RA1. RIB Development
(D. W. Stracener)
A number of interesting radioactive beams, such as 17F, 33Cl, and 56Ni, can be efficiently ionized using negative-ion sources coupled to an ISOL production target. In each of the cases listed, an isobaric contaminant dominates, resulting in low-purity radioactive beams delivered to the experiment if no purification technique is available. In a previous HRIBF Newsletter we reported on one technique developed to purify a 56Ni beam contaminated with 56Co (see also Liu, Beene, Havener, and Liang, Appl. Phys. Lett. 87, 113504 (2005)). This technique utilizes selective nonresonant laser photodetachment to neutralize and substantially suppress the isobaric contamination. A laser beam having the appropriate photon energy and overlapping the ion beam can be used to selectively neutralize the contaminant if the electron affinity of the contaminant is lower than the electron affinity of the desired radioactive ions. In order to increase the laser-ion interaction time, and thus improve the photodetachment efficiency, the laser-ion beam overlap is made inside a gas-filled RFQ where the ion residence time can be a few milliseconds.
This technique is being developed using negative-ion beams of stable isotopes at the Ion Source Test Facility I (ISTF-1). The main focus of this off-line facility in recent years has been the development of negative-ion sources for RIB production. In the Ni/Co system, the Co contaminant in the negative-ion beam was reduced by a factor of 1000 while only 22% of the Ni ions were neutralized. This will result in an improvement in the 56Ni beam purity from less than 10% to about 99%.
The usefulness of this technique was recently demonstrated for suppressing oxygen contamination in fluorine beams and sulfur contamination in chlorine beams. The electron affinities for negative ions of these elements are: F (3.41 eV), O (1.43 eV), Cl (3.62 eV) and S (2.08 eV). The tests were made using a laser beam with a photon energy of 2.35 eV (523 nm) interacting with negative-ion beams of stable isotopes for each element. Initial results show that 95% of the negative oxygen ions and 99% of the negative sulfur ions were neutralized, while no photodetachment of fluorine and chlorine ions was observed. The negative-ion transmission through the gas-filled RFQ ranged from 27% up to 52%. Further tests will be needed to determine the optimum laser power, laser beam size, and laser-ion beam interaction time, and to improve the ion beam transmission.