RA1. RIB Development
(D. W. Stracener)

In the last few months, a significant effort was made to improve the ⁷Be beam intensity, including finding the best matrix for the sputter target and improving the reliability of the ion source. The ⁷Be atoms are separated from the lithium production target using chemical separation techniques and then added to a sputter target containing a metal powder. Tests at the OLTF with different metal powders show that the best integrated yield of ⁷Be was achieved using copper powder (slightly better than silver powder). Further improvements to the ion source have been made and should result in better reliability and a decrease in the downtime due to problems with the cesium delivery system. Also, changes in the geometry of the sputter target holder should result in higher ⁷Be concentrations in the target matrix. Recently completed experiments using a Be beam are described in a separate article in this Newsletter.

A couple of candidate target materials for RIB production targets were tested off-line to ensure compatibility with the target holder and ion source materials and to determine the optimum operating temperature. With the goal of making proton-rich vanadium beams, titanium oxide powder was pressed into pellets and tested in a hot-plasma ion source. The maximum target temperature was 1700° C and above this temperature the ion source efficiency dropped dramatically. An on-line test showed that the release efficiency of vanadium atoms from this target at 1650° C is small, resulting in low beam intensities. Also, no activity was found in molecular sidebands (e.g. oxides, chlorides, or sulfides). Thorium oxide powders were also pressed into pellets and tested off-line. The maximum operating temperature for this target material was 1950° C. The pellets were slightly denser (about 10%) after operating at this temperature for about three days, which may indicate that the release efficiency will decrease during the lifetime of these targets. The yields of radioactive fission fragments from this target will be compared to those achieved using uranium carbide targets.

In September, we will have another development run using a laser ion source in collaboration with a group from the University of Mainz. The goal of this work is to develop a RIB production ion source using all solid-state lasers. Specifically, during this run, ionization schemes for Sn, Ni, Ge, Cu, and Sr will be investigated. The results of these tests will be reported in the next Newsletter. Also, in the next Newsletter, we will report on the recently completed on-line tests for proton-rich Se beams from a liquid Ge target and the yields of fission fragments from the thorium oxide target.