A metastable isotope is theoritically expected to be slightly heavier than its ground state, due to excitation energy manifesting as excess mass. That difference of masses can even be measured experimentally (see Stephan Pomp at 7 minutes 57 during the Joint ICTP-IAEA Workshop on Simulation of Nuclear Reaction Data with the TALYS Code, 17 October 2023).
In ENDF-6 format, masses are expressed with Atomic Weight Ratio (AWR) "defined as the ratio of the mass of the material to that of the neutron." [ENDF-102] For metastable isotopes, in practice (in existing evaluated files), this AWR does not include the excess mass due to excitation. However, in ENDF-6 format, excitation energy is available (in eV) through the ELIS parameter.
For example, for Cd115m in JEFF-3.3 :
AWR = 113.9180
ELIS = 1.810000E+05 eV
With a neutron mass of 939.56542052 MeV/c², the all-included AWR of Cd115m is expected to be :
113.9180 + 1.810000E+05/1E6/939.56542052 = 113.9182
DRAGR do not include the excitation energy in AWR and therefore the masses are very slightly off for metastable isotopes. The impact of this error is expected to be very small.
A metastable isotope is theoritically expected to be slightly heavier than its ground state, due to excitation energy manifesting as excess mass. That difference of masses can even be measured experimentally (see Stephan Pomp at 7 minutes 57 during the Joint ICTP-IAEA Workshop on Simulation of Nuclear Reaction Data with the TALYS Code, 17 October 2023).
In ENDF-6 format, masses are expressed with Atomic Weight Ratio (AWR) "defined as the ratio of the mass of the material to that of the neutron." [ENDF-102] For metastable isotopes, in practice (in existing evaluated files), this AWR does not include the excess mass due to excitation. However, in ENDF-6 format, excitation energy is available (in eV) through the ELIS parameter.
For example, for Cd115m in JEFF-3.3 :
AWR = 113.9180
ELIS = 1.810000E+05 eV
With a neutron mass of 939.56542052 MeV/c², the all-included AWR of Cd115m is expected to be :
113.9180 + 1.810000E+05/1E6/939.56542052 = 113.9182
DRAGR do not include the excitation energy in AWR and therefore the masses are very slightly off for metastable isotopes. The impact of this error is expected to be very small.