Titanium:Sapphire Laser Ion Source

In the last ten years the resonant ionization with laser light has become a very often used technique for the production of radioactive ion beams at on-line mass separator facilities. Atomic radionuclides are produced by bombardment of a target with a high energetic proton beam. The radionuclides are resonantly excited and ionized by laser light. This ionization process ionizes just the desired element. The produced ion beam passes afterwards a mass separator, to select just the desired isotope. Because of the ionization method, the laser ion source offers a higher elemental selectivity than other ion sources like the surface or the plasma ionization ion sources. The applicability of a laser ion source is limited by surface ionized isobars which are produced in the source by the high temperatures which cannot be prevented at on-line working conditions.

Principle of a laser ion source - click for bigger version

At the RISIKO off-line mass separator at Mainz university a new type of laser ion source is under development (see LIST). In this development a new self-constructed pulsed titanium:sapphire (Ti:Sa) laser system for multistep resonant ionization is used.

Laser system at the RISIKO off-line mass separator
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The requirements for a pulsed laser system used at a laser ion source are: small linewidth for high selectivity, high pulse power, short pulse duration, good beam quality, wide tuning range and easy operation. Because of the high peak intensity it is possible to saturate the optical transitions in a huge volume. This results in a high efficiency and in a reduction of the background by the ratio of pulse duration to time between two pulses.

Ti:Sa laser in operation
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Resonator of a Ti:Sa laser
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The shown laser has been developed to achieve the mentioned demands. A titanium:sapphire crystal is used for the active medium, which offers a tunium range from 600-1000 nm. At the moment a wavelength range fom 725 to 920 nm is accessible by using four different mirror sets. The wavelength selection is done by a three plate birefringent filter and a etalon. To realize multistep resonant excitation, three identical lasers haven been constructed. To syncronize all three lasers, which are pumped by one frequency doubled Nd:YAG pump laser, a Q-switch is installed inside the resonator. The elalon is computer controlled, the wavelenght determination is realized with a commercial wavemeter.To expand the achievable wavelenght range, the efficient freaquency doubling and tripling is possible. So the wavelenght ranges from 240 to 306 nm and 362 to 460 nm are accessible which allows the ionization of many elements with the Ti:Sa laser system.

The typical output power of our lasers is 1.5 W when pumped with a pump power of 10 W @ 532 nm and 10 kHz repetition rate. Doubled are typical 100 mW and tripled 10 mW avaiable. The pulses are 35 ns long, but even pulse lenghts shorter than 20 ns are possible. The linewidth is about 3-5 GHz and the beam shape is TEM00.

Tripling unit for the Ti:Sa laser system
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