|This year's American Physical Society meeting in Atlanta,
March 21 to March 25, celebrates the centennial of that distinguished scientific society
and is billed as "the largest physics meeting ever." Among the dozens of papers
being presented by Berkeley Lab researchers is a new technique for the study of unstable
atomic nuclei developed by I-Yang Lee and his colleagues in the Nuclear Science Division.
"Using the 8-pi Detector at the 88-Inch Cyclotron, we've succeeded in getting gamma-ray spectra from low-Z, neutron-rich nuclei at high resolution, by looking at collision fragments of the target instead of the beam," says Lee. "It's a new way of studying this kind of nuclei, and we've had some interesting surprises."
Z stands for atomic number, the number of protons in a nucleus. Above Z equal to 20, neutron-rich nuclei are common; as the number of protons in the nucleus increase, the number of neutrons increases faster. But smaller neutron-rich nuclei exist only fleetingly in nature and must be created to be studied, typically by bombarding a target with an accelerated beam of ions.
"The most abundant isotope of magnesium has 12 protons and 12 neutrons," says Lee, noting that a magnesium nucleus with 16 neutrons is exceedingly rare, having a half-life of less than a day. "But when we split a vanadium nucleus with a beam of carbon ions, one of the fragments is heavy magnesium, with high excitation. As this fragment decays we can observe the energy and distribution of the emitted gamma rays to learn about its nuclear shell properties, angular momentum, shape, and so on."
The catch is, Lee explains, typically the nuclei of interest are studied as fragments in the cyclotron beam, meaning they are traveling at high velocity relative to the gamma-ray detector. The resulting gamma-ray spectrum is smeared by the Doppler effect, because gamma rays traveling in the same direction of the fragment have more energy when they hit the detector, while gamma rays in the opposite direction have less. Moreover, the cross-section is low: with relatively few nuclei of interest in the beam, the sparse gamma-ray signal is hard to resolve.
Lee calls this a two-step reaction, in which nuclei are created in a collision at the target and reacted downstream in a second target. He and his team decided to study one-step reactions, looking at fragments that remained in the target and decayed in place.
"Because the fragments have little velocity, we get sharp gamma-ray lines," he says. "Also the cross-section is larger, with more fragments of interest and more gamma rays emitted." In addition to magnesium (Z equal to 12), Lee and his colleagues have studied neutron-rich isotopes of vanadium (Z equal to 23), titanium (Z equal to 22), calcium (Z equal to 20), and argon (Z equal to 18). One surprise has been the very high spin of the target fragments observed, with angular momenta up to 12 h-bar (quantized units of angular momentum), instead of six to eight h-bar as expected.
"In these experiments at the 8-pi we've established that it's possible to do gamma-ray spectroscopy of neutron-rich nuclei using target fragmentation reactions," Lee says. "We're looking forward to doing experiments with Gammasphere in the year 2000, which will allow us to observe nuclei even richer in neutrons and farther from stability."
For more information on the study of nuclear structure and reactions in Berkeley Lab's Nuclear Science Division, visit http://www-library.lbl.gov/docs/LBNL/397/64/Overviews/NSR.html.