Large Nuclei

Small nuclei that become excited and deformed lose their energy by breaking up into smaller fragments. A larger nucleus, with 150 or more nucleons, stores most of its excitation energy as rotational energy. As they slow down and de-excite, these nuclei lose energy and return to their unexcited shape. What do these nuclei emit, and how would you characterize the energy spectrum?

Answer

Small nuclei that become excited and deformed prefer to lose their energy by breaking up into helium nuclei (alpha particles) or C-12 nuclei whenever possible. In fact, researchers often talk about “nuclear molecules” composed of these two entities. The larger nuclei, with more than 150 nucleons, usually spin faster when energy is added, and the result of a higher angular momentum state is a nucleus that is more deformed. As they de-excite, up to about 40 gamma rays are emitted by descending an “excitation ladder,” producing a characteristic gamma ray emission spectrum. From this spectrum one can determine the nuclear angular momentum states and the nucleus’s deformation shape. Super deformed nuclei were discovered in this way.

Rotational motion of quantum objects such as atoms and molecules has a long and distinguished physics history. Quantized rotational motion of molecules was first recognized from the absorption spectra of infrared light in 1912. The occurrence of rotational motion of atomic nuclei first became a topic of interest in the late 1930s in an effort to explain observed nuclear excitation spectra by physicists Edward Teller and John Wheeler in about 1938.

Quantum mechanics dictates the shapes. Upon excitation, the nucleus first deforms into a shape like a rugby football, with a length-to-height ratio of about two to one. Mg-24 appears to behave as if two C-12 nuclei are its major components and seems to behave as a super deformed nucleus in this rugby football shape. The next state would have an elongated hyper deformed shape as a result of perhaps six alpha particles lined up along the long axis. This nucleus is highly unstable, and this nuclear sausage would produce an unmistakable debris pattern.

Recent detailed investigations of several Pb isotopes have yielded surprises. The angular distribution and polarization of the gamma rays show that they were not electric quadrupole (E2) transitions but magnetic dipole (M1). Classically, M1 radiation is pictured as being emitted from a rotating current loop, with the field oscillating at the same frequency as the frequency of rotation. Similar gamma-ray emission bands have recently been identified in other nuclei in the mass region around 110, where the nuclei also are nearly spherical. These spectra have a pattern that is typical of transitions between rotation states, which poses an awkward problem: how can we explain these regular patterns of M1 gamma rays? Apparently there is much more to understand.