ORNL, partners launch first experiments using new facility to make cosmic isotopes on Earth

Two decades in the making, a new flagship facility for nuclear physics opened on May 2, and scientists from the Department of Energy's Oak Ridge National Laboratory have a hand in 10 of its first 34 experiments. ORNL researchers and their partners at other national laboratories and universities launched the first experiment on May 11.

The Facility for Rare Isotope Beams, or FRIB, a DOE Office of Science user facility at Michigan State University, will produce more than 1,000 new rare isotopes. These are not your parents' isotopes, such as the americium-241 in smoke detectors or the fluorine-18 in PET scanners.

FRIB's rare isotopes have abnormal proton-to-neutron ratios, so they are unstable and prone to decay. Many exist for mere fractions of a second, such as magnesium-40, calcium-55 and nickel-78. Until now, they were made only in stellar explosions and neutron star mergers. Now, what is designed to be the world's most powerful heavy-ion accelerator makes them here on Earth.

"FRIB will launch a new era of discovery," said Witold Nazarewicz, FRIB's chief scientist and a distinguished professor of physics at MSU. "ORNL has provided long-term expertise and unique instrumentation that will help FRIB deliver new knowledge about what holds atomic nuclei together, how elements are created and how to use nuclei for societal benefits."

First up

FRIB's first experiment is led by Heather Crawford of DOE's Lawrence Berkeley National Laboratory with partners at ORNL, the University of Tennessee, Knoxville, and elsewhere. The experiment employs the FRIB Decay Station initiator, or FDSi, a modular, multidetector system that is extremely sensitive to rare isotope decay signatures.

"The FDSi is an assembly of the best detectors available in the scientific community within an integrated infrastructure for FRIB nuclear decay studies," said ORNL's Mitch Allmond, who manages the project. A key component of the FDSi originated from seminal work on germanium detector design by ORNL's David Radford, including the CLover Array for Radioactive ION Beams, or CLARION.

FDSi allows scientists to quickly mix and couple detectors to capture different types of nuclear data. "All of these individual detectors existed in different groups until now," Allmond added.

FDSi is now at FRIB and is coupled to the Versatile Array of Neutron Detectors at Low Energy, or VANDLE, a neutron spectroscopy instrument developed under Robert Grzywacz, a UTK physics professor with a joint appointment at ORNL. VANDLE provides energy information about neutrons. FDSi's CLARION-VANDLE instrument duo is the first stop on FRIB's beam line.

Its last stop is the Modular Total Absorption Spectrometer, or MTAS, a one-ton gamma-ray detector. "It is FRIB's big catcher's mitt," Grzywacz said. As the beam lands in MTAS, the detector efficiently measures all its energy as one lump sum. MTAS was developed for beta-decay studies of neutron-rich and proton-rich nuclei under ORNL's Krzysztof Rykaczewski. The instrument is installed at FRIB for experiments planned for June to probe the decay of calcium-55, an important isotope in astrophysics.

Other ORNL instruments and resources available to FRIB's international user community of nearly 1,600 researchers include:

• GODDESS detector system. Led by ORNL's Steven Pain, GODDESS allows measurements of diverse reactions driving element creation in stars. At its heart is the Oak Ridge Rutgers University Barrel Array, an arrangement of position-sensitive silicon detectors to spot charged particles.
• Jet Experiments in Nuclear Structure and Astrophysics, or JENSA. Led by ORNL's Kelly Chipps, JENSA is a unique gas jet system for nuclear reaction studies. Chipps spearheaded its use with radioactive beams.
• SEparator for CApture Reactions, or SECAR. This recoil separator allows direct measurements of reactions that power exploding stars. ORNL's Michael Smith led a 1991 proof-of-principle demonstration that recoil separators could measure such reactions. Smith and Chipps are members of SECAR's project team.
• Computational Infrastructure for Nuclear Astrophysics, or CINA. Developed at ORNL by Smith and Eric Lingerfelt, it is the first cloud computing system for nuclear astrophysics. CINA helps scientists understand how their FRIB results relate to cosmic events.

Electromagnetic switchyards

To accelerate stable beams of heavy ions, cryomodules holding superconducting radio-frequency resonators guide positively charged atoms through segments of FRIB's heavy-ion accelerator, a 1,600-foot-long pipe bent into a paper-clip shape.

Inside the accelerator, the ions pick up speed. When they reach half the speed of light, they smash against the target, which breaks the nuclei into pieces, each having fewer protons and neutrons than nuclei in the original stable beam. An array of superconducting electromagnets purifies the resulting beams of radioactive isotopes at FRIB. "You basically create a cocktail of many residual products," Allmond said.

Some rare isotopes are pulled out for applications. Others continue through a series of magnets that filter out undesirable isotopes. Electromagnetic "switchyards" steer purified beams to end stations that maintain them for experiments.

Scientists can accelerate, stop and reaccelerate the beams for experiments at a wide range of desired energies.

ORNL's DNA in FRIB

Many FRIB experiments grew out of pioneering work in ORNL's Physics Division to create beams of radioactive ions.

"Oak Ridge National Laboratory has deep roots in studies of ion and radioactive ion beams," ORNL Physics Division Director Marcel Demarteau said at FRIB's grand opening. He spoke after a ribbon cutting by Energy Secretary Jennifer Granholm.

On March 18, 1962, the first light-ion beam circulated in the Oak Ridge Isochronous Cyclotron. In the 1970s, the addition of a tandem accelerator enabled heavy-ion beams. By 1997, a novel reconfiguration allowed for the first-ever radioactive ion beams. Renamed the Holifield Radioactive Ion Beam Facility, or HRIBF, it became a resource for nuclear physicists worldwide. Nazarewicz, FRIB's chief scientist, served as HRIBF's scientific director from 1999 to 2012.

"HRIBF was a key component of the worldwide program with rare isotopes and played a crucial role in the pathway to FRIB. It was the only U.S. facility providing the capability of accelerated beams of short-lived nuclei," Nazarewicz said.

"We did things there which were simply impossible anywhere else," Grzywacz said. Those feats included the first measurement with a reaccelerated unstable beam in North America and the first acceleration of neutron-rich fission fragments of tin-132 to confirm its doubly magic nature, which gives a nucleus extraordinary stability.

HRIBF users had access to high-quality beams of 200 rare isotopes. They made seminal measurements with instrument arrays. Moving detector readouts to electronic systems, they pioneered digitization for nuclear physics.

In 2012, HRIBF was decommissioned but the instrumentation was still available for use. The researchers have since upgraded instruments for installation at other facilities, such as Japan's RIKEN, Argonne National Laboratory's ATLAS and FRIB.

In 2016, the American Physical Society declared HRIBF a historic physics site. Its DNA lives on in FRIB.

ORNL's Chipps, chair of the FRIB User Organization, also spoke at FRIB's opening. "FRIB will position us to gain incredible new insights into the nature of the universe around us," she said.