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March 31, 2006
Catching Some Rays
On Track to the Launch of GRETINA, the World's Most Powerful Gamma-Ray Detector

To understand the fate of radioactive elements in the shock wave of an exploding star, or to learn the limits of stability for superheavy elements, or to answer many other interesting questions about what happens when atomic nuclei collide, fuse, spin, or break into pieces, a good way to start is by catching some gamma rays.

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I-Yang Lee with the Gammasphere

The record for sensitivity and precision in detecting gamma rays emitted during nuclear events has long been held by the Gammasphere detector, developed at Berkeley Lab's Nuclear Science Division (NSD) and now housed at Argonne National Laboratory. But even before Gammasphere was commissioned in December, 1995, some of its creators , including NSD's I‑Yang Lee, were contemplating a far more sensitive detector. It would come to be known as the Gamma Ray Energy Tracking Array, or GRETA.

GRETA's first stage, dubbed GRETINA, will incorporate the detector's essential elements and will be capable of expansion to full size; even by itself, GRETINA will greatly outperform Gammasphere in many applications. A collaboration of national laboratories and universities led by Lee at Berkeley Lab is steadily working to complete GRETINA in 2010.

Crystal Power

"Both Gammasphere and GRETINA are based on high-purity germanium crystals, a material that combines efficiency for  detecting incident gamma rays, high resolution, and the capacity to cleanly distinguish higher-energy from lower-energy events," Lee explains.

In the detector the crystals are cooled by liquid nitrogen and arranged facing inward, forming a sphere that surrounds the area where a beam of nuclei from an accelerator strikes a target to create specific nuclear events.

But there's a catch, Lee says. "The biggest pure germanium crystals it's possible to grow commercially are only about three inches long and three inches wide. That's not big enough to stop many gamma rays. A gamma ray can bounce off an electron inside the crystal and keep on going right through it, depositing only some of its energy. So the information is incomplete and could be misleading."

In the Gammasphere, the solution to this problem is a technique called Compton suppression. By surrounding the crystals with a shield of scintillators to catch deflected (Compton-scattered) gamma rays, these can be identified, "and we can reject all the gamma rays that don't stop in the germanium," Lee explains. "Unfortunately that means throwing away 80 percent of what could be useful signals."

GRETINA will recover those lost signals through the principle of gamma-ray energy tracking. The germanium detectors will be close-packed, with no intervening scintillator material; a gamma ray that passes through one of them will likely stop in one of its neighbors. By summing the signals deposited by a single gamma ray in both detectors, its full energy is recovered.

The prototype design called for modules with three germanium crystals, each divided into 36 segments.

"The problem is that more than one gamma ray — often as many as 20 — may hit the array of detectors at nearly the same time," Lee says. "To tell them apart, we divide each detector into 36 sectors, each with its own electronics."

The crystals are carved in a hexagonal shape, with each of the six sides divided into six segments. Says Lee, "With the capability of recording up to 36 signals from a single crystal, each within a centimeter-sized cube, we can achieve resolution of a millimeter or two in the position of each gamma ray and an accurate record of its energy."

Lee says this leap in detector power has required three kinds of technological advance: "in computing power, in fast electronics, and in crystal segmentation. When we began GRETA 10 years ago, none of these were available."

Working groups have been established to address these core technical challenges and other issues within the GRETINA collaboration, which is presently comprised of six national laboratories and nine universities, including Lawrence Livermore, Argonne, and Oak Ridge National Laboratories, and Michigan State and Washington Universities.

Detector development is chaired by Augusto Macchiavelli of Berkeley Lab's NSD. Segmented detectors — although with only two segments — were first tried with Gammasphere; the technique is now used by germanium crystal manufacturers. David Radford of Oak Ridge chairs the electronics group; much of the development of fast-signal digitizers has been carried out by Berkeley Lab's Engineering Division.

Mario Cromaz of Berkeley Lab's NSD leads the software group to develop programs that can record and process the flood of electronic signal information. Kai Vetter of Lawrence Livermore has made important contributions to the research and development effort, including development of algorithms and testing of detector prototypes.

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A test module with three segmented crystals has been subjected to rigorous tests.

