5.21. [ Forced Collisions ] section

[Forced collisions] can improve tally statistics of secondary particles generated from a certain target in which the collision probability is very low. It is especially useful in the analyses of secondary particle yields in thin target or in thick target irradiated with particles having low cross sections. When specified particle comes into a region defined as the forced collision region, the particle is divided into two particles. One has a weight by (penetration probability) \(\times\) (its weight), this particle pass through to the next region. The other has a weight of (1 - penetration probability) \(\times\) (its weight), and it is forced to collide with a target in the region. For neutral particles, the transmission probability is calculated according to the cross section of the incident particle, and the collision position is determined stochastically so that it is uniform within the specified region. In the case of charged particles, on the other hand, the forced collision region is divided into a number of segments, the energy loss and collision probability in each segment is determined in advance, and the collision positions are sampled according to these probabilities. The number of the segments is the same for all forced collision regions and is defined by nfcseg (D=50) in the [parameters] section. Note that the accuracy of the simulation becomes worse when [forced collisions] is used in a region in which the collision probability without [forced collisions] is greater than approximately 10%.

Regions and factors for the forced collisions can be defined in this section. Non-defined regions are set factor zero.

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

The format is as follows.

[ Forced Collisions ]
  part =  proton  neutron
  reg                   fcl
  1                     1.000000
  11                    0.500000
  ( { 2 - 5 } 8 9 )     0.200000
  ( 11 12 15 )          0.300000
  ( 6<10[1 0 0]<u=3 )   -0.500000
  ...                   ...
  ...                   ...

You set particle as part= in the first line. The default is part=all. part= is the same format as in tally definition.

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

You can use the format ( { 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. By using this format you can set different forced collision factor for each lattice. If the same cell is re-defined, the value, which is defined at first, is used.

The forced collision factor fcl means, 0: no forced collision, \(|fcl| > 1\) is an error, and \(|fcl| \le 1\) is multiply forced collision probability by \(fcl\), instead the weight is reduced by \(1/|fcl|\) times.

We have two options to control the particle transport and multiple scattering with the weight cut off in the forced collisions region. When \(fcl < 0\) , secondary particles produced by forced collisions are treated by the normal process. In this case, weight cut off is not performed. When \(fcl > 0\) , the forced collision is also applied to secondary particles. In this case, weight cut off is performed. Even if a particle is killed by this weight cut off, of course the particle is tallied before killed. There is a possibility that all particles are killed by this weight cut off, if you set the weight cut off and the forced collisions without consideration. For example, it corresponds to tallying tracks of secondary particles and information of particles at a distance from the forced collisions region. When you want to transport secondary particles produced by forced collisions, you should decrease the value of the cutoff weight parameter wc1(i) in the [parameters] section.