# Problem with cross-section (NuFissionMatrix\_XS)

**URL:** <https://openmc.discourse.group/t/problem-with-cross-section-nufissionmatrix-xs/3760>\
**Category:** User Support\
**Created:** [December 19, 2023, 9:36am UTC](https://openmc.discourse.group/t/problem-with-cross-section-nufissionmatrix-xs/3760 "2023-12-19T09:36:27Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![Visionary](https://avatars.discourse-cdn.com/v4/letter/v/13edae/32.png) [@Visionary](https://openmc.discourse.group/u/Visionary)\
**Post date:** [December 19, 2023, 9:36am UTC](https://openmc.discourse.group/t/problem-with-cross-section-nufissionmatrix-xs/3760/1 "2023-12-19T09:36:27Z")

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Dear All,  
I’m curious about the defination about NuFissionMatrix\_XS. Why this cross-section is given as matrix and not a scalar at certain energy group like fission\_XS?  
Best regard

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**Author:** ![paulromano](https://yyz2.discourse-cdn.com/free1/user_avatar/openmc.discourse.group/paulromano/32/486_2.png) [@paulromano](https://openmc.discourse.group/u/paulromano)\
**Post date:** [December 19, 2023, 1:16pm UTC](https://openmc.discourse.group/t/problem-with-cross-section-nufissionmatrix-xs/3760/2 "2023-12-19T13:16:46Z")

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You can find the definition [here](https://docs.openmc.org/en/latest/pythonapi/generated/openmc.mgxs.NuFissionMatrixXS.html#openmc.mgxs.NuFissionMatrixXS). Each element in the matrix tells you the production of neutrons with energies in group g’ originating from neutrons with energies in group g.