Lasers & LIST - Laser Ion Source Trap

Laser ion source

	By sending photons with the right energy, it is possible to excite an electron from a level to another. If chosing the right sequence of photons with which to irradiate an element, it is even possible to excite the electron beyond the ionisation potential and then produce an ion. This is the technique used in a laser ion source where the photons are produced at the right energy with a frequency tunable laser system. The power required to excite successively a large number of isotopes can only be achieved with a pulsed laser system where the power is compressed in a small bunch and therefore where high power density can be reached.
	This technique is used at LISOL. Two excimer lasers with repetition rate up to 200 Hz are used to pump two dye lasers, one of which can also be used with a doubling cavity. A wide range of frequencies can then be reached (225-800 nm) and many isotopes have already been successfully ionised on-line (Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Ru, Rh, Pd).
	The power of this technique resides in the ionisation of elements that are otherwise not available. Iron, cobalt, nickel and cupper are good examples of this. With no laser ionisation, the recorded gamma spectra are free of the gamma lines coming from the decay of those elements while with laser ionisation, new gamma lines appear.

Laser Ion Source Trap

The concept of the Laser Ion Source Trap (LIST) is to combine the selectivity of a laser ion source with the beam quality of an ion trap. By ionising the element of interest from an atomic beam within a trap, the contamination should be greatly reduced and the beam quality highly enhanced. The bunching of the beam from the trap can also prove to be of great interest.

This technique has first been proposed and developped for common ISOL facilities^[1]. Since then, it has been proposed for gas catcher ion sources such as the one in LISOL or at IGISOL^[A] at JYFL.

The first real observation of the feasibility of coupling a LIST to a gas catcher comes, however, from LISOL. By applying a positive voltage on the SPIG rods, all ions from the gas cell are repelled and the only place where laser ionisation occurs is within the radiofrequency ion guide^[2]. As the SPIG cannot yet be used as a trap, this cannot be accounted as a full LIST, yet this proves the feasibility of the technique.

Most recent publications and theses on this subject

[1] K.D.A. Wendt, K. Blaum , Ch. Geppert , R. Horn , G. Passler , N. Trautmann , B.A. Bushaw. Laser resonance ionization for efficient and selective ionization of rare species. Nuclear Instruments and Methods in Physics Research B 204:325-330 (2003).

[2] T. Sonoda, T.E. Cocolios, Yu. Kudryavtsev. LIST @ LISOL. - to be published.

[A] T. Kessler. Development and application of laser technologies at radioactive ion beam facilities. Ph.D. thesis, JYFL, 2008.