# About flux spectrumn

**URL:** <https://openmc.discourse.group/t/about-flux-spectrumn/6405>\
**Category:** User Support\
**Created:** [July 7, 2026, 6:02pm UTC](https://openmc.discourse.group/t/about-flux-spectrumn/6405 "2026-07-07T18:02:33Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![DinaEl](https://avatars.discourse-cdn.com/v4/letter/d/77aa72/32.png) [@DinaEl](https://openmc.discourse.group/u/DinaEl)\
**Post date:** [July 7, 2026, 6:02pm UTC](https://openmc.discourse.group/t/about-flux-spectrumn/6405/1 "2026-07-07T18:02:33Z")

</div>

Hello,

I am calculating the neutron flux spectrum in OpenMC using an energy filter. My simulation uses about **20 million particles** , but the resulting neutron spectrum still exhibits significant oscillations, particularly in the resonance energy region (around 1–100 eV).

I expected the spectrum to be much smoother with this number of particles.

Is this behavior normal? Could it be related to:

- the statistical uncertainty in narrow energy bins,
- the number of energy groups used,
- or the way the flux tally is processed after the simulation?

What is the recommended approach in OpenMC to obtain a smooth and physically meaningful neutron energy spectrum for comparison with published results?

I have attached the resulting spectrum for reference.

Thank you for your help

 ![spectre_de_neutron](https://global.discourse-cdn.com/free1/uploads/openmc/original/2X/d/d39247ce9c8562c45e6c0368a5a2eeb0602dd520.png)

---

<div class="post-metadata">

**Author:** ![yrrepy](https://yyz2.discourse-cdn.com/free1/user_avatar/openmc.discourse.group/yrrepy/32/2365_2.png) [@yrrepy](https://openmc.discourse.group/u/yrrepy)\
**Post date:** [July 7, 2026, 6:26pm UTC](https://openmc.discourse.group/t/about-flux-spectrumn/6405/2 "2026-07-07T18:26:17Z")

</div>

> What is the recommended approach in OpenMC to obtain a smooth and physically meaningful neutron energy spectrum for comparison with published results?

neutron flux per unit lethargy  
[https://github.com/openmc-dev/openmc-notebooks/blob/develop/flux-spectrum.ipynb](https://github.com/openmc-dev/openmc-notebooks/blob/develop/flux-spectrum.ipynb)  
[https://www.ornl.gov/file/plotting-flux-spectra-keno/display](https://www.ornl.gov/file/plotting-flux-spectra-keno/display)

 ![image](https://global.discourse-cdn.com/free1/uploads/openmc/original/2X/3/3d4bef9871a34cbc0c4f60261acb3d35c2f6486f.png)

If you have enough particles, more energy bins (CCFE-709, UKAEA-1102).  
If not enough particles, XMAS, VITAMIN, etc.

I recommend a stair-step, histogram plot style

 ![image](https://global.discourse-cdn.com/free1/uploads/openmc/original/2X/1/1792be630db9687002ebc4d1734336afbafa31d4.jpeg)

---

<div class="post-metadata">

**Author:** ![DinaEl](https://avatars.discourse-cdn.com/v4/letter/d/77aa72/32.png) [@DinaEl](https://openmc.discourse.group/u/DinaEl)\
**Post date:** [July 7, 2026, 6:30pm UTC](https://openmc.discourse.group/t/about-flux-spectrumn/6405/3 "2026-07-07T18:30:59Z")

</div>

.

I am already using the **XMAS energy group structure** , which is why I was surprised to still observe noticeable oscillations in the spectrum despite using about **20 million particles**.

I will try plotting the neutron flux per unit lethargy with a stair-step histogram as you suggested.

Thank you for your help.