updated input.rst w/ resonance scattering info

docs/source/usersguide/input.rst

@@ -237,6 +237,62 @@ or sub-elements and can be set to either "false" or "true".
 
   *Default*: true
 
+``<resonance_scattering>`` Element
+----------------------
+
+The ``resonance_scattering`` element can contain one or more of the following
+attributes or sub-elements:
+
+  :scatterer:
+    An element with attributes/sub-elements called ``nuclide``, ``method``,
+    ``xs_label``, ``xs_label_0K``, ``E_min``, and ``E_max``. The ``nuclide``
+    attribute is the name, as given by the ``name`` attribute within the 
+    ``nuclide`` sub-element of the ``material`` element in ``materials.xml``,
+    of the nuclide to which a resonance scattering treatment is to be applied.
+    The ``method`` attribute gives the type of resonance scattering treatment
+    that is to be applied to the ``nuclide``.  Acceptable inputs for the
+    ``method`` attribute are ``ARTS``, ``CXS``, ``WCM``, and ``DBRC``.  
+    Descriptions of each of these methods are documented here_.  The 
+    ``xs_label`` attribute gives the label for the cross section data of the
+    ``nuclide`` at a given temperature.  The ``xs_label_0K`` gives the label 
+    for the 0 K cross section data for the ``nuclide``.  The ``E_min`` attribute 
+    gives the minimum energy above which the ``method`` is applied.  The 
+    ``E_max`` attribute gives the maximum energy below which the ``method`` is 
+    applied.  One example would be as follows:
+
+    .. _here: http://dx.doi.org/10.1016/j.anucene.2014.01.017
+
+    .. code-block:: xml
+
+        <resonance_scattering>
+          <scatterer>
+            <nuclide>U-238</nuclide>
+            <method>ARTS</method>
+            <xs_label>92238.72c</xs_label>
+            <xs_label_0K>92238.00c</xs_label_0K>
+            <E_min>5.0e-6</E_min>
+            <E_max>40.0e-6</E_max>
+         </scatterer>
+         <scatterer>
+            <nuclide>Pu-239</nuclide>
+            <method>dbrc</method>
+            <xs_label>94239.72c</xs_label>
+            <xs_label_0K>94239.00c</xs_label_0K>
+            <E_min>0.01e-6</E_min>
+            <E_max>210.0e-6</E_max>
+          </scatterer>
+        </resonance_scattering>
+
+    .. note:: The free gas, constant cross section (``cxs``) scattering model, 
+              which has historically been used by Monte Carlo codes to sample 
+              target velocities, is the default ``method``.  Below ``E_min``, 
+              the ``cxs`` default method is applied and above ``E_max``, the 
+              target-at-rest (asymptotic) kernel is used.  An arbitrary number of
+              ``scatterer`` elements may be specified, each corresponding to a 
+              single nuclide at a single material temperature.
+
+    *Default*: None
+
 ``<run_cmfd>`` Element
 ----------------------