[ Importance ] section
==================================================

The importance for cell can be defined in this section.
If the importance is not defined, it is set as 1.0. 

Maximum 6 [importance] sections are allowed to be defined in an 
input file.

The format is as follows.

.. code-block:: text

  [ Importance ]
    part =  proton  neutron 
    reg                   imp
    1                     1.000000    
    11                    5.000000    
    ( { 2 - 5 } 8 9 )     2.000000    
    ( 11 12 15 )          3.000000
    ( 6<10[1 0 0]<u=3 )   6.000000 
    ...                   ...
    ...                   ...

Particle is defined as part= at the first line.
If the part is not defined, default value is defined as part=all. 
The format to describe particles is the same as in tally definition. 
However, it can distinguish ityp only, each nucleus is not specified. 

If you want to change the order of region number (reg) and (imp), set as imp reg. 
You can use the skip operator non.

You can use the format like ( { 2 - 5 } 8 9 )}, and you can use the lattice and universe style as ( 6 < 10[1 0 0] < u=3 ). 
But you need to close a value by (  )  if it is not a single numeric value.
The importance of bottom level is a product by each importance at each level. 
In PHITS, importance of a specific cell at bottom level can be defined by above format. 
By using the format, we can define different importance into each lattice. 
If the importance is double-defined, the first defined importance is valid. 

If you set large importance to particles which have strong penetration through matter such as neutrino, PHITS calculation takes time too much. 
To avoid this problem, neutrino is excluded from part=all. 
You must give attention about it. 

Some rules can be used to define an importance of a cell in a repeated structures and lattices. 
For example, cells 5, 6, and 7 on a bottom level are included by cells 11, 12, and 13 on upper level, we can define the importance as 

.. code-block:: text

  [ Importance ]
    reg               imp
    ( 5 6 7 < 11 )    2.0
    ( 5 6 7 < 12 )    4.0
    ( 5 6 7 < 13 )    8.0
    ( 11 12 13 )      1.0

or

.. code-block:: text

  [ Importance ]
    reg           imp
    ( 5 6 7 )     1.0
    11            2.0
    12            4.0
    13            8.0

Above two definitions give same results, but in the latter case, the importance for cells 5, 6, and 7 are displayed as 1.0 at the importance summary.