A New Key to Unlocking the Nucleus
Berkeley Lab's Nuclear Science Division, a leader in building high-resolution gamma-ray detectors for studying nuclear decays, is now heading up a multi-institutional collaboration to build the most sensitive such device ever, the proposed Gamma-Ray Energy Tracking Array, or GRETA.
GRETA's first stage is GRETINA, which will use seven detector modules instead of the 30 modules GRETA will eventually require. The business end of each module is an array of four closely packed, segmented germanium crystals. Signals from gamma rays hitting the crystals are collected by each segment and conveyed to outside circuit boards for computer analysis.
Late in December GRETINA's first module arrived at Berkeley Lab from its manufacturer, Canberra Eurysis in France. Before ordering the remaining modules, at a cost of $1.3 million each, the first module is being rigorously tested to make sure it lives up to its specifications.
"First we have to test for mechanical tolerance," says I-Yang Lee, who heads the GRETINA project. "The crystals have to fit together with less than half a millimeter of play, and there can't be any deformation under pressure, or while the crystals are being cooled down or warmed up."
Mechanical tests were performed by the Engineering Division's Bob Connors using a new Zeiss ACCURA coordinate measuring machine (CMM) to measure tolerances with a touch probe, a snorkel-like device on which a mechanical finger is mounted that can reach out to feel surfaces and edges and measure their positions with an accuracy of three micrometers (millionths of a meter) per meter. Supported on air bearings, the articulated touch probe moved around and over the module in response to programmed instructions, or as Connors flew it by hand using a joystick.
After physical measurements were taken in both warm and cool modes, the module was transferred to Bldg 88 for tests of the crystals' energy resolution, the accuracy of the layout of segments, and the electrical response to gamma rays detected at numerous points within each crystal.
"Thirty-six quadrilateral segments are formed on the surface of each crystal by implanting boron ions in the germanium," Lee explains. This segmentation is the key to GRETINA's sensitivity. Ideally, no signal will ever have to be discarded because every gamma ray can be tracked, including those that deposit only part of their energy in one crystal and the rest in an adjacent crystal.
The final step in testing is to characterize the module's position sensitivity and electrical responses. "Each of GRETINA's crystals is capable of resolving 300,000 points to within a millimeter," says Paul Fallon, the Nuclear Science Division's deputy director. "We have created a model of what we ought to see at key points, and we do coincident measuring. What we see gives us the specific characteristics of the individual crystals in the array."
GRETINA's first module has passed all tests so far, on track to ordering the remaining modules, with construction to begin this summer and completion scheduled for 2010.
For more about GRETINA go here.
— Paul Preuss
|Published by the Berkeley Lab Communications Dept., Creative Services Office