A Moving Experience

Going mobile makes detector more versatile

by Dave Jacqué

Gammasphere, listed in the Guinness Book of World Records as the world's most sensitive gamma-ray detector, was already a seasoned traveler, having crossed the United States from Lawrence Berkeley National Laboratory to Argonne by truck.

But recently the 20-ton instrument went truly mobile so that it can be moved around the experiment hall where it resides to meet the needs of physicists.

Previously, Gammasphere was rooted to a spot between an instrument called the Fragment Mass Analyzer (FMA) and a beamline from ATLAS, the Argonne Tandem-Linac Accelerator System. Now, with a couple of days' work, Gammasphere can be moved about 20 feet across the floor to a different beamline.

“ The ability to move Gammasphere gives the user community a substantial amount of flexibility,” said Kim Lister, who leads the Low Energy Research Group in Argonne's Physics Division. “Gammasphere can be an impediment to some kinds of experiments that require the FMA, and vice versa.”

“ The things we're looking for with the FMA are so exotic, they may be produced only once every couple of hours,” Lister said. “And you can't just crank up the beam intensity to make more, as that would probably damage the sensitive Gammasphere detectors.”

Moving Gammasphere away from the FMA allows experimenters to use new kinds of auxilliary detectors in conjunction with the device and makes existing instruments easier to use. For example, Gammasphere was recently used in combination with the Compact Heavy Ion Counter, or CHICO. Designed and built by researchers at the University of Rochester's Nuclear Structure Research Laboratory specifically for use with Gammasphere, CHICO sat in the center of Gammasphere to measure the mass and scattering angles of heavy ions while Gammasphere sought out the gamma rays they emitted. In Gammasphere's previous position, wedged between a concrete wall and the FMA, the skills of a contortionist were required to install and adjust CHICO. In the new location, CHICO was accessible from almost any angle.

Mechanical engineer Bruce Zabransky and chief technician John Rohrer, both of Argonne's Physics Division, handled much of the planning and preparation for Gammasphere's first trek across the experiment hall. Preparations included building an exact scale model of the experiment hall floor and the obstacles Gammasphere had to avoid. A forest of data cables and liquid nitrogen lines had to be disconnected, and without these lifelines, the relocation team had only a few hours to move Gammasphere. If its germanium crystals are allowed to warm above a certain point, Gammasphere loses a significant portion of its energy resolution and selectivity for rare events.

“ The only remedy is to anneal the detectors, which involves taking them all out and baking them in our annealing factory,” Lister said. “The whole process would take about three months.” Rolling on heavy-duty industrial casters across a steel-plate “dance floor” that provided a smooth rolling surface, Gammasphere made its way 20 feet to its new location. The move required about eight hours and a crew of 20.

Lister said Gammasphere will probably move back and forth across the experiment hall once a year or so. The scheduling will depend on the needs of the user community.

The efficiency and high resolution of Gammasphere would be major assets for research with the proposed Rare Isotope Accelerator (RIA). RIA, proposed in the Department of Energy's Strategic Plan but not yet funded, will enable broad-based physics research with intense, high-quality energetic beams of short-lived isotopes of all chemical elements, from the very lightest to the very heaviest. The isotopes will be available over a range of energies, from thousands of electron volts per particle for radioactive decay studies and trapping, through millions of electron volts (MeV) for astrophysics, to tens of MeV for reactions studies and hundreds of MeV for fast fragment physics. Gammasphere is optimized for studies at the lower energies, where it will remain an important tool.

Gammasphere's unique sensitivity will enhance research in several of RIA's target areas, Lister said, so it's likely to move from beamline to beamline once or twice a year at RIA just as it now does at ATLAS.