7. New Results from Laser Ion Source Development at HRIBF
(Y. Liu, spokesperson)
Resonant ionization laser ion sources (RILIS) promise significant improvement over conventional ion sources in generating pure radioactive ion beams (RIB). We have made new progress in developing a RILIS for HRIBF.
A Ti:Sapphire laser complete with 2nd and 3rd harmonic units has been received and installed at the off-line ion source test facility 2 (ISTF-2). This laser is the first of three Ti:Sapphire lasers that we will need for a RILIS. It is manufactured by Photonics Industries International, Inc. and its prototype was first tested at ISTF-2 in 2006. Pumped by 18 W of pulsed 532 nm laser at 10 kHz from a Nd:YAG pump laser, it can delivery about 2 W fundamental peak power near 800 nm and more than 300 mW and 100 mW peak power in frequency doubled and tripled outputs, respectively. It is also continuously tunable from 700 nm to 960 nm, with typical linewidths of 1-3 GHz. Fig.7-1 shows a photo of the Ti:Sapphire laser together with the Nd:YAG pump laser.
Figure 7-1: The new Ti:Sapphire laser and its pump laser. Also shown are two additional Ti:Sapphire lasers from Mainz University, which are also pumped by the Nd:YAG laser.
In a RILIS, a particular isotope can be selectively ionized by laser radiation via stepwise atomic resonant excitations followed by ionization in the last transition. In order to yield a useful RIB current, maximized ionization efficiency is required. The RILIS efficiency is often limited by insufficient laser power to saturate the last ionization step. It is thus important to find schemes that lead to ionizing an excited atom resonantly through an autoionization (AI) or Rydberg state, which are much more efficient than non-resonant transitions to the continuum. Therefore, an important focus is to develop the most efficient ionization schemes for a range of elements of interest.
Ionization schemes for four new elements - Co, Ho, Tb and Dy - have been investigated in recent experiments conducted at ISTF-2 in collaboration with the LARISSA group led by Klaus Wendt of the University of Mainz and, for the first time, the TRIUMF ISAC laser ion source group led by Jens Lassen. The experiments were performed with our hot-cavity laser ion source, our new Ti:Sapphire laser, and two additional Ti:Sapphire lasers with harmonic generation units from the University of Mainz, which are also shown in Fig.7-1. Since there were no known resonant ionization schemes for the four elements using Ti:Sapphire lasers, searches for high lying Rydberg and AI states in these elements were necessary. With the new Photonics Ti:Sapphire laser, we were able to study the atomic spectroscopy of each element over a wide wavelength range. The Photonics Ti:Sapphire laser's continuous tunability proved to be extremely useful for such studies. Consequently, many AI states were observed for each element, most of them for the first time. Three-photon resonant ionization through AI states was achieved for Co. For Ho, Tb, and Dy, two-photon and three-photon ionization schemes were studied. Although no Rydberg states that led to resonant ionization were observed, many AI states were found in all three actinides. Fig. 7-2 shows some of the resonant ionization schemes established for these elements. Analysis of the experimental atomic spectroscopic data is in progress.
Figure 7-2: Resonant ionization schemes for Co, Ho, Dy, and Tb.
The ionization efficiency for Co and Ho has been measured using liquid samples that contained a known amount of the neutral atoms. Using the three-photon resonant ionization schemes shown in Fig.7-2, the overall ionization efficiency was found to be more than 20% for Co and about 40% for Ho. The 40% efficiency for Ho is the highest RILIS efficiency ever reported for any element.