The nuclear structure information needed to describe each fission fragment is obtained from the RIPL-3 library. Nuclear structure data are always evolving, depending on the availability of new nuclear structure experiments. CGMF does not calculate the excited states in a given nucleus, but instead fully depends on the ENSDF or somewhat equivalenty, RIPL3 libraries. The study of isomeric ratios or more generally specific \(\gamma\) decay chains strongly depends on the quality of the underlying nuclear structure information.
Preparing the nuclear structure file for CGMF¶
A special discrete level file (
cgmfDiscreteLevels.dat) for use with CGMF is produced from the RIPL-3 library, and includes adjustments for incomplete information. The Jupyter notebook
transformLevelDataFile.ipynb and the python class
Nucleus.py are used for this purpose.
- For each nucleus, the level data are first read from the RIPL-3 complete datafile;
- The levels are then “fixed” for missing or unassigned spin and parity;
- Finally, the gamma transitions are “fixed”.
Fixing the level scheme¶
If the spin or/and parity of a level is negative, then the level is kept and its (\(J,\pi\)) values are chosen according to the distribution assumed in the continuum following the Kawano-Chiba-Koura (KCK) level density systematics [J. Nucl. Sci. and Tech. 43, 1 (2006)]. Assuming that the parities are evenly distributed, the missing level parity is chosen randomly.
In some cases, RIPL-3 would provide choices of spin and parity for a uncertain level. In that case, the first option will be selected.
In the case of the ground-state being completely unknown, default values will be selected according to:
- even-even: \(0^+\)
- odd-odd: \(1^+\)
- even-odd: \(1/2^+\)
Such arbitrary decisions can have an non-negligible impact when studying specific \(\gamma\) decay chains in a particular nucleus. In that case, extra caution should be put in interpreting the results of CGMF by studying the origin of the nuclear structure information available (or not) for this nucleus.
Fixing the \(\gamma\) transitions¶
In addition to incomplete level schemes, the nuclear structure data from RIPL-3/ENSDF can have incomplete decay chains.
In the case of the first excited state, if no decay data is available, it is assumed that the level decays 100% into the ground-state emitting a \(\gamma\) ray of energy corresponding to the excitation energy of this first level.
is this the right thing to do? Not always!
In the case of higher excited levels, the spin and parity are chosen according to the (\(J,\pi\)) values of levels below in energy, and to which the current uncomplete level could be reached through an E1 transition.
More sophisticated level decay schemes should be employed.