Uniqueness of 56Fe as a primary standard for atomic masses

Abstract

A recent evaluation of atomic masses confirms that 56Fe has uniquely the 'least' atomic mass per baryon, ZAM/A. Thus, with 56Fe as the primary standard (2656M=56) all other atomic masses would be greater than the mass number A and the corresponding 'mass excesses' would be not only 'positive' but also 'maxima'. Moreover, a mass excess relative to 56Fe is potentially the largest 'portion' of the atomic mass that can be transformed into energy (assuming conservation of baryon number); it therefore has a unique physical significance. Accordingly, a plot of the mass excesses against A and Z would provide a unique 'potential-energy surface' which should be useful in the representation of nuclidic transformations that involve a change in A(eg alpha decay) as well as those with A=constant.