To first approximation, the mass of a nucleus is the sum of its constituent proton and neutron masses. But the roughly 1% correction for nuclear binding energy embodies important information about the configuration of the particular nucleus. Binding energies are especially interesting, and correspondingly hard to measure, in two cases: short-lived isotopes with unusually large or small neutron numbers N for their proton numbers Z, and the transuranic nuclei (Z > 92) approaching the so-called island of stability expected to be found somewhere near Z = 120 and N = 184. (See figure 1.)

Nowadays the best measurements of nuclear masses are done in Penning traps — small, cylindrically symmetric cavities in which charged particles can be kept away from the walls by static electric and magnetic fields and stored for long periods (see Physics Today, May 1985, page 17). Beyond its custodial function,...

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