The function is_stable() is misnamed.

[karnigen]

I believe there is a naming inconsistency with the function is_stable(). Currently, it returns true for unstable nuclides, which is counterintuitive. A more logical behavior would be to return true for stable nuclides and false for unstable ones.

I suggest renaming the function to is_unstable() to better reflect its current behavior. Alternatively, we could keep the name is_stable() but invert the return values.

Python is_stable() wraps c++ isStable() that simply returns STA of MF=1 MT=451.
Meaning of STA from ENDF documentation 1.1.

STA Target stability flag:

• STA=0, stable nucleus;
• STA=1 unstable nucleus. If the target is unstable, radioactive decay data should be given in the decay data sub-library (NSUB=4).

The isStable() function returns true for stable nuclids for MF=8 MT=457, as expected (returns NST).
It's counterproductive to receive inconsistent results from the same function in different environments.

[whaeck]

My apologies for replying so late on this issue, but I have been on vacation since December 17 till today and I did not have access to my professional email during that period.

On to the issue at hand: you are correct, the STA variable is being incorrectly interpreted in the current release version of ENDFtk. This specific problem has been addressed in the current develop version (see PR #205). To summarise, the following changes were made in that particular PR:

• the STA() function in C++ and STA property in Python still returns the value of the stability flag
• an isUnstable() function in C++ and is_unstable property in Python has been added and returns true for unstable nuclides
• isStable() now returns the result of ! isUnstable()

We should be releasing a new version of ENDFtk that provides a large number of updates and a few minor interface changes and corrections (including this particular one) soon. For a full overview of all changes, you can look here: https://github.com/njoy/ENDFtk/blob/develop/ReleaseNotes.md