# What is the energy distribution of a default fixed source

**URL:** <https://openmc.discourse.group/t/what-is-the-energy-distribution-of-a-default-fixed-source/5674>\
**Category:** User Support\
**Created:** [July 14, 2025, 11:51am UTC](https://openmc.discourse.group/t/what-is-the-energy-distribution-of-a-default-fixed-source/5674 "2025-07-14T11:51:42Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![ozgur.hanci](https://avatars.discourse-cdn.com/v4/letter/o/e9c0ed/32.png) [@ozgur.hanci](https://openmc.discourse.group/u/ozgur.hanci)\
**Post date:** [July 14, 2025, 11:51am UTC](https://openmc.discourse.group/t/what-is-the-energy-distribution-of-a-default-fixed-source/5674/1 "2025-07-14T11:51:42Z")

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Hi all,

I am simulating a fuel assembly for a detector simulation. I did not specify any energy distribution or no monoenergetic parameter. So, is the default distribution a Watt distribution? Does the code below ensures that (from source.cpp)?

```auto
 // Determine external source energy distribution
    if (check_for_node(node, "energy")) {
      pugi::xml_node node_dist = node.child("energy");
      energy_ = distribution_from_xml(node_dist);
    } else {
      // Default to a Watt spectrum with parameters 0.988 MeV and 2.249 MeV^-1
      energy_ = UPtrDist {new Watt(0.988e6, 2.249e-6)};
    }

```

I am preparing a document so the knowledge is needed, thank you for your guidance!

Have a nice day.

---

<div class="post-metadata">

**Author:** ![Ignacio](https://avatars.discourse-cdn.com/v4/letter/i/eb9ed0/32.png) [@Ignacio](https://openmc.discourse.group/u/Ignacio)\
**Post date:** [July 15, 2025, 9:29am UTC](https://openmc.discourse.group/t/what-is-the-energy-distribution-of-a-default-fixed-source/5674/2 "2025-07-15T09:29:20Z")

</div>

Yes, it is clear in [the documentation](https://docs.openmc.org/en/stable/usersguide/settings.html#external-source-distributions):

> By default, if no energy distribution is specified, a Watt fission spectrum with  
> a = 0.988 MeV and b = 2.249 MeV-1 is used.