Add missing documentation on <source> in depletion chain file format (#3590)

Co-authored-by: Paul Romano <paul.k.romano@gmail.com>
Co-authored-by: April Novak <novak@berkeley.edu>

Author: 3 people

docs/source/io_formats/depletion_chain.rst

@@ -56,6 +56,27 @@ attributes:
 
 .. _io_chain_reaction:
 
+--------------------
+``<source>`` Element
+--------------------
+
+The ``<source>`` element represents photon and electron sources associated with
+the decay of a nuclide and contains information to construct an
+:class:`openmc.stats.Univariate` object that represents this emission as an
+energy distribution. This element has the following attributes:
+
+  :type:
+    The type of :class:`openmc.stats.Univariate` source term.
+
+  :particle:
+    The type of particle emitted, e.g., 'photon' or 'electron'
+
+  :parameters:
+    The parameters of the source term, e.g., for a
+    :class:`openmc.stats.Discrete` source, the energies (in [eV]) at which the
+    particles are emitted and their relative intensities in [Bq/atom] (in other
+    words, decay constants).
+
 ----------------------
 ``<reaction>`` Element
 ----------------------

openmc/data/decay.py

@@ -591,7 +591,7 @@ def decay_photon_energy(nuclide: str) -> Univariate | None:
     openmc.stats.Univariate or None
         Distribution of energies in [eV] of photons emitted from decay, or None
         if no photon source exists. Note that the probabilities represent
-        intensities, given as [Bq].
+        intensities, given as [Bq/atom] (in other words, decay constants).
     """
     if not _DECAY_PHOTON_ENERGY:
         chain_file = openmc.config.get('chain_file')