Add detsim city documentation

Author: gondiaz

docs/source/detsim.rst

@@ -1,5 +1,53 @@
 Detsim
 ==========
 
-.. caution::
-  This page is currently not created, we are working on it. If you would like to contribute on this documentation, reach `me <[email protected]>`_!
+**Detsim**, from **Detector Simulation**, is a the city that simulates the *detector response* for *fast-simulated* events.
+A *fast-simulated* event consist of a collection of hits of position and energy desposition in the gaseous volume (left figure below). The *detecto respose* are the signals measured in the light-sensors in the form of time ordered waveforms (right figure below).
+
+.. image:: images/detsim/fastsim_output.png
+    :width: 45%
+.. image:: images/detsim/fullsim_output.png
+    :width: 45%
+
+
+.. _Detsim input:
+
+Input
+-----
+
+Detsim input are NEXUS fast simulation tracks, these files must contain only the following non-empty information.
+
+ * ``/MC/hits``
+
+.. _Detsim output:
+
+Output
+------
+
+ * ``/MC/``: the data in the nexus input file is copied in the output file
+ * ``/Run/event_map``: mapping between nexus event id and IC event id. This mapping is needed because **detsim** might split nexus events into multiple IC events
+ * ``/Run/events``: IC event number and simulated timestamps based on **rate** configuration parameter
+ * ``/Run/runInfo``: run number of each event
+ * ``/pmtrd``: the PMT waveforms for each PMT
+ * ``/sipmrd``: the SiPM waveforms for each SiPM
+ * ``/Filters/active_hits``: nexus events without hits inside the active volume are filtered
+
+.. _Detsim config:
+
+Config
+------
+
+These are the **detsim** parameters in each part of the city flow
+
+.. image:: images/detsim/parameters.png
+    :width: 55%
+
+
+Workflow
+--------
+
+The city flow is explained in the following diagram
+
+.. image:: images/detsim/workflow.png
+    :width: 75%
+