# Time Filter in k-eigenvalue calculation

**URL:** <https://openmc.discourse.group/t/time-filter-in-k-eigenvalue-calculation/2710>\
**Category:** User Support\
**Created:** [February 10, 2023, 1:55pm UTC](https://openmc.discourse.group/t/time-filter-in-k-eigenvalue-calculation/2710 "2023-02-10T13:55:57Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![lorenzoloi](https://avatars.discourse-cdn.com/v4/letter/l/43a26b/32.png) [@lorenzoloi](https://openmc.discourse.group/u/lorenzoloi)\
**Post date:** [February 10, 2023, 1:55pm UTC](https://openmc.discourse.group/t/time-filter-in-k-eigenvalue-calculation/2710/1 "2023-02-10T13:55:57Z")

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Hello everyone,

I saw this [example](https://github.com/fusion-energy/neutronics-workshop/tree/main/tasks/task_17_detectors_examples) where TimeFilter was used in a fixed source calculation for detection purposes. In this case, the physical meaning is clear to me. Considering to score the flux: the neutron starts from the source at t=0, he travels through the media (considering interactions) and it is recorded the passage in a given cell at a given time instant.

I have a question related to the usage of the Time Filter in a k-eigenvalue calculation: what is the physical meaning in this setup?  
Are the source neutrons the one given by the fission source?  
If yes, are the source neutrons from fission transported through the materials without multiplication? (I mean, when a neutron is absorbed, it do not contribute anymore to the TimeFilter score).

Thank you very much

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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:** [February 13, 2023, 12:45pm UTC](https://openmc.discourse.group/t/time-filter-in-k-eigenvalue-calculation/2710/2 "2023-02-13T12:45:01Z")

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> [@lorenzoloi](#):
>
> what is the physical meaning in this setup?  
> Are the source neutrons the one given by the fission source?

Yes, the source neutrons are the neutrons that are born from fission, so the time filter would be measuring “time since fission”.

> [@lorenzoloi](#):
>
> If yes, are the source neutrons from fission transported through the materials without multiplication?

In a k-eigenvalue simulation, neutrons that are born from fission are not immediately transported but are saved for the next generation in a “bank”. Neutrons that are born from other reactions (e.g., (n,2n)) are immediately transported however.