5.3.1. <source>: Multi-source

Using this multi-source function, multiple sources with different source definitions, such as different particle species, geometries, or energy distributions, can be specified. Each source definition begins with <source> = value, which specifies the relative weight of such type of source. totfact = defines a global scaling factor.

Tally results in PHITS calculations are normalized to per source particle, or per unit weight in a more strict sense. For the totfact setting, if it is given a positive value, number of source particles are generated based on the intensity ratio of each source definition; if negative, same number of source particles are generated for each source definition, but their weights are adjusted to match the intensity ratio of all source definitions. If one source definition has a much lower intensity than the others, yet such source definition contributes significantly to the tally results, it is recommended to set totfact < 0 to improve calculation efficiency. [1]

Table 5.3.2 <source>

value

explanation

The relative weight of such source definition. This is necessary when multiple sources are defined.
Table 5.3.3 totfact

value

explanation

(D=1)

Global scaling factor.
Table 5.3.4 iscorr

value

explanation

(D=0)

Multi-source correlation option. 0:Uncorrelated multi-source. 1:Correlated multi-source. In this case, source particles from each multi-source definition are generated as an event. This option is useful for estimating detector response by nuclear reactions producing several secondary particles. Note that the locations of all sources generated in an event are not the same, unlike the case of iscorr = 2,3 . 2:Correlated multi-source. In this case, the locations of all sources generated in an event are the same. This mode is useful for simulating a nuclear reaction occurring at an arbitrary location. 3:Correlated multi-source. In this case, the locations of all sources generated in an event are the same, and the direction of the second source is opposite to that of the first. This mode is useful for simulating a particle decaying into two particles at an arbitrary location.

For iscorr = 1, 2, 3, <source> should be defined as an integer absolute value. As for normalization, if you want to consider multiple generated radiation as a single source, you should specify the sum of the numbers defined in <source> with totfact. For example, to simulate the \((X, 1p2n)\) reaction as a single source, use <source> = 1 for the proton source, <source> = 2 for the neutron source, and totfact = 3. Conversely, if you want to consider each radiation produced as a single source, totfact is the same as for iscorr = 0.