Gammasphere, world's most powerful gamma ray detector, dedicated

December 8, 1995

By Lynn Yarris, [email protected]

It can truly be said that the Dec. 1 dedication of Gammasphere was an earthshaking event. With a crowd gathered in the High Bay at the 88-Inch Cyclotron waiting for the ceremony to begin, the Hill was rocked by a 3.5-magnitude earthquake centered about four miles north of Berkeley along the Hayward fault.

No damage was done to what is now -- even at partial capacity--the world's most powerful instrument for detecting gamma rays. Only 79 of Gammasphere's 110 high-resolution germanium crystal detectors are in place but experiments on the Lab's newest national user facility are already underway.

When Gammasphere is operating at full power (sometime early next year), its honeycomb array of germanium crystal detectors and surrounding bismuth-germanate (BGO) scintillation counters will make it a hundred times more sensitive than any previous generation of gamma ray detector systems. This leap in technology should open new vistas in the study of nuclear structure.

Berkeley Lab Director Charles Shank hosted the dedication ceremony, which featured speeches from the individuals who were instrumental in bringing Gammasphere into being -- Frank Stephens, the Nuclear Science Division physicist who has guided the project and was named its first director; James Symons, former NSD director under whose watch Gammasphere was constructed; Robert Janssens, chairman of the Gammasphere Users Committee; and Dennis Kovar of DOE's Division of Nuclear Physics, which funded the project.

During the ceremony, Stephens handed out plaques in recognition of individual contributions to the realization of Gammasphere. The plaques contained an inscription that read, in part, "This array represents a major step forward in the design and fabrication of world class gamma detectors."

Gammasphere was conceived by nuclear scientists at Berkeley Lab in 1987 and formally proposed to DOE in 1988. Funding for the $20 million construction project, which had strong scientific support, was approved in 1991. A major purpose of Gammasphere is to enable scientists to observe the gamma radiation emitted by rapidly spinning "superdeformed" atomic nuclei. Such observations yield information on nuclear structure that could not be obtained from conventional studies.

For all that is known about the structure of the atomic nucleus, there are still a number of important unanswered questions. Scientists especially want to know more about what happens to atomic nuclei under the extreme physical conditions that can exist on earth in accelerators, or in white dwarfs, neutron stars, and other exotic objects in the cosmos.

One of the reasons for building Gammasphere here is that its design is based on Berkeley Lab's High Energy Resolution Array (HERA), which in 1985 became one of the first detectors to employ a system for suppressing Compton radiation. In observations of rapidly spinning superdeformed nuclei, the critical gamma rays are those emitted as the nuclei return to normal. The spectral lines of these weak emissions can be obscured by radiation from Compton scattered photons -- gamma photons that strike electrons and lose some of their energy. Like HERA but with much greater selectivity, Gammasphere can detect and reject signals from scattered photons.

Gammasphere was built with the active participation of scientists at Argonne, Lawrence Livermore, and Oak Ridge national laboratories, among other institutions. It is scheduled to run about 50 percent of the time at the 88-Inch Cyclotron, which was selected in 1990 by DOE as the initial host accelerator. Gammasphere was built as a transportable system and may in the future be moved to other locations.