"After simulation studies and measurements on test modules provided by the manufacturer, Canberra Eurisys, we decided on a new design that optimizes detector efficiency, cost, and reliability," Lee says. "Each module will have four crystals instead of three, and the field-effect transistors in the electronics won't be refrigerated. The so-called 'warm FET' design will greatly reduce down-time when repairs are needed. Now we're ready," he says. "We have the technology in hand."

The Road Ahead

The Department of Energy defines several "critical decision" points for new experimental facilities. GRETINA's deputy project manager, Sergio Zimmermann of Berkeley Lab's Engineering Division, notes that at each of these stages (labeled CD-0 through CD-4) DOE requires a certain level of development, "including project execution plans, safety analyses, firm budgets, and realistic schedules. With approval at CD-3, the budget, schedule, and technical scope of the project are baselined, and changes require specific procedures."

GRETINA is in the interesting position of pursuing different development stages simultaneously. Part of the design has been approved (CD-2A), although not all details have been locked; meanwhile, because the segmented crystal modules are complex and require a long lead-time before completion, the construction phase has already begun (CD‑3A), with the purchase of the first three modules.

Says Zimmermann, "We are on good track to achieve the remaining critical decisions." 

GRETINA's first finished module will be delivered in November, 2006. After rigorous testing, production of two more modules will commence. The remaining modules and construction of the entire GRETINA system will begin in July, 2007. When GRETINA is completed in 2010 it will be commissioned at Berkeley Lab's 88-Inch Cyclotron. 

GRETINA will consist of seven identical modules with four germanium crystals in each, for a total of 28 detectors in all. The crystals have irregular hexagonal cross sections; two crystals in each module are of one matching shape and the other two are of a slightly different matching shape. The modules will be mounted in an aluminum sphere, focused inward on the experiment's interaction region; even this relatively modest number of detectors will outperform the Gammasphere. More modules can be added until the full GRETA array of 30 modules (120 crystals) is reached.

After commissioning at the 88-Inch Cyclotron, GRETINA will travel to other major nuclear research laboratories, each with a different experimental set-up. At Oak Ridge, experiments will be done with radioactive beams. Argonne will use GRETINA with a linear accelerator, and Michigan State will use it with coupled cyclotrons.

A beam striking a target (left) causes some nuclei in the beam and target to fuse; a rapidly spinning compound nucleus thus formed may immediately fission (upper path) or may emit protons, neutrons, and gamma rays before reaching a perhaps temporarily stable ground state (at right). Captured gamma rays reveal the structure and state of the nucleus.

Experiments with neutron-rich nuclei are planned at Oak Ridge to study the astrophysical r-process (r is for rapid), the route by which many elements heavier than iron are formed. Fusion in ordinary stars can't produce elements heavier than iron. When a star explodes, however, it can release a flood of neutrons that collide with matter in the neighborhood to form neutron-rich nuclei. These rapidly decay to stable forms, which comprise about half the elements heavier than iron. 

"At Oak Ridge they make beams of neutron-rich nuclei by hitting a uranium target, then accelerate the beams to low energy," Lee explains. "The best way to determine the properties of these r-process nuclei is by measuring the gamma rays they emit as they rapidly decay. GRETINA will be particularly useful in these studies because of its higher efficiency with low-intensity beams." 

The radioactive beams produced at Michigan State University present a different challenge. High-velocity, neutron-rich nuclei are produced by "projectile fragmentation" when a heavy-ion beam from the coupled cyclotrons is sent through a thin foil; the desired nuclear species is separated from other fragments by magnetic fields.

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A cutaway sketch of GRETINA

"High-velocity beams cause problems because of a phenomenon called Doppler broadening," Lee says. Like the pitch of a passing siren, the apparent frequency of a gamma ray emitted by a speeding nucleus is smeared out in the detector. To correct the measurement, the angle at which the gamma ray strikes the detector must be known precisely. "GRETINA will be able to determine the angle of the hit to high accuracy, 40 times better than existing detectors."

GRETINA will be this country's premier gamma-ray instrument for nuclear science. Only the AGATA demonstrator will approximate GRETINA's sensitivity and resolution (AGATA is a detector similar to GRETA, under construction by a consortium of 10 European nations). For studies of unusual nuclei GRETINA, and eventually GRETA, will have few rivals.

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