|Even before the discovery of nuclear fission, theorists began
thinking of an atomic nucleus as something more complicated than a bag of hard particles.
The liquid-droplet model of the late 1930s pictured the nucleus as a quivering blob of
fluid; inherent was the possibility that it could rotate and deform.
Decades of study have resulted in far more subtle and sophisticated models which incorporate particle-like, fluid-like, and even gas-like components -- but the nucleus continues to surprise researchers. Depending on factors such as its mass, temperature, ratio of neutrons to protons, and angular momentum, the nucleus can exhibit a range of intricate and often puzzling behaviors.
One important influence on shape is "magic numbers." In the shell model of the nucleus, protons and neutrons fill shells from lower to higher energies; the first shell can accomodate up to two nucleons, the second eight, the third 20, and so on. Nuclei with full shells of protons, neutrons, or both are said to have magic numbers and, if unexcited, are usually spherical. Nuclei lying between the magic numbers are typically deformed even when at rest and are usually prolate, that is, football-shaped.
As nuclei spin, the balance of factors is perturbed, and at very high angular momenta nuclei may adopt odd shapes resembling peanuts, bananas, jumping jacks, or sea urchins, among others.
Not all these short-lived states proposed by theorists have been observed by experimenters. If they can be, usually it is because of radiation in the form of gamma rays, emitted by deformed nuclei as they revert to their normal shapes and lowest energy ground states.
"Nuclei spinning at 60 to 80 h-bar" -- h-bar is the quantized unit of angular momentum -- "are in principle accessible for observation," says Paul Fallon of the Nuclear Science Division (NSD), "although at spins over 70, they tend to fission." Fallon notes that the study of nuclei at high angular momenta started with pioneering experiments done at Berkeley Labs HILAC accelerator in the 1960s.
"Today its a wide, active field of nuclear science, with lots of room for many labs and many instruments," says NSDs David Ward, noting that there are active research programs on every continent, including more than half a dozen U.S. universities and national laboratories, and that students around the world have been attracted to this line of research.
Only about a millionth of a billionth of a meter across and spinning a billion trillion times a second, atomic nuclei have been described as "among the giddiest systems in nature." A high-spin nucleus, created in the 88-Inch Cyclotron by colliding an accelerated beam of ions with a thin foil target, sheds its excess rotational energy by emitting as many as 30 gamma rays in various directions, until -- about a billionth of second after the collision of beam and target -- it reaches the ground state.
After the departure of Gammasphere, the premier instrument in the world for studying spinning nuclei, which left the 88-Inch Cyclotron for Argonne National Laboratory in the fall of 1997, the 8-pi Spectrometer was installed in its place. Funded by Canada's National Science and Engineering Research Council and by Atomic Energy of Canada, Limited, the 8-pi was built by a consortium including scientists from Chalk River, McMaster University, the University of Toronto, and the Université de Montréal.
Like Gammasphere, the 8-pi is designed to record the position, energy, and timing of gamma rays emitted as a spinning nucleus slows down. Seventy-two bismuth germanate crystals surround the grapefruit-sized vacuum chamber where collisions of the beam with the target occur, their faces forming what would be a closed spherical shell -- except for 20 small, evenly distributed openings at the vertices of some of these faces, through which detectors of high-purity germanium can peer.
The shell of dense bismuth germanate gives an overview of each nuclear event, selecting high-spin events in which many gamma rays are emitted. It functions as what physicists call a calorimeter, measuring the total energy of the deposited gamma rays.
The pure germanium looks at the gamma rays with high energy resolution, determining in detail the path of nuclear decay. Because it completely surrounds the interaction region, the detector can measure the position, number, and timing of individual gamma ray hits, giving clues to the spin state of the nucleus before and after gamma rays were emitted.
"Three or four nuclei with different excitation energies may be created by the beam-target interaction," says Ward. "The detector functions as a kind of channel selector to tell us which nucleus weve made."
Which nucleus is made depends on what the target is made of and what ions are accelerated in the beam. The 88-Inch Cyclotron is uniquely qualified to create a variety of nuclei of different masses and spin states. Says Fallon, "The 88-Inch can accelerate almost anything" -- ample amounts of ions as light as hydrogen to as heavy as uranium.
"Our overriding goal is to determine the nucleuss rotational degrees of freedom," says Fallon, "to boil it down to basic symmetries and simple concepts." He notes that gamma-ray emission is governed by quantum rules; the pattern of gamma-ray energies is related to particular arrangements of neutrons and protons and associated with specific nuclear shapes.
Ward and Fallon say that experiments on the 8-pi Detector are not limited to the shapes of rapidly spinning nuclei; many other questions of nuclear physics may be addressed as well. Teams from all over the world have lined up to use the 8-pi Detector at the 88-Inch Cyclotron. Experiments started in the spring of 1998, and are now running as often as one every week.
Researchers using the 8-pi detector have devised a new way of studying unstable nuclei. Typically the nuclei of interest are studied as fragments in the cyclotron beam, meaning they are traveling at high velocity relative to detector, and the resulting spectrum is smeared. To eliminate this smearing, researchers are looking at fragments that remained in the target and decayed in place. The results have been encouraging.