Gas catchers & SPIG - SextuPole Ion Guide

Gas catchers

The idea of a gas catcher is simple in principle: collect the reaction products in a noble buffer gas (He or Ar), use the flow of the gas to transport those to an exit hole and get a beam with nice properties for further studies. Applying those simple principles can give rise to many complications but also many applications!

Basic properties of a gas catcher

Thanks to the low chemical reactivity of the noble gases, the extraction of the reactions products should be chemically indiscriminate. In the very least, it is perfectly suited to study the refractory elements and also many that are not available at ISOL facilities (like iron, cobalt, nickel, ...)

The limits in this chemical transparency comes from the contaminants present in the buffer gas. Due to work in air and to limits of purification of commercially available gases, some ppb (particles per billion) of various gases (nitrogen, oxigen, water, ...) are present in the gas cell. Those are then reacting with the reaction products and form maolecules that cannot be studied in the selected mass range. Several steps have been taken at LISOL to reduce this effect. The first is to flush the cell while heating it for several hours after each work performed on it; the second is the implantation of an extra gas purifier to have a better control of the gas purity; finally, the addition of a potential difference between the gas cell and the SPIG induces the dissociation of some molecules.^[1]

The pressure of the buffer gas is also a factor of great importance in a gas catcher. Too low pressure does not allow for a sufficient stopping power while too high pressure makes the environement too thick and the reaction products cannot be efficiently extracted. A pressure of 500 mbar is the optimal running condition for the LISOL setup. Note that this parameter depends on the recoil energy of the reaction products, the volume of the gas catcher, the size of the exit nozzle and the vacuum conditions required outside the gas cell.

Different purpose, different gas catcher

Not all experiment are the same and not every one of them requires the same conditions to run in an optimal fashion. Their are three different types of gas cell that have been developped (or are under development) at LISOL.

	Fusion cell This cell has a limited inside volume to minimise the extraction time and study isotopes as exotic as possible. This setup is typically used to study exotic nuclei with N=Z and other neutron-deficient nuclei by fusion-evaporation reactions from both a stable beam and a stable thin target.
	Fission cell This cell has a large volume to catch as much of the recoils as possible. The targets are surrounded by an aluminium foil to reduce the diffusion of the plasma induced by the reaction to the exit of the cell where it would trigger many recombinations of electrons and ions. This setup is mostly used to study neutron-rich nuclei from the p-induced fission of 238U.
	Shadow cell This setup is still under development. Its original purpose was to decouple the reaction and ionisation regions to obtain better selectivity by limiting the recombination of electrons to ions close to the exit nozzle. It allows also for the study of electrodes in the cell to collect surviving ions, responsible for the limit in selectivity. This last study resulted in the first proof of the feasibility of combining a gas catcher to a Laser Ion Source Trap (LIST).

Surviving ions and laser ions

Despite the claims about gas catchers on chemical independance, it turns out that some form of chemical effects remain, whether it is related to the contaminants in the gas or to particular resonances between atomic levels of the element and the buffer gas. Using a spontaneous fission source (²⁵²Cf)m the survival of different elements has been studied by comparing the detected production to the expected yields. No hard conclusion could be found but some relation to the ionisation potential of the element can be seen.

Survival efficiency of all the isotopes that could be studied using the spontaneous fission source 252Cf.	Comparison of the survival efficiency to the ionisation potential of the studied elements.

This property is of particular importance when discussing the selectivity of the ion source (the number of ions produced with laser ionisation compared to the same number without the laser ionisation). If the element of interest has a high survival efficiency, the laser enhancement will be limited. This limitation triggered the development of the Shadow gas cell and later on of the combinaison of a LIST to the gas catcher.

SPIG - SextuPole Ion Guide

The SextuPole Ion Guide (SPIG) is a structure made of 6 rods (130mm long) placed in a circle around the beam path at the exit nozzle of the gas cell. A radiofrequency signal is applied on the rods with each neighbouring rod 180 degrees out of phase. The result is an overall cooling and focusing of the beam to the center of its path and an improved emittance for further transport and handling.

The use of a small negative potential difference (-210V) from the gas catcher to the SPIG is used to dissociate molecules formed between the isotopes of interest and the contaminants found in the buffer gas.

The use of a small positive potential difference (+40V) from the gas catcher to the SPIG was the mean of discovery of the feasibility of coupling a gas catcher to a LIST.

The SPIG is also a complicated device with many parameters that can vary from one experiment to the other. It also has a strong influence on the selectivity as it can catch argon ions from outside the gas cell or collect activity on the rods to give it back later in the beam.

Most recent publications and theses on this subject

[1] Yu. Kudryavtsev, B. Bruyneel, M. Huyse, J. Gentens, P. Van den Bergh, P. Van Duppen, L. Vermeeren. A gas cell for thermalizing, storing and transporting radioactive ions and atoms. Part I: off-line studies with a laser ion source. Nuclear Instruments and Methods in Physics Research B 179:412-435 (2001).

[2] Yu. Kudryavtsev, M. Facina, M. Huyse, J. Gentens, P. Van den Bergh, P. Van Duppen. Beams of isotopes produced at LISOL by laser ionization after thermalization of energetic ions in a gas cell. Nuclear Instruments and Methods in Physics Research B 204:336-342 (2003).

[3] M. Facina, B. Bruyneel, S. Dean, J. Gentens, M. Huyse, Yu. Kudryavtsev, P. Van den Bergh, P. Van Duppen. A gas cell for thermalizing, storing and transporting radioactive ions and atoms. Part II: on-line studies with a laser ion source. Nuclear Instruments and Methods in Physics Research B 226:401-418 (2004).

[A] M. Facina. A gas catcher for the selective production of radioactive beams through laser ionzation. Ph.D. Thesis, KULeuven, 2004.