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February, 2007
Keys to the Nucleus: Testing the First GRETINA Module

The best way to study the existence of the heaviest elements, nucleosynthesis in exploding stars, and other phenomena peculiar to the atomic nucleus is to create customized nuclei in an accelerator like Berkeley Lab's 88-Inch Cyclotron, then capture and analyze the gamma rays these nuclei emit when they disintegrate. The Lab's Nuclear Science Division (NSD) has been a leader in building high-resolution gamma-ray detectors and was the original home of the Gammasphere, the world's most sensitive. Now NSD is leading a multi-institutional collaboration to build Gammasphere's successor, the proposed Gamma-Ray Energy Tracking Array, or GRETA.

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The GRETA gamma-ray detector will use 30 modules, each containing four segmented germanium crystals. GRETINA, the first stage of GRETA, uses seven of these modules.

GRETA's first stage is GRETINA, which will use just seven detector modules, less than a quarter of the 30 modules GRETA will eventually require. Each of the 80-centimeter, 45-kilogram modules will be mounted in an aluminum sphere, pointing inward. The business end of each module is an array of four close-packed germanium crystals. Dewars of liquid nitrogen cool the crystals, and signals from gamma-rays hitting them are conveyed by a network of flexible integrated circuits to outside circuit boards for computer analysis.

Late in December, 2006, GRETINA's first module arrived at Berkeley Lab from its manufacturer, Canberra Eurysis in France. Before ordering the rest, at a cost of $1.3 million each, the mandate is to make sure the first module lives up to its specifications.

"First we have to test for mechanical tolerance," says I-Yang Lee, who heads the GRETINA project. "To pack the modules so they can form a virtually perfect hollow sphere means they have to fit together with less than half a millimeter of play." The detector operates in a vacuum at the temperature of liquid nitrogen, but to repair neutron damage they must periodically be annealed at high temperature. Lee says, "There can't be any deformation under pressure, or while the crystals are being cooled down or warmed up."

The mechanical tests were performed in Building 77, where the Engineering Division's Bob Connors uses 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.

"We measured features on the module's rigid mounting flange and constructed a coordinate system, then scanned thousands of points along the crystals and other key parts. The CMM has a measuring envelope of 1,600 millimeters in the X coordinate, 3,000 in the Y, and 1,400 in the Z, which is plenty large enough for the detector," Connors says. The module in its frame stands on a 12.5-metric-ton slab of granite. Supported on air bearings, the articulated touch probe moves around and over the module in response to programmed instructions, or with Connors flying it by hand with a joystick.

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Spacer image Engineering's Bob Connors matches the actual physical measurements of the first module to the GRETINA design specifications, using a Zeiss coordinate measuring machine.

As the probe moves and touches different points on the module it builds up a 3-D map of the device, a map which can be directly superimposed on a computer-aided design (CAD) plan of the module residing in the computer. Any deviations from design specs are immediately apparent on the monitor.

Connors says one of the most challenging measurements was the surface of the partial sphere formed by the concave ends of the four germanium crystals. "In the past, measuring each point in absolute space on a surface like this would have taken us hours, but with the new CMM we can take each measurement virtually instantly," Connors says.

After physical measurements of the module 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 on the crystals, 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, each segment slightly separated from the others," Lee explains. "A gamma ray passing through a crystal, or stopping in it, scatters Compton electrons when it collides with germanium atoms. Its energy and origin can be determined by tracking and summing the charges collected by the boron-doped surface of each segment."

Segmentation is what gives GRETA (and GRETINA) the ability to far outperform Gammasphere in sensitivity. Ideally, no gamma-ray signal will ever have to be discarded, because every gamma ray can be tracked and its trajectory reconstructed, including those that deposit only part of their energy in one crystal and the rest in an adjacent crystal.  

The exact position of each segment's edges is determined using a collimated source of gamma rays from radioactive americium-241. As the pencil beam of gamma rays is passed across the detectors, electrical signals are measured from individual sectors. The width of the gamma-ray peak is a measure of the crystal's energy resolution in each sector.

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The crystals nested in each of GRETA's modules are machined to form a spherical surface, which will surround the interaction point where exotic nuclei are created and then disintegrate.

The final step in testing is to characterize the module's position sensitivity and electrical responses, information that will be used to gauge its performance against that of all the modules that will be used in GRETINA and eventually GRETA. Signals from all segments in each crystal are measured together. Detailed pulse shapes from each sector are determined using two detectors, one that measures the energy of incoming gamma rays and another that measures the energy of gamma rays that pass through and exit the crystal.

"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. "But since it takes us a day to measure the electrical response at each point, we have to settle for measuring a subset. We have created a model of what we ought to see at key points, based on the design and what we learn from our initial tests. We do coincident measuring of the subset. What we see gives us the specific characteristics of the individual crystals in the array."

With GRETINA's first module having passed all tests handsomely, orders will soon be approved for the remaining modules and for construction of the entire device, to begin this summer. Completion of GRETINA, destined to be the country's premier gamma-ray detector for nuclear science, is scheduled for 2010.

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