RA1. RIB Development
(D. W. Stracener)
The last two RIB campaigns have utilized a different uranium carbide target and the RIB yields have been as good as or better than any previously measured from the RIB Injector. The yields of the radioactive beams accelerated in the last few months are listed in the article describing the operations of the RIB Injector.
Uranium carbide (UC) targets used at the HRIBF have been manufactured in the past by applying a thin UC coating onto the fibers of a low-density, rugged carbon matrix. These targets have been used successfully for as long as 50 days to produce high yields of fission fragments via proton-induced fission. The production beam is 54-MeV protons with an average current of 10 microAmps. However, the manufacturing process is complicated and therefore, costly and it has been difficult to consistently produce high-quality targets.
The new uranium carbide targets are fabricated from a uranyl nitrate solution mixed with graphite powders and, so far, the process seems to produce UC targets with a more consistent quality. The UC targets are manufactured at ORNL in collaboration with Jim Kiggans in the Materials Science and Technology Division. The process is simpler than the previously-used technique resulting in lower costs and more reproducible results.
Each of these new UC disks is 0.15 cm thick with a diameter of 1.5 cm and a density of 2.2 g/cm3. The RIB production target consists of 13 of these disks to achieve a target thickness of 4.2 g/cm2, which is optimized for use with a 54-MeV proton beam. The UC disks are similar to pressed-powder targets used at other ISOL facilities but the fabrication process used here at ORNL is slightly different. The uranyl nitrate crystals are added to an organic binder solution and a flaky graphite mixture is added. This suspension is mixed using a Spex mill and then dried slowly at temperatures around 50 C. The graphite mixture consists of both natural graphite and synthetic graphite with particulate sizes of 6.4 micron and 33 microns, respectively. After drying, the powder is milled to make the particulate size more uniform and to break up any agglomerates. The target disks are then pressed from this powder at relatively low pressures and sintered in a vacuum furnace at temperatures around 1800 C to drive off the volatile components of the binder and to convert the uranium to uranium carbide. Further work is needed to determine the UC particulate size in the final target.
In the next few months, we plan to investigate RIB release efficiencies and useful lifetimes of these targets as a function of target density, UC particulate size, uranium to carbon ratio, and the characteristics of the graphite used in the fabrication process.