertFalse(aborted.any())
+
+        # Compute the dot product between initial and final directions
+        rayleigh_scatters = (photon_stop.histories & (1 << 4)) > 0
+        cos_scatter = (self.photons.directions[rayleigh_scatters] * photon_stop.directions[rayleigh_scatters]).sum(axis=1)
+        theta_scatter = np.arccos(cos_scatter)
+        h = histogram.Histogram(bins=100, range=(0, np.pi))
+        h.fill(theta_scatter)
+        h = rootify(h)
+
+        # The functional form for polarized light should be (1 + \cos^2 \theta)\sin \theta
+        # according to GEANT4 physics reference manual
+        f = ROOT.TF1("pol_func", "[0]*(1+cos(x)**2)*sin(x)", 0, np.pi)
+        h.Fit(f)
+        self.assertGreater(f.GetProb(), 1e-3)
+