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Spinning the Atomic Nucleus to 60h!
Changes in Structure and the Rotational
Quenching of Nuclear Pairing Correlations.
|A persistent theme in science is to investigate the behavior of the
physical system in question under extreme conditions. The quest to observe increasingly
high angular momentum states in atomic nuclei has driven the field of high-spin nuclear
spectroscopy for many years. With each step forward in detector technology, physicists
have been able to observe an increasingly rich variety of phenomena.
A recent GAMMASPHERE experiment on the rare-earth nucleus 156Dy has made the most comprehensive high-spin study of excitations in the first "normal-deformed" minimum of the potential energy surface. It has been possible to observe rotational states up to a new world record of ~60h, see upper figure, at an excitation energy of 30 MeV! The gamma rays emitted from these levels are very weak, < 0.01% of the total gamma ray flux de-exciting the nucleus 156Dy. The ability of GAMMASPHERE to detect such weak signals over the previous generation of detector systems can be seen in the middle figure.
The lower panel shows that the rotational excitations in 156Dy can be split into three distinct angular momentum groups. The low moment of inertia class (spin I = 0 - 16h) corresponds to the ground-state rotational band and vibrational structures. Here the nucleus displays superfluid properties with nucleons teaming up in time-reversed orbits, or "Cooper pairs". But collective rotation of the nucleus tries to break these correlated fermions apart (the Coriolis-antipairing effect, see inset). This leads to the second class of states (I = 18 - 38) where one, then two, then three specific pairs of nucleons have been broken apart by the Coriolis field (backbending). With increasing rotational frequency and spin, it is thought that a transition may occur from a superfluid to a normal phase in a manner analogous to the quenching of superconductivity in metals by a sufficiently high magnetic field. The third class of states (I = 40 - 60) indicates another structure change occurs, most likely associated with a prolate to oblate shape change of the nucleus, but which also may involve the transition to a region where the pairing field is significantly weakened. Indeed, calculations which assume that pairing correlations are quenched provide an excellent description of this high-spin change in structure. However, such a comparison while indicative, cannot be taken alone as conclusive proof of the long sought pairing phase transition. If, where, and exactly how the superfluid to normal phase transition occurs in the finite system of the atomic nucleus remains an important and unfinished business.