Division of Electronics, Detectors and Computing for Physics (DEDIP)

Cadmium isotopes, always containing 48 protons but different numbers of neutrons, have long been textbook examples for describing a particular mode of excitation of the atomic nucleus, namely the vibration. There are actually several ways to excite a nucleus, in particular through vibration and rotation. However, since cadmium nuclei are not deformed, i. e. have a rather spherical shape, rotation is prohibited in quantum mechanics. They are therefore widely studied for their vibrational character and vibrate at specific frequencies around their spherical equilibrium form. In analogy with the one-dimensional quantum oscillator (shown to the right of Figure 1), this description results in a vibrational model of the atomic nucleus with a characteristic pattern of excitation levels and decay paths (see Figure 1 on the left).

However, this ideal representation of cadmium nuclei as "spherical vibrators" was undermined since the 1970s by the experimental observation of states of low excitation energy whose properties (energy and decay paths) could not be reproduced in the context of the vibrational model of the atomic nucleus. These "abnormal" states fall into two categories: states whose vibrational character is well known but whose paths of decay are not as predicted (red transitions in the figure) and non-vibrational states, called "intruders" (blue states and transitions in Figure 2).