A world-class gamma ray spectrometer called GRETINA has returned to Argonne National Laboratory (CSA CSM) for a second run expected to last 18 months. Scientists will use the instrument in conjunction with Argonne’s Fragment Mass Analyzer, where it will enable studies of nuclei at or near the proton drip line and along the rp process, one of the pathways through which stars synthesize heavier elements. GRETINA was previously installed in the Argonne Tandem Linac Accelerator System (ATLAS), from 2013 to 2015, contributing to a number of findings, including one that confirmed that a particular nucleus is shaped like a pear.

“We want to go to the point where nuclei no longer exist,” Mike Carpenter, an Argonne physicist, “so we can determine the shapes at the drip line and compare what we observe to theoretical models. We want to understand the structure of nuclei up to the point where they are ready to essentially fall apart.”

The promise of high-precision instruments like GRETINA excites scientists like Carpenter and Argonne’s Shaofei Zhu, who will now have a richer view of the dynamics of stretched nuclei or those whose components can barely hold together. “GRETINA is one of the first systems to use gamma-ray energy tracking to trace the path of gamma rays emitted from nuclear reactions through crystals, resulting in unprecedented energy resolution and efficiency,” says Zhu, user liaison for ATLAS. “This technology has tremendous potential for high-sensitivity imaging, which has applications in medical diagnostics and treatments, homeland security—such as cargo inspections—and, of course, nuclear physics.”

Coupled with Gammasphere, Argonne will have complementary instruments that allow scientists to see what a nucleus looks like in exquisite detail, not by imaging but by studying the gamma rays—high-energy light particles—that provide an indirect view of the shapes of the nucleus. Gammasphere, the world’s most powerful spectrometer for nuclear structure research, is especially useful for collecting gamma ray data created by the fusion of heavy ions. However, unlike Gammasphere, GRETINA can merge the partial energies of a gamma ray that scatters between two crystals, increasing its sensitivity.

One of the pursuits of Zhu and Carpenters’s team is to understand the occurrence and physics of “super deformation” in nuclei. Scientists know that nuclei can go into very elongated modes in the fission process, but now they also know there are ways to make them super-elongated with input of energy and angular momentum as well. “GRETINA will enable us to pin down where these super-deformed modes exist and how they transition into normal nuclear shapes,” says Carpenter, who leads Gammasphere research. “With this instrument, we’re able to study the structure and stability of nuclei under various conditions. The new capabilities provided by gamma ray tracking give us large gains in sensitivity for a large number of experiments, particularly those aimed at nuclei far from beta stability.”

GRETINA is about the size of a large SUV. Like Gammasphere, it is built from large crystals of hyper-pure germanium, an element with chemical properties similar to those of carbon, silicon, tin and lead. The instrument consists of 28 highly segmented coaxial germanium crystals, each divided into 36 electrically isolated elements. GRETINA combines four crystals in a single cryostat, enabling the device to maintain an extremely low temperature to form a quad-crystal module. The configuration allows GRETINA to detect gamma rays from the decay of nuclei. Nuclear physicists can then infer the size and shape of the nuclei based on the energies of the emitted gamma rays. “Of all the detectors we can use to measure gamma rays, germanium has by far the best energy resolution,” Zhu says.