All nuclear isotopes (a given number of neutrons and protons) have excited
states. By exploring such states, one explores collective behavior of
nuclear matter and the shell-model structure for exotic nuclei. For
example, photon spectra from the decays of excited states for three sulfur
isotopes are exhibited on the right. The exotic isotopes were excited by
passing them near a heavy nucleus, and the in-flight gamma decays were
observed with Sodium-Iodide detectors. This experiment measured sulfur's
transition quadrupole moment.