ORNL Review Vol. 34, No. 2, 2001

Beam Technologies Enable HRIBF Experiments

The principal developer of beam-production technologies that have brought praise and awards to experimentalists at ORNL's Holifield Radioactive Ion Beam Facility (HRIBF) is Gerald Alton, leader of the Advanced Concept R&D Group in the HRIBF Section of ORNL's Physics Division. He and current group members Yuan Liu, Sid Murray, Charles Reed, and Cecil Williams have been the unsung heroes in enabling world-class experiments in nuclear astrophysics and nuclear structure physics at ORNL. Dan Bardayan received the Publication of the Year Award from UT-Battelle and the American Physical Society's Dissertation of the Year Award for 2000 for one of these experiments. HRIBF operations staff members, including Dan Stracener and Paul Mueller, were vital allies in implementing and validating the ideas coming out of Alton's group.

The Advanced R&D Group's responsibilities include selection of target materials, design of high-release-efficiency targets and development of efficient sources for their ionization. Alton came up with the ideas that led to the production of the fluorine-17 (¹⁷F) beam used by Bardayan, group leader Michael Smith, and their colleagues. Many radioactive beam scientists believed that a ¹⁷F beam could never be produced at the needed intensity at ORNL, but the developments by Alton's group helped to produce this "impossible beam" and prove them wrong.

To make an ¹⁷F beam, Alton had the idea of using highly permeable targets made from thin fibers of refractory oxides to allow fast diffusion of ¹⁷F ions out of the target and fast transport to the ion source before they decay. The oxygen-16 in the oxide target is transmuted to ¹⁷F by deuteron beams from the Oak Ridge Isochronous Cyclotron (ORIC). Oxide materials were selected by assessing their chemical equilibrium compositions and determining the highest temperature to which they could be heated. The best candidates proved to be aluminum, zirconium, and hafnium oxides (Al₂O₃, ZrO₂, and HfO₂). The preferred target material is Al₂O₃, because the release product is aluminum fluoride. On the other hand, both ZrO₂ and HfO₂) are more refractory than Al₂O₃, meaning that they can stand more beam-deposited heat. The speed of release strongly increases with temperature, so a hotter target can be important.

Dan Stracener and his HRIBF operations colleagues found that a combination of HfO₂ + Al₂O₃ target material gave the best results. The HfO₂ is the production target and absorbs the beam energy while the Al₂O₃ provides the Al for forming the aluminum fluoride molecules. These molecules are rapidly transported to a special ion source conceived by Alton, called the kinetic ejection negative ion source, which dissociates the molecules and negatively ionizes ¹⁷F efficiently as required for direct acceleration with the tandem accelerator.

"The key to solving the target problems is the use of highly permeable fibrous or thin-layered composite materials with dimensions chosen so that the species of interest can diffuse from the material within its lifetime," Alton says. "We were the first to propose the use of custom-dimensioned target materials to optimize release from the target. We select refractory chemical compounds by performing thermal and chemical analyses and then format them with the dimensions appropriate for the fast release of short-lived species."

More recently, Alton’s group developed a target for producing beams of neutron-rich nuclei for nuclear structure studies (see ORNL's Search for Rare Isotopes). This target consists of a stack of 12 to 14 carbon-fiber disks that are 2 millimeters thick and 15 millimeters in diameter. These carbon-fiber disks are coated with ~ 10 microns of depleted-uranium carbide, using a process developed for this purpose by McDermott Technology, Inc.

Gerald Alton’s group developed target materials for producing radioactive ion beams. The above two scanning electron micrographs show hafnium oxide fibers for producing fluorine-17 beams. The bottom two micrographs show uncoated carbon fibers (left) and a carbon-fiber disk coated with depleted-uranium carbide, using a process developed for this purpose by McDermott Technology, Inc.

When a 40-MeV proton beam from ORIC bombards the uranium carbide coating of the disks, the uranium-238 fissions, forming a wide range of fission fragments. More than 132 radioactive isotopes from 28 elements have been extracted from this target so far. The energy of the fission products aids their diffusion from the target to the ion source. There an electron beam plasma ion source is used to knock off electrons from the fragments, making them positively charged. These positive ions are run through a cesium cell where they pick up a pair of electrons each, making a negative ion beam of 200 keV. The tandem accelerator can boost the energy of this beam to 350 or 400 MeV.

These target and ion source developments have enabled experiments to be done with, for example, tellurium-132 at intensity levels of up to 2 x 10⁷ particles per second. The fibrous targets in combination with the kinetic ejection ion source produced beam intensities required to complete pioneering studies of reactions important in understanding element formation in stellar explosions.

"We have tried to move the state of the art in the technologies of both target and ion-source design from 'black art' toward science," Alton says. "These developments will have a strong impact on present and future radioactive ion beam facilities, based on the isotope separator on-line technique used at the HRIBF."

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