4.2. Important PHITS settings for using [T-Dchain]

This section seeks to make note of some of the important settings which should be set within PHITS for accurate/best results in DCHAIN. This is covered some within the [T-Dchain] portion of the PHITS manual, but this section seeks to concisely summarize the important settings and explain their meanings.

In the briefest terms possible, there must be a [Volume] section assigning volumes to the regions tallied in [T-Dchain] for reg meshes (it is calculated automatically for xyz and tet meshes; [T-Dchain] does not support r-z meshes), and the following settings in the [Parameters] section should be set: jmout= 1, e-mode= 0, and igamma= 3. Additionally, file(21) must be set equal to the path to your DCHAIN data folder, <PHITS-install>/dchain-sp/data/; if your PHITS installation uses the default recommended paths, the default value of this is likely set correctly already.

Setting jmout= 1 tells PHITS to convert the provided materials in the [Material] section to units of atom density when being outputted to the DCHAIN input file (TGNNDS in Card 9b). This is quite important because its default value is set to 0 which just echos the values provided to the [Material] section back to the DCHAIN input file, and these are likely to be incorrect as the cell’s density is not accounted for and the [Material] entries are often provided as mass or atom fractions, not absolute atom densities. Setting jmout= 1 is vital to ensure the volume normalized results are correctly calculated.

Along a similar line, providing volumes to the [Volume] section is vital for the absolute results (such as total activity in Bq) to be scaled correctly. This also affects the totfact parameters values printed to the .pht output file’s [Source] sections used to scale the source’s strength to its real physical value for secondary PHITS dose assessment and shielding calculations. If the volume of a tallied region is not provided, it assumes a default value of 1 cm\(^3\). Note that this only applies to region reg meshes in PHITS; volumes are automatically calculated for 3D grid xyz and tetrahedral tet meshes.

Setting e-mode= 0 sets the PHITS event generator mode to “simple sampling mode” (non-event generator mode), and this is its default value. When e-mode= 0, reaction products from neutron reactions under 20 MeV do not contribute to the [T-Yield] tally constructed automatically by [T-Dchain]; this is the desired behavior as these reaction products are instead calculated inside of DCHAIN using the neutron flux tallied by [T-Dchain]’s automatically constructed [T-Track] tally in conjunction with its own dedicated activation cross section libraries. If set to 1 or 2, these reaction products will then be double-counted unless in DCHAIN JMODE is set to ignore one of the two quantities (.dtrk fluxes or .dyld yields). If explicitly wishing to test the PHITS event generator modes in DCHAIN, please keep this in mind. Given that e-mode is often enabled for certain types of calculations (like detector response), take care when copying this value from the [Parameters] section of one PHITS input file to another which contains a [T-Dchain] tally.

Setting igamma= 3 results in [T-Dchain]’s [T-Yield] tally including isomers in addition to ground state nuclides. By default igamma is set to 2 which does not include isomer production. DCHAIN’s ISOMTR parameter controls how isomers are handled within DCHAIN. If igamma= 3 in PHITS, [T-Dchain] automatically sets ISOMTR= 0 in the DCHAIN input, meaning the isomers are treated individually. Otherwise, [T-Dchain] sets ISOMTR= 2 by default, in which case DCHAIN will assume for any nuclide with isomeric states that production of them is evenly split between the ground and metastable states. It is recommended to allow PHITS to handle this distribution with igamma= 3.