.. _sec-outgoingparticlebias:

[ Outgoing Particle Bias ] section
==================================================

This section describes the configuration of variance reduction used to accelerate the calculation of the probability that outgoing particles will fly within a specific angular range after a reaction.
Currently, this feature only supports Rutherford scattering simulated by [Track Structure] function.

Up to six sections can be defined. The format is as follows:

.. code-block:: text

  [ Outgoing Particle Bias ]
    part = alpha
    reg    x    y    z    r 
    (1-4)  10   10   20   4
    5      10   20   20   10

When a particle specified by part is scattered or generated within a cell specified by reg, the fraction of differential scattering cross-section corresponding to a circle of radius **r** and center coordinates (x, y, z) is calculated.
One particle is then emitted toward that circle with a statistical weight corresponding to that fraction.
The remaining weight is assigned to the second particle, which is sampled randomly across all the other directions.
As a result, since one particle is directed toward the specified direction, you can effectively reduce the variance for tallies placed in that direction.

When calculating the transport of particles toward a target direction, please note the following.
Unlike T-point, particles are treated as normal particles after splitting and are subject to subsequent scattering.
Therefore, they are not guaranteed to reach the target circle.
Similar to [Forced collisions], if the corresponding **wc2** parameter is not lowered, the split particle may be cutoff by the weight cutoff immediately after scattering.

**part** parameter specifies the particle species in the first line.
If omitted, the default is **part = all** .
The format for specifying particles (including specific nuclei) is the same as that used in tally specifications.
To change the order of region number ( **reg** ), target coordinates ( **x, y, z** ), target radius ( **r** ), and importance ( **imp** ), specify the order in the subsequent lines. The **non** keyword can be used to skip columns.
You can group regions with the same values using the format **( { 2 - 5 } 8 9 )** .
Lattice and universe structures, such as **( 6 < 10[1 0 0] < u=3 )** , are also supported.
Any specification that is not a single number must be enclosed in parentheses ( ).
The bias of a hierarchy is the bias of the higher levels. However, you can setup the bias of a specific cell at the lowest level using the format above. This allows you to setup bias for individual lattice elements. If the same cell is defined multiple times, the value defined first will be adopted.