At the Limits of Spin: Hyperdeformed Nuclei
The response of atomic nuclei to increasing angular momentum (or spin) and excitation energy is one of the most fundamental topics of nuclear structure research. Studies at the extremes of angular momentum and excitation energy probe the competing modes of nuclear excitation and the rich variety of shapes that occur. Rotational motion provides a striking example of emergent phenomena in many-body quantum systems and is a sensitive tool to study the underlying microscopic structure. GRETA will be essential to push these measurements beyond what can be done today. The sensitivity gained from using GRETA for a high angular-momentum experiment is illustrated in Figure 1 for the case of 108Cd, where a weakly populated rotational structure, with an intensity of ~10-3 of the total fusion cross section has been seen [Cla01] at high angular momentum in an experiment with Gammasphere. This structure corresponds to a strongly deformed prolate shape, possibly exceeding a 2:1 major-to-minor axis ratio. This rotational band is at the limit of observation for present arrays. GRETA will allow the study of structures that are 10 times weaker. (Similar gains are expected for many of the cases discussed above for studies of very heavy nuclei.) The order of magnitude gains in sensitivity will be essential, for example, to enable measurements of predicted high-spin hyperdeformed (HD) nuclei, e.g. [Cha01], which are associated with a third minimum in the potential energy surface of rapidly rotating nuclei. This extreme deformation (approaching a 3:1 major-to-minor axis ratio) is predicted to occur at the very limits of sustainable angular momentum before the nucleus undergoes fission. In 108Cd, this corresponds to a spin of ~70ħ [Cha01], and a band beyond the reach of today’s experimental sensitivity, but within the reach of GRETA. Reaccelerated radioactive beams at FRIB, e.g. 94Kr, will give access to neutron-rich Cd isotopes up to 114Cd, where hyperdeformed structures are predicted to be favored at even lower spins.
* Recently, gamma-ray decays have been reported from excited states in element Z = 109 and Z = 107, produced in the 243Am(48Ca, xn) 115 reaction [Rud14, Gat15].
Figure 1: Simulations compared to data for the example of the highly-deformed rotational band in 108Cd illustrating GRETA’s gain in resolving power to observe weakly-populated, high angular-momentum rotational states The top panel shows the measured Gammasphere spectrum [Cla01] (black) together with the corresponding simulated spectrum (red). This is the observational limit for current detector arrays - a band with intensity of 10-4 would not be seen today (middle panel). GRETA, shown in the bottom panel, would clearly resolve such a structure.