Add attenuation method to Material class

Refs #25957

Framework/Algorithms/src/SampleCorrections/MCInteractionVolume.cpp

@@ -18,21 +18,6 @@ using Kernel::V3D;
 
 namespace Algorithms {
 
-namespace {
-
-/**
- * Compute the attenuation factor for the given coefficients
- * @param rho Number density of the sample in \f$\\A^{-3}\f$
- * @param sigma Cross-section in barns
- * @param length Path length in metres
- * @return The dimensionless attenuated fraction
- */
-double attenuation(double rho, double sigma, double length) {
-  using std::exp;
-  return exp(-100 * rho * sigma * length);
-}
-} // namespace
-
 /**
  * Construct the volume encompassing the sample + any environment kit. The
  * beam profile defines a bounding region for the sampling of the scattering
@@ -119,11 +104,7 @@ double MCInteractionVolume::calculateAbsorption(
     for (const auto &segment : path) {
       const double length = segment.distInsideObject;
       const auto &segObj = *(segment.object);
-      const auto &segMat = segObj.material();
-      factor *= attenuation(segMat.numberDensity(),
-                            segMat.totalScatterXSection(lambda) +
-                                segMat.absorbXSection(lambda),
-                            length);
+      factor *= segObj.material().attenuation(length, lambda);
     }
     return factor;
   };

Framework/Kernel/inc/MantidKernel/Material.h

@@ -103,6 +103,10 @@ public:
   double
   absorbXSection(const double lambda =
                      PhysicalConstants::NeutronAtom::ReferenceLambda) const;
+  /// Compute the attenuation at a given wavelegnth over the given distance
+  double attenuation(const double distance,
+                     const double lambda =
+                         PhysicalConstants::NeutronAtom::ReferenceLambda) const;
 
   /**
    * Returns the linear coefficient of absorption for the material in units of

Framework/Kernel/src/Material.cpp

@@ -243,11 +243,21 @@ double Material::absorbXSection(const double lambda) const {
   }
 }
 
+/**
+ * @param distance Distance (m) travelled
+ * @param lambda Wavelength (Angstroms) to compute the attenuation (default =
+ * reference lambda)
+ * @return The dimensionless attenuation coefficient
+ */
+double Material::attenuation(const double distance, const double lambda) const {
+  return exp(-100 * numberDensity() *
+             (totalScatterXSection() + absorbXSection(lambda)) * distance);
+}
+
 // NOTE: the angstrom^-2 to barns and the angstrom^-1 to cm^-1
 // will cancel for mu to give units: cm^-1
 double Material::linearAbsorpCoef(const double lambda) const {
-  return absorbXSection(NeutronAtom::ReferenceLambda) * 100. * numberDensity() *
-         lambda / NeutronAtom::ReferenceLambda;
+  return absorbXSection(lambda) * 100. * numberDensity();
 }
 
 // This must match the values that come from the scalar version

Framework/Kernel/test/MaterialTest.h

@@ -85,6 +85,8 @@ public:
     TS_ASSERT_DELTA(material.incohScatterXSection(lambda),
                     material.incohScatterXSection(), 1e-04);
     TS_ASSERT_DELTA(material.absorbXSection(lambda), 5.93, 1e-02);
+    const double distance(0.05);
+    TS_ASSERT_DELTA(material.attenuation(distance, lambda), 0.01884, 1e-4);
   }
 
   // highly absorbing material
@@ -124,6 +126,8 @@ public:
 
     const double totXS = TiZr.totalScatterXSection();
     TS_ASSERT_DELTA(TiZr.totalScatterLengthSqrd(), 25. * totXS / M_PI, 1.e-4);
+    const double distance(0.05);
+    TS_ASSERT_DELTA(TiZr.attenuation(distance), 0., 1e-4);
   }
 
   /** Save then re-load from a NXS file */