Mass excess

Energy scale of nuclear reactions

The ¹²C standard provides a convenient unit (the dalton) in which to express nuclear mass for defining the mass excess. However, its usefulness arises in the calculation of nuclear reaction kinematics or decay. Only a small fraction of the total energy that is associated with an atomic nucleus by mass–energy equivalence, on the order of 0.01% to 0.1% of the total mass, may be absorbed or liberated as radiation. By working in terms of the mass excess, much of the mass changes which arise from the transfer or release of nucleons is effectively removed, highlighting the net energy difference.

Nuclear reaction kinematics are customarily performed in units involving the electronvolt, which derives from accelerator technology. The combination of this practical point with the theoretical relation E = mc² makes the unit megaelectronvolt over the speed of light squared (MeV/c²) a convenient form in which to express nuclear mass. However, the numerical values of nuclear masses in MeV/c² are quite large (even the proton mass is ~938.27 MeV/c²), while mass excesses range in the tens of MeV/c². This makes tabulated mass excess less cumbersome for use in calculations. The 1/c² factor is typically omitted when quoting mass excess values in MeV, since the interest is more often energy and not mass; if one wanted units of mass, one would simply change the units from MeV to MeV/c² without altering the numerical value.