RA1. RIB Development
The RIB development group has been involved in several efforts at the On-Line Test Facility (OLTF) and at the High-Power Target Laboratory (HPTL) to develop new radioactive ion beams and to improve the quality of the beams that are currently available at the HRIBF.
A group led by Jon Batchelder (ORAU) has made several on-line tests to optimize a target and ion source for the production of 97Ag beams. The search for an isomer in 97Ag is a PAC-approved experiment. The 97Ag nuclei will be produced in the reaction of a 54Fe beam with an enriched 50Cr target. These tests have included measurements of silver holdup times using different materials for the target holder and catcher foil, including molybdenum, tantalum, graphite, and niobium.
Another project, led by Ron Goans (a student from University of Tennessee), is seeking to understand the differences in yields of neutron-rich tin and germanium isotopes from uranium carbide targets that are produced using various techniques. Our data from the IRIS1 injector indicate that the RIB intensities of neutron-rich Ge and Sn isotopes are higher when UC/RVC targets are used than the intensities observed when we are using targets fabricated from UC powders. The UC/RVC targets are produced by coating a low-density, highly-porous graphite matrix with a layer of uranium carbide, resulting in a target density of about 1 g/cm3. We have not used these targets lately since the fabrication process is complex and it has been difficult to consistently produce high-quality targets. Over the last couple of years, we have used targets produced by pressing a mixture of UC powder with graphite powder. These targets, with densities of about 2.2 g/cm3, have performed quite well in terms of longevity and RIB yields with the exception of Ge and Sn isotopes. Beams of these two elements are purified by extracting positively charged sulfide molecules of these elements. Sulfur is added to the system via the introduction of hydrogen sulfide. Ron is investigating different methods of introducing sulfur into the system and looking at differences in the holdup times of the elemental ions and the molecular ions as a function of target material, target temperature, and sulfur concentration.
Another student, Cara Jost (Mainz), is continuing her investigation of a technique to purify neutron-rich strontium beams by selectively trapping the rubidium contaminants on surfaces in the transfer line between the UC target and the ion source. It is known that Group I elements can be trapped on a quartz surface at relatively high temperatures (up to 1200° C). Cara has measured holdup times of several elements (both stable and radioactive) at different temperatures for a variety of materials (quartz, sapphire, tantalum) and geometries, including "straight-thru" tubes and "blocked" tubes where the average number of interactions with the wall is much higher.
At the HPTL, we have recently completed an on-line measurement using a
proton beam from ORIC to irradiate a niobium silicide target for the
production of 25Al and 26Al.
The normalized yields were lower than we
had previously measured from a SiC fiber target. This is partly
explained by the high density (4.2 g/cm3) and low porosity of the
niobium silicide targets. We will produce another set of niobium
silicide targets that are more suitable for the release of short-lived
isotopes. Further investigations are needed to determine the best
targets for these aluminum beams. To this end, we have purchased a
couple square meters of silicon carbide cloth woven from fibers with a
nominal diameter of 10 microns. These materials are certified to have
very low oxygen content, which is important since silicon oxides are
quite volatile at the HRIBF ion source operating temperatures.