High Spin Gamma-Ray Spectroscopy
Gamma-rays are highly energetic photons emitted by a nucleus during its decay from an excited state. They occur from bound states and only from decays within the nucleus. Observing γ - rays emitted from excited states can tell us about the energy (E), spin or angular momentum (I), parity (π) and lifetime (τ)of the state. In addition they can give use information on the quadrupole moment, magnetic moment and shape of the nucleus.
All even (Z) - even (N) nuclei have, as a consequence of pairing, a ground state spin and parity of Iπ = 0+. Closed-shell nuclei have a spherical shape and excited nuclear states can only be formed either by vibrations of the core or by breaking some of the nucleon pairs. Where the nucleus has a deformed shape, e.g. quadrupole or octupole (see left illustrations) it can exhibit a regular rotational band structure (figure on right).
In order to observe the decay from high spin states nuclei must first be produced in this highly excited state. One way in which we do this is to use Heavy Ion Fusion Evaporation Reactions ( illustrated schematically above). Using this type of reaction we transfer a large amount of angular momentum to the nucleus e.g. 60ħ. To observe the prompt (10-15secs after formation) radiation we surround the target location with High Purity Germanium detectors. A picture of such an array, "Gammasphere" is shown on the right. An illustration of how a single detector works is shown beneath.
Rotating the nucleus to increasingly higher spin forces the nucleus to adopt a configuration which has the lowest rotational energy. The spin is composed of a collective part and a contribution arising from single particles. Minimising the energy can be achieved by reducing the collective part (pair breaking) or by increasing the nuclear moment of inertia. During rapid rotation pairs of nucleons are broken and their individual angluar momentum aligned. The movie "backbending" (courtesy of Mark Riley at FSU) illustrates this process. Backbending provides a clear experimental signature for high spin physics.
One research area our group is pursuing is the search for triaxial nuclei and wobbling rotational modes. At low spins, the signatures for a nucleus with a rigid triaxial shape is similar to that for a soft temporary g deformation. But, at high spins there is a more definitive signature - rotational bands associated with a wobbling mode akin to that of a spinning asymmetric top. More details of our research into triaxial nuclei can be found in Experimental Programme.
Gammasphere: An array of large volume HPGe detectors with over 100 elements.
A HPGe detector. The incoming g-ray is detected in the Germanium crystal. Escaping g-rays detected by scintillating material around the Germanium are ignored.
High spin decay schemes. A rotational band in a collective nucleus is shown left. A non-collective decay scheme is shown on the right.