RA1. RIB Development
(D. W. Stracener)
We have recently completed two separate tests using laser beams to improve the quality of our radioactive ion beams. The first is an initial test of an ion source where the ions are produced using three independently-tuned Ti:Sapphire lasers to selectively ionize the element of interest. This project is described in another article in this Newsletter. In the other development project a laser is used to purify a 56Ni beam by eliminating a large fraction of the 56Co contamination from the low energy beam that is extracted from the ion source.
Accelerated beams of 56Ni have a wide range of interest in the nuclear structure and astrophysics communities. Some time ago, we developed a target and ion source to deliver 56Ni beams. A nickel pellet is irradiated with 42-MeV protons and the 56Ni is produced via the (p,p2n) reaction on 58Ni. In addition, 56Co is produced via the (p,2pn) reaction and the cross-section for the 56Co production is about an order of magnitude larger than for the 56Ni production. This irradiated sample is then rotated into a Cs-sputter ion source and negative-ion beams are produced. The sputter efficiencies of nickel and cobalt are similar, so the composition of the A=56 beam is about 90% cobalt and 10% nickel.
We have demonstrated an efficient way to purify the Ni beam by selectively neutralizing the negative Co ions using a CW Nd:YAG laser interacting with the beam in a gas-filled RF quadrupole (RFQ) ion guide. The photons are absorbed and the additional electrons are excited into the continuum. The technique of laser photo-detachment was demonstrated in the Co/Ni system over a decade ago by Berkovits et al. (NIM A281 (1989) 663 and NIM B52(1990) 378).
The electron affinities are 0.661 eV for Co and 1.156 eV for Ni and the fundamental wavelength of a Nd:YAG laser is 1064 nm (1.165 eV). This photon energy is high enough to efficiently neutralize negative Co ions and, with much lower efficiency, negative Ni ions will also be neutralized. The beam energy from the ion source is 5 keV and the energy is reduced to <50 eV before entering the He-filled RFQ ion guide, which is 40 cm long. In the RFQ ion guide, ion energy is further reduced by collisions with the buffer gas molecules. This results in a laser-ion interaction time on the order of 1 ms.
In a test with stable isotopes (see Fig.RA1-1) 95% of the Co ions were neutralized and 10% of the Ni ions were neutralized. For this test the laser beam power was 5 W with about 50% transmission through the RFQ. The figure shows the relative beam intensities of nickel and cobalt ions with the laser beam interacting with the ion beam (Laser On) and with the laser beam physically blocked (Laser Off).
Several improvements must be realized before the technique can be implemented on the RIB Injector as a means to deliver purified beams of 56Ni. The transmission of the ion beam through the RFQ ion guide must be improved. A new RFQ ion guide system must be designed for beams with energies of about 20 keV. Higher laser power is needed to further suppress the negative Co ions in the beam. While it will certainly be possible to implement this technique in a second, more spacious radioactive beam injector, we are trying to devise a way to use this purification technique on the present RIB Injector.