RA1. RIB Development
(D. W. Stracener)
After a short experiment in July to test a silicon carbide target, the HPTL facility was shutdown to allow for the installation of the high voltage platforms and conduits for the IRIS-2 facility. This silicon carbide target consisted of solid disks (about 1 mm thick) separated by gaps of about 4 mm to allow for radiation to the walls of the target holder. The simulations show that the production beam power can be significantly increased by allowing for heat dissipation via radiation when compared to the maximum beam power allowed when the dissipation of heat from the center of the targets is solely dependent on conduction through the silicon carbide disk. Early in 2008 we will be able to continue our target development at the HPTL and our initial tests will include a measurement of the release of radioactive aluminum isotopes from a niobium silicide target. We will also continue the investigation of silicon carbide targets to determine the maximum beam power that can be used on various target geometries. Our tests have shown that the silicon fiber targets have a higher release rate for aluminum as a function of beam power, but the silicon carbide disks can withstand up to ten times more production beam current, resulting in higher extracted beam currents of radioactive aluminum for experiments.
The "new" uranium carbide targets that were described in a previous Newsletter have performed quite well over the last few months and they have enabled the HRIBF to complete a good number of experiments with neutron-rich beams. These targets also helped the HRIBF achieve a record year in terms of the number of hours of radioactive beams delivered to experiments. During 2007 the production beam currents were gradually increased to the point where we now routinely use between 12 and 15 microAmps of 54-MeV protons on these uranium carbide targets. At these production beam currents, the useful lifetime of an ion source with a uranium carbide target is about 30 days.
As mentioned in the previous Newsletter, we made some initial tests of the release of fission fragments from a high-density (10.5 g/cm3) uranium carbide disk. The yields were lower than yields from low-density (2 g/cm3) targets by up to three orders of magnitude in some cases. We have recently received some uranium carbide samples that were produced using a similar technique but the densities are much lower at about 6 g/cm3 and in the near future we plan to test these targets with low production beam currents at the OLTF. Development of these high-density targets is a result of a collaboration between Will Talbert (TechSource), Jerry Nolen (ANL), and John Greene (ANL).
Another area of research at the OLTF has focused on the development of higher quality strontium beams, which are typically contaminated with rubidium isotopes. This effort is being led by Cara Jost, a student from Mainz. She is investigating ways to purify the strontium beams by looking at the formation and ionization efficiency of strontium oxide and strontium fluoride molecules and measuring the efficiency of formation of negative ion beams of strontium from these molecular beams passing through a Cs-vapor charge exchange cell. Another idea under investigation is the use of a selective adsorption technique where a quartz tube is used along a section of the transfer line between the target and the ion source. At some temperatures quartz will preferentially trap atoms of alkali metals (rubidium) while releasing atoms of other elements. This ion source coupled to a uranium carbide target will be tested on-line early in 2008.