Description

I encountered a KeyError while trying to access specific sections of an ENDF material. Below is a summary of the issue:

Steps to Reproduce:

1. Load a material using endf.Material(), for example:

   import endf
   mat = endf.Material('n-092_U_235.endf')

2. The following warnings appear:

   /path/to/venv/lib/python3.12/site-packages/endf/material.py:228: UserWarning: MF=31, MT=452 ignored
     warn(f"{MF=}, {MT=} ignored")
   /path/to/venv/lib/python3.12/site-packages/endf/material.py:228: UserWarning: MF=31, MT=456 ignored
     warn(f"{MF=}, {MT=} ignored")
   /path/to/venv/lib/python3.12/site-packages/endf/material.py:228: UserWarning: MF=35, MT=18 ignored
     warn(f"{MF=}, {MT=} ignored")
   

3. Attempt to access a specific section:

   print(mat.section_text[12, 75])

   This raises the following error:

   KeyError: (12, 75)
   

4. Attempting to access other sections like:

   mat.section_data[3, 16]

   works as expected and returns data.

Expected Behavior:

mat.section_text[12, 75] should return the corresponding section data, or provide a clearer error if this section is not available.

Actual Behavior:

A KeyError is raised when trying to access section_text for certain sections.

System Information:

• Python version: 3.12
• endf version: 0.1.4 through pip
• ENDF file used: n-092_U_235.endf

Additional Context:

The warnings related to MF=31 and MF=35 might be linked to the issue, but it's unclear if this is the cause.