Photon spectrum difference between PHITS and OpenMC

Hi all,

I executed the Broomstick problem with OpenMC 0.12.2 and PHITS 3.24, using ENDF/B-
VIII.0, and found that the results for the photon spectrum differ, particularly for the U235 case with density 18.95 g/cm³ and 1MeV neutron source. For the neutron spectrum, no significant differences are observed.

I observed it when comparing the results obtained with an old version of OpenMC that I had downloaded a while ago (0.12.0-dev), and in this case they are consistent with the values ​​reported by PHITS.

For all 0.12.X stable versions the same results are obtained, so I guess that the difference could be in the development between stable versions 0.11.0 and 0.12.0 (I tried to execute the model with OpenMC 0.11.0, but I get a Segmentation Fault).

Attached are two graphics where you can appreciate the differences mentioned.

Thanks in advance,


Hi @Norbert_Schmidt. After thinking about this some, I believe the cause of the difference you are seeing is the result of a method used in OpenMC where the total photon yields from neutron reactions are scaled slightly higher to account for production of delayed photons from fission (hence why you are seeing this in U235). This method makes sense for reactor calculations when you assume the reactor is operating at steady state (and have a relatively stable population of fission products producing delayed photons). For other systems, it may not make sense. If you want to turn this off for the sake of comparison to PHITS or another code, add the following to your code:

settings = openmc.Settings()
settings.delayed_photon_scaling = False

Here is the pull request where this was added to OpenMC:

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Thank you very much @paulromano for your answer! Indeed that was the reason for the apparent differences between the results (I didn’t know that the delayed_photon_scaling option is declared as True by default). The new results can be seen in the next figure:

Thanks again and best regards,