RUI: Stable Triaxial Deformation in A~165 and 110 Nuclei

United States Naval Academy- Do Not Use, Annapolis MD

Investigators

Abstract

The atomic nucleus has been observed to exhibit several different shapes in different isotopes. In particular, they are known to have spherical, prolate (like a rugby ball), oblate (like a doorknob), and exotic octupole (like a pear) shapes. Each of these forms has at least one axis of symmetry for its mass distribution. That is, if the nucleus is rotated about its symmetry axis through any angle, it looks exactly the same. In fact, rotating nuclei about an axis perpendicular to this symmetry axis allows for the study of evolving shapes with decreasing spin and energy. This is done through the examination of long chains of gamma rays emitted by the rotating nuclei as they slow down. Theoretical calculations suggest that an unusual shape may be observed where there is no such axis of symmetry. The mass of this nucleus is distributed unequally along the three axes of length, width, and height. For this reason, these nuclei are said to be triaxial or asymmetric in shape. Although theory suggests this shape exists in many regions of the nuclear chart, direct experimental evidence of its existence is scarce. However, if a nucleus retains a triaxial shape and is rotated rapidly, a sequence of gamma-ray decays may be observed that is characteristic of a wobbling motion. One may envision the motion of a spinning, asymmetric top that precesses and wobbles as it slows down in order to picture the wobbling motion of a triaxial nucleus. Indeed, an isotope of lutetium (163Lu) was recently discovered to exhibit this wobbling motion and is perhaps the best example of an asymmetric nucleus to date. Other examples have been found in neighboring lutetium nuclei, but none have been observed in any other element. Is the wobbling motion confined to these nuclei, or is this shape more widely seen in the nearby region? What exactly are the necessary conditions for a nucleus to exhibit this unusual shape? These are some of the questions this proposal will try to answer. A search will begin for wobbling motion in the tantalum isotopes 165Ta and 167Ta. These nuclei have two more protons then the lutetium nuclei, which show evidence for triaxial shapes. Current theoretical investigations suggest that the number of protons should not greatly affect the presence of wobbling. Instead, theory predicts that nuclei having approximately 72 protons and 94 neutrons are the key factor; however, no evidence of wobbling is observed in any nuclei other than in lutetium (with 71 protons). The 165Ta and 167Ta have 73 protons as well as 94 and 96 neutrons, respectively. Therefore, they are prime candidates for possibly finding evidence of wobbling beyond the lutetium nuclei. These nuclei will be created in reactions that will leave them in very high-spin states and the gamma rays emitted will be detected with large arrays of gamma-ray detectors at Argonne National Laboratory and Yale University. In addition, a search for triaxial shapes in ruthenium nuclei with large neutron excess will begin when Argonne National Laboratory begins accelerating radioactive isotopes following the fission of a californium source. Once again, theory suggests asymmetric shapes for these nuclei at high spin and experiments will be performed to attempt to find conclusive evidence for this unusual shape. Undergraduate students from the US Naval Academy will be intimately involved with each project as they will participate in the experiments, analyze the data, and present results at various conferences. The opportunity to use world-class facilities and contribute to the frontiers of nuclear structure research will hopefully propel these students into scientific careers.