Using accelerators to make nuclear reactions (see Chapter 7), scientists can create nuclei which have very high angular momentum. Nuclei respond to this rotation, which can be as fast as a hundred billion billion revolutions per second, in a rich and varied way. These nuclei lose some of their excitation energy and almost all of the initial angular momentum by the emission of gamma rays. The gamma ray flash is finished in less than 10-9 seconds, during which 30 or more gamma rays can be emitted.
A number of preferred pathways in the de-excitation process occur. They relate to favorable arrangements of protons and neutrons and can often be associated with specific symmetries or nuclear shapes. If a sufficient fraction of the decay flows down a particular quantized pathway or band, then the associated structure becomes observable and can be studied in detail. Scientists have recently built arrays of over 100 gamma ray detectors to study the details of some of these rare pathways. One such array is Gammasphere, shown below. Gammasphere was built by groups from Lawrence Berkeley National Laboratory, Argonne National Laboratory, and Oak Ridge National Laboratory. It was used at Berkeley Labs 88-Inch Cyclotron for two years before being moved to Argonne. During its tenure in Berkeley, many exciting discoveries were made, including:
- Details of superdeformation in nuclei. Superdeformation occurs when quantum shell effects help stabilize a football shape (2:1 axis ratio) in certain nuclei. Superdeformed nuclei, prevalent in several regions of the chart of the nuclides, have been found to display some amazing properties.
- Identical Bands. Scientists have discovered that sequences of ten or more identical photons are asso ciated with different bands in neighboring nuclei. This comes as a great surprise; it has long been believed that the gamma-ray emission spectrum for a specific nucleus represents a unique fingerprint. Explaining identical bands is now in the hands of shell model theoreticians.
- Magnetic Rotation. Magnetic rotation occurs in nearly spherical nuclei. It is characterized by sequences of gamma-rays reminiscent of collective rotational bands but with a quite different character. Namely, each photon carries off only one (rather than two) units of angular momentum and couples to the magnetic rather than electric properties of the nucleons. This is a new form of quantal rotor that is not fully understood at present.
Now Gammasphere has been moved to Argonne National Laboratory where it will be exploring a new range of phenomena. In particular, it will investigate the structure of nuclei far from the line of stability. At Argonne, scientists are studying the fundamental question: why certain combinations of protons and neutrons form stable nuclei while other combinations do not. In other words, it should help us understand what causes the limits on the range of 270 stable isotopes that appear in the chart of the nuclides.