Pursuing the Mysteries
of Matter

Nearly two hundred researchers from institutions all over the world have come to LBL to work with a detector that opens a new window on the unseen, enigmatic interactions that take place inside the atomic nucleus. This new detector, the Gammasphere, will provide a 100-fold increase in sensitivity over previously existing detector systems.

Much of the research at the Gammasphere focuses on "superdeformed" nuclei. These unusual football-shaped nuclei rotate with much more regularity than ordinary nuclei, making them ideal for studying subtle features of nuclear rotation.

Germanium semiconductor detectors fan out from the target chamber of the Gammasphere, which will provide a 100-fold increase in sensitivity over previously existing detector systems.

Though protons and neutrons were discovered (respectively) in 1909 and 1934, exploring just how they combine and behave in forming an atomic nucleus remains a daunting challenge. Nuclei are extremely minute -- the diameter of a nucleus is about 1/100,000th the diameter of the whole atom, and they can be made to rotate extremely rapidly, using particle accelerators such as LBL's 88-inch cyclotron. Gammasphere detects gamma rays given off as one of these nuclei spins down, allowing experimenters to learn about nuclear structure and properties.

The Gammasphere consists of an array of germanium semiconductor detectors that surrounds the spherical surface of small target chamber. A beam from the 88-inch cyclotron is directed into this chamber onto a metal foil target, creating collisions between nuclei. When two nuclei collide off-center, they fuse to create a rapidly spinning, heavier nucleus. The rate of spin is extraordinarily fast -- in its one-billionth of a second lifetime, a superdeformed nucleus will spin about as many times as the earth has spun since its creation some 4 billion years ago.

The laws of quantum mechanics dictate that the spinning nuclei cannot slow down smoothly, but instead must shed energy by hopping down from one energy level to the next.

Nuclei having different numbers of protons and neutrons, or different arrangements of these particles, emit their own characteristic spectrum or "band" of gamma rays which Gammasphere detects with unprecedented sensitivity. These unique signatures paint a picture of the properties of the nucleus, including the size, shape, symmetry, and flow patterns. Such information could help solve one of the reigning mysteries of nuclear science -- the phenomenon called "band twinning" in which identical gamma spectra are obtained from superdeformed nuclei containing different numbers of protons and neutrons.

Gammasphere is now being phased into full scale operation, and should have all of its 110 detectors installed by the Fall of 1995. In its "early implementation" phase it has operated with 30 to 55 detectors for almost 5000 hours at the 88-Inch Cyclotron, which has had less than 2% downtime and 7% tuning time -- a remarkable performance.

-- Jeffery Kahn