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.

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