Editing Input syntax manual (section)

=== set cfe ===

 '''set cfe''' ''LN'' [ ''TN LG TG'' ]
Defines the minimum mean distance for scoring the collision flux estimator (CFE) for photons and neutrons. Input values:

{|
| <tt>''LN''</tt>
| : minimum mean distance for scoring the CFE for neutrons [in cm] (default value: 20.0)
|-
| <tt>''TN''</tt>
| : minimum mean time interval for scoring the CFE for neutrons [in s]
|-
| <tt>''LG''</tt>
| : minimum mean distance for scoring the CFE for photons [in cm] (default value: 20.0)
|-
| <tt>''TG''</tt>
| : minimum mean time interval for scoring the CFE for photons [in s]
|}

<u>Notes:</u>
*The use of delta-tracking necessitates the use of CFE for scoring the integral reaction rates. 
**The scoring is based on both real and virtual collision to improve the statistics in low density regions (and short time intervals).
*The minimum mean distance is the statistical mean-free-path (mfp) of collisions that contribute to the CFE. 
**Collisions are more frequent if the physical mfp is shorter.
*In time-dependent simulations it may be more convenient to define the minimum mean time between two collisions, to get sufficient statistics for short time bins.
*Adjusting the distance affects both statistics and running time, but it should be noted that <u>no studies have been performed on what the optimal value should be</u>.
*Only one criterion can be provided for each particle type. 
**If distance is given, time must be set to "<tt>-1</tt>" and vice versa.
*For more information on tracking modes and CFE, see the detailed descriptions on [[delta- and surface-tracking]] and [[Result estimators#Implicit estimators|result estimators]].
*The collision flux estimator methodology is described in a related paper.<ref name="cfe">Leppänen, J.
''"On the use of delta-tracking and the collision flux estimator in the Serpent 2 Monte Carlo particle transport code."'' Ann. Nucl. Energy [http://www.sciencedirect.com/science/article/pii/S0306454916311367 '''105''' (2017) 161-167].</ref>
*In version 2.1.27 and earlier the name of this input option was "set minxs".